Echinoderm, pronounced ih KY nuh durm, is the general name of certain spiny-skinned sea animals. There are about 6,000 kinds of echinoderms. Starfish, brittle stars, sand dollars, sea urchins, and sea cucumbers are among the most common kinds. All echinoderms have an internal bony skeleton. Their spines are a part of the skeleton. The echinoderm phylum (large animal group) is the only major phylum made up entirely of sea animals.
Adult echinoderms have radial symmetry. Their body parts are arranged around the center of the animal like the spokes of a wheel around the hub. Echinoderm bodies are usually divided into five sections with the mouth in the center.
Echinoderms are the only animals that have many tiny tubelike structures called tube feet. The tube feet project from the body in rows. Echinoderms use the tube feet for moving, feeding, breathing, and sensing. The outer tip of each tube often forms a suction disk for gripping hard surfaces. Within the echinoderm's body, a tiny bulb attached to the tube foot forces water into it to make it lengthen. An internal system of water-filled canals connects the tube feet to each other and to a sievelike plate that usually opens to the sea water. The entire system of tube feet and canals is called the water vascular system.
Echinoderms reproduce by laying eggs that develop into larvae and swim freely. The larvae have bilateral symmetry (two similar halves). The larvae sink to the ocean bottom and change into the adult, radial form.
Scientific classification. Echinoderms make up the echinoderm phylum, Echinodermata.
Wednesday, February 21, 2007
Mollusks
Mollusk, pronounced MAHL uhsk, is a soft-bodied animal that has no bones. Snails, slugs, clams, mussels, oysters, squids, and octopuses are mollusks. Most mollusks have a hard shell that protects their soft bodies. Some, such as cuttlefish and squids, have no outside shell. A special shell grows inside their bodies. This shell is called a cuttlebone in cuttlefish and a pen in squids. A few kinds of mollusks, including octopuses and certain slugs, have no shell at all. For additional information on mollusk shells and how they are formed.
All mollusks have a skinlike organ called the mantle. In mollusks with outside shells, the mantle makes the shell. The edges of the mantle release liquid shell materials and add them to the shell as the mollusk grows. In mollusks with no outside shell, the mantle forms a tough cover around the body organs.
Mollusks live in most parts of the world. Some kinds of mollusks live in the deepest parts of oceans. Others are found on the wooded slopes of high mountains. Still others live in hot, dry deserts. Wherever mollusks live, they must keep their bodies moist to stay alive. Most land mollusks live in damp places such as under leaves or in soil.
Mollusks are used mainly for food. People in many parts of the world eat mollusks every day. Most Americans do not eat them nearly so often. The most popular kinds used as food in the United States are clams, oysters, and scallops. Mollusk shells are made into many useful products, including pearl buttons, jewelry, and various souvenir items. Perhaps the best-known mollusk products are the pearls made by pearl oysters.
Some mollusks are harmful to people. For example, certain small, freshwater snails of the tropics carry worms that cause an often fatal disease called schistosomiasis. Shipworm clams drill into rope, wooden boats, and wharves and cause millions of dollars worth of damage a year.
Mollusks make up the largest group of water animals. There are about 50,000 known kinds of living mollusks, and scientists find about 1,000 new species every year. The fossils of about 100,000 other species of mollusks have also been found.
The mollusks make up a phylum (major division) of the animal kingdom. The scientific name of the phylum is Mollusca, a Latin word meaning soft-bodied. To learn where the phylum fits into the animal kingdom, see ANIMAL.
There are seven classes (large groups) of mollusks. They are (1) univalves or Gastropoda, (2) bivalves or Bivalvia or Pelecypoda, (3) octopuses and squids or Cephalopoda, (4) tooth shells or Scaphopoda, (5) chitons or Polyplacophora, (6) Monoplacophora, and (7) Aplacophora.
Univalves or gastropods (Gastropoda) are the largest class of mollusks. They include limpets, slugs, snails, and whelks. Most kinds of univalves have a single, coiled shell. The name univalve comes from Latin words meaning one shell. But some kinds of univalves, including garden slugs and the sea slugs called nudibranchs, have no shells after the larval stage.
The name Gastropoda comes from Greek words meaning belly and foot. Gastropods seem to crawl on their bellies, but actually they use a large, muscular foot. The foot spreads beneath the body, and its muscles move in a rippling motion that makes the animal move forward. Most sea snails and some land snails have a lidlike part called an operculum on the back of the foot. When danger threatens, the snail draws back into its shell and the operculum closes the shell opening.
Certain kinds of univalves have two pairs of tentacles (feelers) on their heads. One pair helps the animals feel their way about. Some species have an eye on each of the other two tentacles. Others have no eyes at all. A univalve also has a ribbon of teeth. This ribbon, called a radula, works like a rough file and tears apart the animal's food. Most univalves that eat plants have thousands of weak teeth. A few kinds eat other mollusks, and have several dozen strong teeth.
Bivalves (Bivalvia or Pelecypoda) form the second largest class of mollusks. They include clams, oysters, mussels, scallops, and shipworms. All bivalves have two shells held together by hinges that look like small teeth. The shells of bivalves are usually open. When the animals are frightened, strong muscles pull the shells shut and hold them closed until danger has passed.
Bivalves have a strong, muscular foot. Many kinds of these animals move about by pushing the foot out and hooking it in the mud or sand. Then they pull themselves up with the foot. Some bivalves, such as the geoduck and razor clam, use the foot to dig holes. They push the foot downward into mud or sand. First the foot swells to enlarge the hole, and then it contracts and pulls the shell into the burrow. The Pholas clam can dig holes even in hard clay or soft rock.
Bivalves have no head or teeth. They get oxygen and food through a muscular siphon (tube). The siphon can be stretched to reach food and water if the animal is buried in mud or sand. Bivalves feed on plant cells material, which is filtered from the water by the gills.
Octopuses and squids (Cephalopoda) are the most active mollusks. The argonaut, cuttlefish, and nautilus also belong to this group. All the species in the group live in the ocean.
The word Cephalopoda comes from Greek words meaning head and foot. A cephalopod seems to be made up of a large head and long arms that look like feet. Octopuses and squids have dome-shaped "heads" surrounded by arms. Octopuses have eight arms, and squids have eight arms and two tentacles. The arms grow around hard, strong, beaklike jaws on the underside of the head. These jaws tear the animal's prey, and are far more dangerous than the arms. Octopuses use their arms and squids use their tentacles and arms to capture prey and pull it through their jaws. Octopuses and squids eat fish, other mollusks, and shellfish.
Tooth shells (Scaphopoda) have slender, curving shells that resemble tusks. These mollusks are often called tusk shells. The word Scaphopoda comes from Greek words that mean boat and foot. A tooth shell has a pointed foot that looks somewhat like a small boat. All tooth shells live in the ocean, where they burrow in the mud or sand. The top of the shell sticks up into the water. Tooth shells have no head or eyes. They feed on one-celled organisms that are swept into the mouth by tentacles.
Chitons (Polyplacophora) have flat, oval bodies covered by eight shell plates. The plates are held together by a tough girdle. The name Polyplacophora comes from Greek words that mean many, shell, and bearer. This name refers to the eight overlapping pieces of a chiton's shell. Chitons have a large, flat foot. They can use the foot to move about, but they usually cling firmly to rocks. When they are forced to let go of the rocks, they roll up into a ball. Chitons have a small head and mouth, but they have no eyes or tentacles. Their long radula has many teeth, which some chitons use to scrape seaweed from rocks for food.
Monoplacophora live in the deep parts of the ocean, and most are found only as fossils. The name Monoplacophora comes from Greek words meaning single, shell, and bearer. Monoplacophorans have one shell that is almost flat, like a limpet shell. They are unusual because they have several pairs of gills, six or more pairs of kidneys, and many ladderlike nerve centers. Like other mollusks, they have a mantle. They also have a radula. Little is known about their habits.
Aplacophora are rarely seen, wormlike mollusks covered with small spines. The name Aplacophora comes from Greek words that mean no shell.
All mollusks have a skinlike organ called the mantle. In mollusks with outside shells, the mantle makes the shell. The edges of the mantle release liquid shell materials and add them to the shell as the mollusk grows. In mollusks with no outside shell, the mantle forms a tough cover around the body organs.
Mollusks live in most parts of the world. Some kinds of mollusks live in the deepest parts of oceans. Others are found on the wooded slopes of high mountains. Still others live in hot, dry deserts. Wherever mollusks live, they must keep their bodies moist to stay alive. Most land mollusks live in damp places such as under leaves or in soil.
The importance of mollusks
Mollusks are used mainly for food. People in many parts of the world eat mollusks every day. Most Americans do not eat them nearly so often. The most popular kinds used as food in the United States are clams, oysters, and scallops. Mollusk shells are made into many useful products, including pearl buttons, jewelry, and various souvenir items. Perhaps the best-known mollusk products are the pearls made by pearl oysters.
Some mollusks are harmful to people. For example, certain small, freshwater snails of the tropics carry worms that cause an often fatal disease called schistosomiasis. Shipworm clams drill into rope, wooden boats, and wharves and cause millions of dollars worth of damage a year.
Kinds of mollusks
Mollusks make up the largest group of water animals. There are about 50,000 known kinds of living mollusks, and scientists find about 1,000 new species every year. The fossils of about 100,000 other species of mollusks have also been found.
The mollusks make up a phylum (major division) of the animal kingdom. The scientific name of the phylum is Mollusca, a Latin word meaning soft-bodied. To learn where the phylum fits into the animal kingdom, see ANIMAL.
There are seven classes (large groups) of mollusks. They are (1) univalves or Gastropoda, (2) bivalves or Bivalvia or Pelecypoda, (3) octopuses and squids or Cephalopoda, (4) tooth shells or Scaphopoda, (5) chitons or Polyplacophora, (6) Monoplacophora, and (7) Aplacophora.
Univalves or gastropods (Gastropoda) are the largest class of mollusks. They include limpets, slugs, snails, and whelks. Most kinds of univalves have a single, coiled shell. The name univalve comes from Latin words meaning one shell. But some kinds of univalves, including garden slugs and the sea slugs called nudibranchs, have no shells after the larval stage.
The name Gastropoda comes from Greek words meaning belly and foot. Gastropods seem to crawl on their bellies, but actually they use a large, muscular foot. The foot spreads beneath the body, and its muscles move in a rippling motion that makes the animal move forward. Most sea snails and some land snails have a lidlike part called an operculum on the back of the foot. When danger threatens, the snail draws back into its shell and the operculum closes the shell opening.
Certain kinds of univalves have two pairs of tentacles (feelers) on their heads. One pair helps the animals feel their way about. Some species have an eye on each of the other two tentacles. Others have no eyes at all. A univalve also has a ribbon of teeth. This ribbon, called a radula, works like a rough file and tears apart the animal's food. Most univalves that eat plants have thousands of weak teeth. A few kinds eat other mollusks, and have several dozen strong teeth.
Bivalves (Bivalvia or Pelecypoda) form the second largest class of mollusks. They include clams, oysters, mussels, scallops, and shipworms. All bivalves have two shells held together by hinges that look like small teeth. The shells of bivalves are usually open. When the animals are frightened, strong muscles pull the shells shut and hold them closed until danger has passed.
Bivalves have a strong, muscular foot. Many kinds of these animals move about by pushing the foot out and hooking it in the mud or sand. Then they pull themselves up with the foot. Some bivalves, such as the geoduck and razor clam, use the foot to dig holes. They push the foot downward into mud or sand. First the foot swells to enlarge the hole, and then it contracts and pulls the shell into the burrow. The Pholas clam can dig holes even in hard clay or soft rock.
Bivalves have no head or teeth. They get oxygen and food through a muscular siphon (tube). The siphon can be stretched to reach food and water if the animal is buried in mud or sand. Bivalves feed on plant cells material, which is filtered from the water by the gills.
Octopuses and squids (Cephalopoda) are the most active mollusks. The argonaut, cuttlefish, and nautilus also belong to this group. All the species in the group live in the ocean.
The word Cephalopoda comes from Greek words meaning head and foot. A cephalopod seems to be made up of a large head and long arms that look like feet. Octopuses and squids have dome-shaped "heads" surrounded by arms. Octopuses have eight arms, and squids have eight arms and two tentacles. The arms grow around hard, strong, beaklike jaws on the underside of the head. These jaws tear the animal's prey, and are far more dangerous than the arms. Octopuses use their arms and squids use their tentacles and arms to capture prey and pull it through their jaws. Octopuses and squids eat fish, other mollusks, and shellfish.
Tooth shells (Scaphopoda) have slender, curving shells that resemble tusks. These mollusks are often called tusk shells. The word Scaphopoda comes from Greek words that mean boat and foot. A tooth shell has a pointed foot that looks somewhat like a small boat. All tooth shells live in the ocean, where they burrow in the mud or sand. The top of the shell sticks up into the water. Tooth shells have no head or eyes. They feed on one-celled organisms that are swept into the mouth by tentacles.
Chitons (Polyplacophora) have flat, oval bodies covered by eight shell plates. The plates are held together by a tough girdle. The name Polyplacophora comes from Greek words that mean many, shell, and bearer. This name refers to the eight overlapping pieces of a chiton's shell. Chitons have a large, flat foot. They can use the foot to move about, but they usually cling firmly to rocks. When they are forced to let go of the rocks, they roll up into a ball. Chitons have a small head and mouth, but they have no eyes or tentacles. Their long radula has many teeth, which some chitons use to scrape seaweed from rocks for food.
Monoplacophora live in the deep parts of the ocean, and most are found only as fossils. The name Monoplacophora comes from Greek words meaning single, shell, and bearer. Monoplacophorans have one shell that is almost flat, like a limpet shell. They are unusual because they have several pairs of gills, six or more pairs of kidneys, and many ladderlike nerve centers. Like other mollusks, they have a mantle. They also have a radula. Little is known about their habits.
Aplacophora are rarely seen, wormlike mollusks covered with small spines. The name Aplacophora comes from Greek words that mean no shell.
Arthropods
Arthropod, pronounced AHR thruh pahd, is any animal that belongs to the major division, or phylum, of the animal kingdom called the Arthropoda. This term is formed from two Greek words, and means jointed feet. Actually, the legs, rather than the feet, are jointed. All the Arthropoda, or arthropods, have jointed legs. Among the most important groups of arthropods are the following: (1) insects, including cockroaches, beetles, bees, butterflies, and many others; (2) crustaceans, including such well-known animals as crabs, lobsters, shrimps, and barnacles; (3) arachnids, including mites, ticks, spiders, and scorpions; (4) chilopods, or centipedes; and (5) diplo-pods, or millipedes. The arthropod phylum contains more than three-fourths of all the different kinds of animals. Insects make up the largest class of arthropods in terms of the number of species.
The bodies of arthropods, as well as their legs, are made up of sections. Among some primitive arthropods, each section of the body has its own pair of legs. Most of these legs are used for swimming or walking. In some types of arthropods, certain legs have developed special shapes and uses. Some serve as sucking organs, some are jaws, some serve as weapons of offense and defense, and some are sense organs. Insects lack most of the pairs of legs found in other arthropods. They have only three pair. One pair is attached to each segment of an insect's chest or thorax. Insects also may have one or two pairs of wings.
Arthropods have an outside shell, or exoskeleton, that contains a stiff, horny material called chitin. Certain arthropods, such as flies and moths, have only thin, weak shells. Others, including crabs and lobsters, have thick, strong shells. Nearly all arthropods have a kind of heart and blood system and usually a well-organized nervous system. Some arthropods have simple eyes, some have compound eyes, and some (including many insects) have eyes of both types.
The bodies of arthropods, as well as their legs, are made up of sections. Among some primitive arthropods, each section of the body has its own pair of legs. Most of these legs are used for swimming or walking. In some types of arthropods, certain legs have developed special shapes and uses. Some serve as sucking organs, some are jaws, some serve as weapons of offense and defense, and some are sense organs. Insects lack most of the pairs of legs found in other arthropods. They have only three pair. One pair is attached to each segment of an insect's chest or thorax. Insects also may have one or two pairs of wings.
Arthropods have an outside shell, or exoskeleton, that contains a stiff, horny material called chitin. Certain arthropods, such as flies and moths, have only thin, weak shells. Others, including crabs and lobsters, have thick, strong shells. Nearly all arthropods have a kind of heart and blood system and usually a well-organized nervous system. Some arthropods have simple eyes, some have compound eyes, and some (including many insects) have eyes of both types.
Nematodes
Roundworm, also called nematode or eelworm, is any of more than 12,000 species of worms. Many species of roundworms live freely in soil, water, dead plants, or dead animals. All other roundworms are parasites. They live and feed on living plants and animals, which serve as hosts. Some parasitic species cause serious diseases in human beings and other hosts.
Roundworms range in size from microscopic to more than 3 feet (90 centimeters) long. They have slender, round bodies with tapered ends. Roundworms have remarkable powers of reproduction and are extremely numerous. Researchers have found more than 90,000 roundworms in a single rotting apple.
Nearly all species of roundworms reproduce by laying eggs. Some species produce great quantities of eggs. For example, females of the species Ascaris lumbricoides each lay about 200,000 eggs per day for at least 10 months. Among some species of roundworms, the eggs hatch into tiny young that look like the adults. Eggs of other species hatch into young called larvae, which gradually transform into adults. Species of roundworms that do not lay eggs give birth to larvae.
Parasitic roundworms may infect a host in a number of ways. Some species enter the host when the host swallows food that contains the roundworm, its eggs, or its larvae. Among other species, the larva burrows into the skin of the host. In other species of roundworms, the larva is taken up by an insect, such as a fly or a mosquito, and transmitted through the bite of that insect to the host.
At least 14 species of roundworms cause infection in human beings. A. lumbricoides, which inhabits the small intestine, infects about 65 million people throughout the world. It causes a disease called ascariasis. Symptoms of this disease include pneumonia and intestinal pain. The roundworm Trichuris trichiura infects the large intestine and occurs in about 350 million people worldwide. It causes trichuriasis, a disease characterized by diarrhea. Other common roundworms that cause disease in humans include filariae, hookworms, pinworms, and trichinae.
Roundworms range in size from microscopic to more than 3 feet (90 centimeters) long. They have slender, round bodies with tapered ends. Roundworms have remarkable powers of reproduction and are extremely numerous. Researchers have found more than 90,000 roundworms in a single rotting apple.
Nearly all species of roundworms reproduce by laying eggs. Some species produce great quantities of eggs. For example, females of the species Ascaris lumbricoides each lay about 200,000 eggs per day for at least 10 months. Among some species of roundworms, the eggs hatch into tiny young that look like the adults. Eggs of other species hatch into young called larvae, which gradually transform into adults. Species of roundworms that do not lay eggs give birth to larvae.
Parasitic roundworms may infect a host in a number of ways. Some species enter the host when the host swallows food that contains the roundworm, its eggs, or its larvae. Among other species, the larva burrows into the skin of the host. In other species of roundworms, the larva is taken up by an insect, such as a fly or a mosquito, and transmitted through the bite of that insect to the host.
At least 14 species of roundworms cause infection in human beings. A. lumbricoides, which inhabits the small intestine, infects about 65 million people throughout the world. It causes a disease called ascariasis. Symptoms of this disease include pneumonia and intestinal pain. The roundworm Trichuris trichiura infects the large intestine and occurs in about 350 million people worldwide. It causes trichuriasis, a disease characterized by diarrhea. Other common roundworms that cause disease in humans include filariae, hookworms, pinworms, and trichinae.
Rotifers
Rotifer, pronounced ROH tuh fuhr, is a type of tiny multicellular animal that lives in water. The largest rotifers are about 1/26 inch (1 millimeter) long. Rotifers have cylinder- or vase-shaped bodies. Most species live in lakes, rivers, or streams. Some live in the ocean.
The name rotifer means wheel bearer and refers to the circles of hairlike projections called cilia on the animal's head. The cilia create a circular water current that draws food to the rotifer. This water current also enables most species of rotifers to "swim." Other species spend their entire lives attached to such objects as stones and leaves.
In many rotifer populations, the male has no role in reproduction. The female produces young by herself. This kind of reproduction, called parthenogenesis, produces only female offspring.
Scientific classification. Rotifers make up the phylum Rotifera.
The name rotifer means wheel bearer and refers to the circles of hairlike projections called cilia on the animal's head. The cilia create a circular water current that draws food to the rotifer. This water current also enables most species of rotifers to "swim." Other species spend their entire lives attached to such objects as stones and leaves.
In many rotifer populations, the male has no role in reproduction. The female produces young by herself. This kind of reproduction, called parthenogenesis, produces only female offspring.
Scientific classification. Rotifers make up the phylum Rotifera.
Platyhelminthes
Flatworm is a kind of worm. Some flatworms live freely on land or in water. Others live as parasites in human beings or other animals. Many flatworms, especially the larger species, have a flat body.
Flatworms have a simple body structure. A layer of cells called the epidermis covers the animal's body. An inner layer of cells forms an intestine in most flatworms. A tightly packed mass of cells called the parenchyma fills the body between the epidermis and intestine. Muscles, glands, nerves, and reproductive organs lie in the parenchyma. The only opening of the intestine is the animal's mouth. The mouth may be at the head end, the rear end, or the underside of the body.
Many flatworms have a smooth, soft body. Many have suckers or other projections on the body. Some flatworms have spines and tiny, needlelike spicules that serve as a kind of skeleton. Most flatworms measure less than 1 inch (2.5 centimeters) long. However, the largest flatworms, called tapeworms, may grow up to 100 feet (30 meters) long.
There are about 13,000 species of flatworms. They may be divided into four groups: (1) turbellarians; (2) monogeneans; (3) trematodes; and (4) cestodes, or tapeworms. Most turbellarians are free-living. They usually are found in sand and mud on the bottom of bodies of water. A few species live on land in moist soil. The other three groups of flatworms are parasites. They live in a wide variety of organisms that serve as hosts.
Almost all flatworms are hermaphroditic--that is, both male and female reproductive organs are found in the same animal. Most turbellarians lay eggs that hatch into tiny young that resemble the adults. In some turbellarians, and in all parasitic flatworms, young--called larvae--hatch from the eggs. The larvae look different from the adults and live in different habitats. For example, the larva of a monogenean has hairlike cilia that enable it to swim. The larva swims until it finds an appropriate fish for a host. The larva attaches to the fish and develops into an adult. The adult, which lives on the skin and gills of the fish, lacks cilia and cannot swim.
Parasitic flatworms cause disease in their hosts. Schistosomiasis, for example, is a tropical disease caused by schistosomes (blood flukes) living in the blood vessels of the abdomen. Adult tapeworms that live in the intestine of human beings do not usually cause much harm. However, tapeworm larvae cause serious diseases that can be fatal if not treated.
Scientific classification. Flatworms make up the phylum Platyhelminthes. The four classes of flatworms are Turbellaria, Monogenea, Trematoda, and Cestoda.
Flatworms have a simple body structure. A layer of cells called the epidermis covers the animal's body. An inner layer of cells forms an intestine in most flatworms. A tightly packed mass of cells called the parenchyma fills the body between the epidermis and intestine. Muscles, glands, nerves, and reproductive organs lie in the parenchyma. The only opening of the intestine is the animal's mouth. The mouth may be at the head end, the rear end, or the underside of the body.
Many flatworms have a smooth, soft body. Many have suckers or other projections on the body. Some flatworms have spines and tiny, needlelike spicules that serve as a kind of skeleton. Most flatworms measure less than 1 inch (2.5 centimeters) long. However, the largest flatworms, called tapeworms, may grow up to 100 feet (30 meters) long.
There are about 13,000 species of flatworms. They may be divided into four groups: (1) turbellarians; (2) monogeneans; (3) trematodes; and (4) cestodes, or tapeworms. Most turbellarians are free-living. They usually are found in sand and mud on the bottom of bodies of water. A few species live on land in moist soil. The other three groups of flatworms are parasites. They live in a wide variety of organisms that serve as hosts.
Almost all flatworms are hermaphroditic--that is, both male and female reproductive organs are found in the same animal. Most turbellarians lay eggs that hatch into tiny young that resemble the adults. In some turbellarians, and in all parasitic flatworms, young--called larvae--hatch from the eggs. The larvae look different from the adults and live in different habitats. For example, the larva of a monogenean has hairlike cilia that enable it to swim. The larva swims until it finds an appropriate fish for a host. The larva attaches to the fish and develops into an adult. The adult, which lives on the skin and gills of the fish, lacks cilia and cannot swim.
Parasitic flatworms cause disease in their hosts. Schistosomiasis, for example, is a tropical disease caused by schistosomes (blood flukes) living in the blood vessels of the abdomen. Adult tapeworms that live in the intestine of human beings do not usually cause much harm. However, tapeworm larvae cause serious diseases that can be fatal if not treated.
Scientific classification. Flatworms make up the phylum Platyhelminthes. The four classes of flatworms are Turbellaria, Monogenea, Trematoda, and Cestoda.
Annelids
Segmented worms are the most highly developed worms. Their body consists of segments that give the worms a ringed appearance. This group includes polychaete worms, oligochaete worms, and leeches.
Polychaete worms, the largest group of segmented worms, live in the sea and along the shore. Many of these worms have tentacles (feelers) on their head and a pair of leglike projections called parapodia on each body segment. The parapodia are used in crawling. They have many setae (bristles) that help the worms grip the surface on which they are moving. Many polychaete worms live among algae or burrow in mud or sand. Some live in tubes attached to the sea floor. A worm makes its tube from sand or from material secreted by its body. Some polychaete worms eat small plants and animals. Others feed on plant and animal remains.
Oligochaete worms include earthworms and many freshwater species. They have a few setae but no parapodia. Most oligochaete worms eat decaying plant matter.
Leeches make up the smallest group of segmented worms. They grow from 3/8 to 12 inches (1 to 30 centimeters) long and have a flat body with a sucker at each end. Most leeches live in water and feed on the blood of fish and other water creatures.
Polychaete worms, the largest group of segmented worms, live in the sea and along the shore. Many of these worms have tentacles (feelers) on their head and a pair of leglike projections called parapodia on each body segment. The parapodia are used in crawling. They have many setae (bristles) that help the worms grip the surface on which they are moving. Many polychaete worms live among algae or burrow in mud or sand. Some live in tubes attached to the sea floor. A worm makes its tube from sand or from material secreted by its body. Some polychaete worms eat small plants and animals. Others feed on plant and animal remains.
Oligochaete worms include earthworms and many freshwater species. They have a few setae but no parapodia. Most oligochaete worms eat decaying plant matter.
Leeches make up the smallest group of segmented worms. They grow from 3/8 to 12 inches (1 to 30 centimeters) long and have a flat body with a sucker at each end. Most leeches live in water and feed on the blood of fish and other water creatures.
Cnidarians
Cnidarian, pronounced ny DAIR ee uhn, is the name of a group of soft-bodied water animals. The group includes the freshwater hydras, hydroids, jellyfish, sea fans, sea anemones, and corals. These animals make up the phylum (large group) called Cnidaria. There are approximately 9,000 species of cnidarians, and most of them live in the sea. Cnidarians are also called coelenterates.
The body of a cnidarian may be shaped like a cylinder, a bell, or an umbrella. The mouth opens at one end and leads to a digestive cavity. Every cnidarian has at least two layers of cells that form its body wall. An outer layer makes up the body covering, and an inner layer lines the digestive cavity. Many cnidarians have a third, or middle, layer consisting of a stiff, jellylike material that helps support the animal.
A medusa, or jellyfish, is a cnidarian that has a bell- or umbrella-shaped body. Its mouth is at the underside of the body. Tentacles with special stinging cells hang downward from the body's ringlike edge. Medusas swim about freely in the sea.
A polyp is a cnidarian that has a body shaped like a hollow cylinder. A polyp lives with one end of its body attached to the sea bottom. The mouth and tentacles extend upward at the other end. Polyps may exist singly or may live together in colonies. Colonies are produced when polyps form buds that detach and become new polyps. Hydras and sea anemones are examples of single polyps, and hydroids and most corals are colony-forming polyps.
Some cnidarians have either medusa or polyp stages, or both, in their life cycles. The medusas are produced from special polyp buds that eventually break free and swim away. Then the medusas produce eggs and sperm that unite and develop into polyps.
The body of a cnidarian may be shaped like a cylinder, a bell, or an umbrella. The mouth opens at one end and leads to a digestive cavity. Every cnidarian has at least two layers of cells that form its body wall. An outer layer makes up the body covering, and an inner layer lines the digestive cavity. Many cnidarians have a third, or middle, layer consisting of a stiff, jellylike material that helps support the animal.
A medusa, or jellyfish, is a cnidarian that has a bell- or umbrella-shaped body. Its mouth is at the underside of the body. Tentacles with special stinging cells hang downward from the body's ringlike edge. Medusas swim about freely in the sea.
A polyp is a cnidarian that has a body shaped like a hollow cylinder. A polyp lives with one end of its body attached to the sea bottom. The mouth and tentacles extend upward at the other end. Polyps may exist singly or may live together in colonies. Colonies are produced when polyps form buds that detach and become new polyps. Hydras and sea anemones are examples of single polyps, and hydroids and most corals are colony-forming polyps.
Some cnidarians have either medusa or polyp stages, or both, in their life cycles. The medusas are produced from special polyp buds that eventually break free and swim away. Then the medusas produce eggs and sperm that unite and develop into polyps.
Porifera
Sponge is an animal that lives at the bottom of oceans and other bodies of water. Sponges do not have heads, arms, or internal organs. They live attached to rocks, plants, and other objects beneath the water's surface. Adult sponges do not move about from one place to another, and many sponges look like plants. For these reasons, people once regarded sponges as plants. But today, scientists classify sponges as animals. Like most animals, sponges eat their food. They cannot manufacture their own food, as do plants.
There are about 5,000 species of sponges. Most of them live in oceans, but a few species are found in lakes, rivers, and other bodies of fresh water. Sponges can live in both shallow and deep water. Most marine (ocean-dwelling) sponges inhabit warm or tropical seas.
Sponges are among the oldest kinds of animals. Fossils have been found of marine sponges that lived more than 500 million years ago. For centuries, people have used sponges for cleaning and bathing. The skeletons of certain sponges make good cleaning tools because they are soft and absorb large amounts of water. Commercial fishing crews still harvest bath sponges in the Gulf Stream and the Mediterranean Sea. However, most cleaning sponges are artificially produced.
Recently, scientists have discovered chemical compounds in sponges that may be used in medicines to fight cancer and other diseases. Because sponges harbor large populations of bacteria in their body tissues, the bacteria may produce many of these compounds. Such important discoveries have led to an increased amount of research involving sponges.
Sponges vary widely in shape, color, and size. Some sponges are round while others are shaped like vases. Many simply follow the shape of the object on which they grow, forming a living crust. Marine sponges range in color from bright yellow, orange, or purple, to gray or brown. Sponges of the same species may be of many different colors. Most freshwater sponges are green, purple, or gray. The smallest sponges measure less than 1 inch (2.5 centimeters) in diameter. The largest grow to more than 4 feet (1.2 meters) in diameter.
Body openings. A sponge has two types of openings on its body surface: (1) small pores called ostia, and (2) a large osculum. The sponge's ostia allow water to enter its body, and the osculum allows water to leave the body. Among more advanced sponges, a network of canals transports water entering through the ostia to all parts of the sponge. The water brings tiny plants and animals into the sponge. These tiny organisms are the sponge's food. Waste products--along with water--leave the sponge through the osculum.
Special cells. The canals that pass into the sponge's body lead into many small chambers. These small chambers in the sponge are lined with cells called choanocytes, also known as collar cells. Each of these cells has a delicate tissue, or collar, that acts like a net to trap food particles. Each collar cell also bears a long threadlike structure called a flagellum. The sponge's flagella whip around, and this action creates the water currents that flow through the body of the sponge.
In addition to collar cells, the sponge's body also contains other types of cells. Some of these cells form tissue that covers the sponge's body and the walls of the canals inside the body. Other types of cells travel freely within the sponge. These cells have many different functions. For example, some heal injuries to the body and others play a major role in reproduction. Still others produce material for the sponge's skeleton.
Skeleton. Sponges have several types of skeletons. Most sponges have a mineral skeleton made up of tiny, needlelike spicules. The spicules may be of either calcium carbonate (limestone) or silica, a glasslike mineral. In bath sponges, the skeleton consists only of fibers of a tan-colored protein called spongin. The skeleton of spongin fibers is what remains after a bath sponge dies and its cells are removed. Many sponges have a skeleton of both mineral spicules and spongin fibers. In other sponges, the skeleton consists of silica spicules, spongin fibers, and a massive base of limestone crystals.
The sponge's skeleton forms a framework that supports and protects the body. Spicules may be organized into bundles that form strong, geometric networks. In many sponges, numerous spicules grow around the osculum. These spicules protect the sponge from animals that try to eat it or enter its body.
Sponges reproduce both sexually and asexually. In sexual reproduction, a new sponge develops from the joining of two sex cells. In asexual reproduction, a new sponge is formed by methods that do not involve sex cells. Most sponges also have the ability to replace lost or injured body parts by growing new ones. This process is called regeneration.
Sexual reproduction in sponges begins when an egg (female sex cell) starts to grow inside the parent sponge's body. At first, the egg absorbs food from surrounding body fluids. Later, it engulfs cells called nurse cells, which provide food reserves. When fully grown, the egg is fertilized by a sperm (male sex cell). Some sponges produce both eggs and sperm. In these species, the egg may be fertilized by a sperm from the same animal.
Other species produce either eggs or sperm only. In these species, another sponge releases sperm into the surrounding water. A sperm enters the parent sponge's body by way of the ostia and canal network and fertilizes the egg.
After the egg is fertilized, it gradually develops into a larva (immature animal form). The larva is covered with cells that have flagella. The flagella beat rapidly, enabling the larva to swim outward through the parent's canal system, aided by water currents. The larva leaves the sponge through the osculum and swims around from a few hours to a few days. It then attaches itself to some suitable surface at the bottom of the body of water and develops into an adult sponge.
Asexual reproduction in sponges may occur in a variety of ways. In every case, however, it involves cells called archaeocytes. These cells have no specialized functions. Instead, they have the capacity to develop into any type of cell in the sponge's body. During asexual reproduction, a group of archaeocytes grow into every type of cell needed to form a new sponge.
Sponges may reproduce asexually by budding. In this process, buds or branches filled with archaeocytes grow on the parent sponge. These growths may break away from or fall off the parent sponge or remain attached to it. The growths eventually develop into new sponges.
Some marine sponges and most freshwater sponges also may reproduce asexually by forming gemmules. Gemmules are budlike structures that consist of a group of archaeocytes within a tough shell of spongin. Many gemmules are reinforced by spicules. Gemmules typically form in response to either cold or hot weather. Protected within the gemmule shell, the archaeocytes can survive periods of drought or freezing temperatures, though the parent sponge may die. Gemmules "hatch" when more favorable weather returns. The archaeocytes then spread out on a solid surface and develop into a new sponge.
Regeneration. The developmental abilities of archaeocytes give sponges remarkable powers of regeneration. Even if large parts of a sponge's body are lost or damaged, they may be replaced or repaired. In laboratory experiments, scientists have pressed sponges through extremely fine cloth so that the bodies of the sponges break up into separate cells or clumps of cells. When these cells are replaced in water, they first migrate together to form rounded cell clusters. Then the cell clusters reorganize to form complete sponges again.
Sponges make up a phylum (major group) of animals called Porifera, which comes from a Latin word meaning pore-bearer. Zoologists divide sponges into three classes, based chiefly on common skeletal features.
Sponges with a limestone skeleton belong to the class Calcarea. Most species in this class inhabit shallow parts of oceans, but some have been found at depths of up to 13,000 feet (4,000 meters). The tiny sponge called Sycon belongs to this group.
A second class, called Hexactinellida, consists of marine sponges with a silica skeleton. These species are commonly called glass sponges. Their spicules form beautiful geometric patterns. Glass sponges live as deep as 23,000 feet (7,000 meters) beneath the ocean's surface. The Venus's-flower-basket is a typical kind of glass sponge.
All freshwater sponges and most of the best-known marine sponges are in the class Demospongiae. Most of these animals have a skeleton of silica or spongin or of both substances. One kind of sponge in this class, the boring sponge, bores into coral, seashells, and other hard structures. This activity helps shape such marine environments as coral reefs and seacoasts. Other marine species in this group of sponges include the red-beard sponge, the sheepswool sponge, and bath sponges.
Some ocean sponges have a skeleton of silica and spongin with a thick base of limestone. Scientists include these sponges, sometimes called coralline sponges, in either the class Calcarea or Demospongiae. Many coralline sponges live in underwater caves. They are closely related to marine sponges that lived hundreds of millions of years ago.
Scientific classification. Sponges make up the phylum Porifera, which is divided into the classes Calcarea, Demospongiae, and Hexactinellida.
There are about 5,000 species of sponges. Most of them live in oceans, but a few species are found in lakes, rivers, and other bodies of fresh water. Sponges can live in both shallow and deep water. Most marine (ocean-dwelling) sponges inhabit warm or tropical seas.
Sponges are among the oldest kinds of animals. Fossils have been found of marine sponges that lived more than 500 million years ago. For centuries, people have used sponges for cleaning and bathing. The skeletons of certain sponges make good cleaning tools because they are soft and absorb large amounts of water. Commercial fishing crews still harvest bath sponges in the Gulf Stream and the Mediterranean Sea. However, most cleaning sponges are artificially produced.
Recently, scientists have discovered chemical compounds in sponges that may be used in medicines to fight cancer and other diseases. Because sponges harbor large populations of bacteria in their body tissues, the bacteria may produce many of these compounds. Such important discoveries have led to an increased amount of research involving sponges.
The bodies of sponges
Sponges vary widely in shape, color, and size. Some sponges are round while others are shaped like vases. Many simply follow the shape of the object on which they grow, forming a living crust. Marine sponges range in color from bright yellow, orange, or purple, to gray or brown. Sponges of the same species may be of many different colors. Most freshwater sponges are green, purple, or gray. The smallest sponges measure less than 1 inch (2.5 centimeters) in diameter. The largest grow to more than 4 feet (1.2 meters) in diameter.
Body openings. A sponge has two types of openings on its body surface: (1) small pores called ostia, and (2) a large osculum. The sponge's ostia allow water to enter its body, and the osculum allows water to leave the body. Among more advanced sponges, a network of canals transports water entering through the ostia to all parts of the sponge. The water brings tiny plants and animals into the sponge. These tiny organisms are the sponge's food. Waste products--along with water--leave the sponge through the osculum.
Special cells. The canals that pass into the sponge's body lead into many small chambers. These small chambers in the sponge are lined with cells called choanocytes, also known as collar cells. Each of these cells has a delicate tissue, or collar, that acts like a net to trap food particles. Each collar cell also bears a long threadlike structure called a flagellum. The sponge's flagella whip around, and this action creates the water currents that flow through the body of the sponge.
In addition to collar cells, the sponge's body also contains other types of cells. Some of these cells form tissue that covers the sponge's body and the walls of the canals inside the body. Other types of cells travel freely within the sponge. These cells have many different functions. For example, some heal injuries to the body and others play a major role in reproduction. Still others produce material for the sponge's skeleton.
Skeleton. Sponges have several types of skeletons. Most sponges have a mineral skeleton made up of tiny, needlelike spicules. The spicules may be of either calcium carbonate (limestone) or silica, a glasslike mineral. In bath sponges, the skeleton consists only of fibers of a tan-colored protein called spongin. The skeleton of spongin fibers is what remains after a bath sponge dies and its cells are removed. Many sponges have a skeleton of both mineral spicules and spongin fibers. In other sponges, the skeleton consists of silica spicules, spongin fibers, and a massive base of limestone crystals.
The sponge's skeleton forms a framework that supports and protects the body. Spicules may be organized into bundles that form strong, geometric networks. In many sponges, numerous spicules grow around the osculum. These spicules protect the sponge from animals that try to eat it or enter its body.
How sponges reproduce
Sponges reproduce both sexually and asexually. In sexual reproduction, a new sponge develops from the joining of two sex cells. In asexual reproduction, a new sponge is formed by methods that do not involve sex cells. Most sponges also have the ability to replace lost or injured body parts by growing new ones. This process is called regeneration.
Sexual reproduction in sponges begins when an egg (female sex cell) starts to grow inside the parent sponge's body. At first, the egg absorbs food from surrounding body fluids. Later, it engulfs cells called nurse cells, which provide food reserves. When fully grown, the egg is fertilized by a sperm (male sex cell). Some sponges produce both eggs and sperm. In these species, the egg may be fertilized by a sperm from the same animal.
Other species produce either eggs or sperm only. In these species, another sponge releases sperm into the surrounding water. A sperm enters the parent sponge's body by way of the ostia and canal network and fertilizes the egg.
After the egg is fertilized, it gradually develops into a larva (immature animal form). The larva is covered with cells that have flagella. The flagella beat rapidly, enabling the larva to swim outward through the parent's canal system, aided by water currents. The larva leaves the sponge through the osculum and swims around from a few hours to a few days. It then attaches itself to some suitable surface at the bottom of the body of water and develops into an adult sponge.
Asexual reproduction in sponges may occur in a variety of ways. In every case, however, it involves cells called archaeocytes. These cells have no specialized functions. Instead, they have the capacity to develop into any type of cell in the sponge's body. During asexual reproduction, a group of archaeocytes grow into every type of cell needed to form a new sponge.
Sponges may reproduce asexually by budding. In this process, buds or branches filled with archaeocytes grow on the parent sponge. These growths may break away from or fall off the parent sponge or remain attached to it. The growths eventually develop into new sponges.
Some marine sponges and most freshwater sponges also may reproduce asexually by forming gemmules. Gemmules are budlike structures that consist of a group of archaeocytes within a tough shell of spongin. Many gemmules are reinforced by spicules. Gemmules typically form in response to either cold or hot weather. Protected within the gemmule shell, the archaeocytes can survive periods of drought or freezing temperatures, though the parent sponge may die. Gemmules "hatch" when more favorable weather returns. The archaeocytes then spread out on a solid surface and develop into a new sponge.
Regeneration. The developmental abilities of archaeocytes give sponges remarkable powers of regeneration. Even if large parts of a sponge's body are lost or damaged, they may be replaced or repaired. In laboratory experiments, scientists have pressed sponges through extremely fine cloth so that the bodies of the sponges break up into separate cells or clumps of cells. When these cells are replaced in water, they first migrate together to form rounded cell clusters. Then the cell clusters reorganize to form complete sponges again.
Kinds of sponges
Sponges make up a phylum (major group) of animals called Porifera, which comes from a Latin word meaning pore-bearer. Zoologists divide sponges into three classes, based chiefly on common skeletal features.
Sponges with a limestone skeleton belong to the class Calcarea. Most species in this class inhabit shallow parts of oceans, but some have been found at depths of up to 13,000 feet (4,000 meters). The tiny sponge called Sycon belongs to this group.
A second class, called Hexactinellida, consists of marine sponges with a silica skeleton. These species are commonly called glass sponges. Their spicules form beautiful geometric patterns. Glass sponges live as deep as 23,000 feet (7,000 meters) beneath the ocean's surface. The Venus's-flower-basket is a typical kind of glass sponge.
All freshwater sponges and most of the best-known marine sponges are in the class Demospongiae. Most of these animals have a skeleton of silica or spongin or of both substances. One kind of sponge in this class, the boring sponge, bores into coral, seashells, and other hard structures. This activity helps shape such marine environments as coral reefs and seacoasts. Other marine species in this group of sponges include the red-beard sponge, the sheepswool sponge, and bath sponges.
Some ocean sponges have a skeleton of silica and spongin with a thick base of limestone. Scientists include these sponges, sometimes called coralline sponges, in either the class Calcarea or Demospongiae. Many coralline sponges live in underwater caves. They are closely related to marine sponges that lived hundreds of millions of years ago.
Scientific classification. Sponges make up the phylum Porifera, which is divided into the classes Calcarea, Demospongiae, and Hexactinellida.
Interesting facts about animals: an introduction to kingdom animalia
Kinds of animals.No one knows exactly how many kinds of animals there are. New kinds are found every year. So far, scientists have identified more than 1 1/2 million types of animals. About 1 million of these are insects. There are about 21,000 kinds of fish, 9,700 kinds of birds, 6,500 kinds of reptiles, 4,000 kinds of amphibians, and 4,500 kinds of mammals.
Largest ears and eyes.The largest ears of all animals are those of the African elephant. Elephant ears grow as large as 4 feet (1.2 meters) across. The largest eyes of all land animals are those of the horse and the ostrich. They measure about 1 1/2 times the size of human eyes.
The flying dragon is another name for the draco lizard. This lizard can spread out folds of skin to form "wings" that it uses to glide through the air from tree to tree. It lives in Asia and the East Indies.
Lives of animals range from several hours to many years. An adult mayfly survives only a few hours or days. Some giant tortoises have lived more than 100 years.
The world's only known poisonous bird is the hooded pitohui, which lives on the island of New Guinea. This brilliantly colored orange-and-black bird has poison on its feathers and skin. This poison serves as a defense against hawks, snakes, and other enemies. It is the same type of poison as that carried by the deadly poison-dart frog of South America.
The hummingbird can fly straight up like a helicopter. It can hover in front of a flower to suck the nectar. The bee hummingbird,which grows to only 2 inches (5 centimeters) long, is the smallest of all birds.
The chameleon's tongue is as long as its body. This lizard swiftly shoots out its tongue to capture insects for food. Certain chameleons can quickly change color and even develop spots and streaks that make them seem to be part of their background.
A tree-climbing crab lives on many tropical islands. It is called the coconut crab because it cracks coconuts with its powerful claws and eats the sweet meat.
The platypus,a mammal, has a bill like a duck and lays eggs as birds do. But it nurses its young with milk as do other mammals. It lives only on mainland Australia and the island of Tasmania.
Largest ears and eyes.The largest ears of all animals are those of the African elephant. Elephant ears grow as large as 4 feet (1.2 meters) across. The largest eyes of all land animals are those of the horse and the ostrich. They measure about 1 1/2 times the size of human eyes.
The flying dragon is another name for the draco lizard. This lizard can spread out folds of skin to form "wings" that it uses to glide through the air from tree to tree. It lives in Asia and the East Indies.
Lives of animals range from several hours to many years. An adult mayfly survives only a few hours or days. Some giant tortoises have lived more than 100 years.
The world's only known poisonous bird is the hooded pitohui, which lives on the island of New Guinea. This brilliantly colored orange-and-black bird has poison on its feathers and skin. This poison serves as a defense against hawks, snakes, and other enemies. It is the same type of poison as that carried by the deadly poison-dart frog of South America.
The hummingbird can fly straight up like a helicopter. It can hover in front of a flower to suck the nectar. The bee hummingbird,which grows to only 2 inches (5 centimeters) long, is the smallest of all birds.
The chameleon's tongue is as long as its body. This lizard swiftly shoots out its tongue to capture insects for food. Certain chameleons can quickly change color and even develop spots and streaks that make them seem to be part of their background.
A tree-climbing crab lives on many tropical islands. It is called the coconut crab because it cracks coconuts with its powerful claws and eats the sweet meat.
The platypus,a mammal, has a bill like a duck and lays eggs as birds do. But it nurses its young with milk as do other mammals. It lives only on mainland Australia and the island of Tasmania.
Wednesday, January 24, 2007
algae; bacteria; protista
Algae, pronounced AL jee, are simple organisms that live in oceans, lakes, rivers, ponds, and moist soil. A single organism of this type is called an alga. Some algae are microscopic and consist of just one cell, and others are large and contain many cells. Some species drift or swim, and others are attached to stones or weeds in the water. Large marine algae are called seaweeds. A few algae live on land, growing on trees or other land plants, soil, and rocks. Others live on sloths or turtles. Still others grow within plants or animals.
All algae contain chlorophyll. They help purify the air and water by the process of photosynthesis. Algae also serve as food for fish and other animals that live in the water.
Some algae multiply rapidly in polluted lakes and rivers. Thick layers of algae, called algal blooms, may form when waste materials, such as sewage and fertilizers, are dumped in the water. The increased algal population sometimes upsets the natural balance of life in water. The water eventually may become extremely low in oxygen and unfit for use by people.
Most botanists classify the blue-green algae, also called cyanobacteria, with bacteria as members of the kingdom Prokaryotae. They classify all other algae in the kingdom Protista.
Some kinds of blue-green algae form slippery, dark coatings on rocks along the shores of rivers, lakes, and oceans. Others occur in soil, forming a slimy layer on wet ground. Lakes with large numbers of blue-green algae look greenish or bluish-green. A few species of blue-green algae may poison fish or cattle and other animals that drink water containing these organisms.
Most blue-green algae can be seen only with a microscope. Some species have only one cell. In others, the cells form strands. The cells of blue-green algae lack a distinct nucleus. Besides chlorophyll, they contain blue or red pigments (coloring matter). The combination of pigments causes some to appear pinkish, brownish, or black. Many species can take nitrogen from the air, convert it to compounds called nitrates, and so help to fertilize soil or water. Most blue-green algae reproduce only by cell division.
All other algae have cells with at least one nucleus. The cells contain chlorophyll and other pigments in specialized cell parts called chloroplasts. These algae are generally grouped according to color--brown, green, or red. They grow and reproduce by cell division. Most kinds can also reproduce sexually.
This broad group of algae includes diatoms and dinoflagellates, most of which have only one cell. Many of these algae occur with marine animals in drifting masses called plankton. Dinoflagellate cells swim by means of two hairlike structures called flagella. Diatoms have cell walls made up of silica. These "skeletons" resist decay and may accumulate on the ocean floor. In some places, they form a whitish material called diatomite, which has many industrial uses.
Brown algae are plentiful along many seashores of temperate zones. Some kinds, called kelps, grow as much as 200 feet (60 meters) long. Algin, a gummy substance obtained from kelp, is used to thicken cosmetics, ice cream, mayonnaise, and other products. Some brown algae are used as fertilizer.
Green algae occur in both fresh and salt water. Most species are microscopic and live in lakes, ponds, and streams. Large quantities of such algae may color an entire lake. Other species are larger and grow along seashores. Many coral beaches of the tropics consist of pieces of green seaweeds filled with lime. Some scientists are experimenting with growing green algae for food.
Red algae are found mostly in subtropical seas, where they sometimes grow with corals. A few species of red algae live in fresh water. Some red algae have blue as well as green and red pigments. Certain red algae are the source of agar, a gelatinlike substance used in laboratories to grow bacteria. In Japan, people eat a red alga that is called nori. It is usually sold dried as papery sheets.
Scientific classification. Blue-green algae belong to the kingdom Prokaryotae. All other algae belong to the kingdom Protista.
Bacteria are simple organisms that consist of one cell. They are among the smallest living things. Most bacteria measure from 0.3 to 2.0 microns in diameter and can be seen only through a microscope. (One micron equals 0.001 millimeter or 1/25,400 inch.) Scientists classify bacteria as prokaryotes.
Bacteria exist almost everywhere. There are thousands of kinds of bacteria, most of which are harmless to human beings. Large numbers of bacteria live in the human body but cause no harm. Some species cause diseases, but many others are helpful.
Helpful bacteria. Certain kinds of bacteria live in the intestines of human beings and other animals. These bacteria help in digestion and in destroying harmful organisms. Intestinal bacteria also produce some vitamins needed by the body.
Bacteria in soil and water play a vital role in recycling carbon, nitrogen, sulfur, and other chemical elements used by living things. Many bacteria help decompose (break down) dead organisms and animal wastes into chemical elements. Other bacteria help change chemical elements into forms that can be used by plants and animals. For example, certain kinds of bacteria convert nitrogen in the air and soil into nitrogen compounds that can be used by plants.
A chemical process called fermentation, used in making alcoholic beverages and cheese and many other foods, is caused by various bacteria. Sewage treatment plants use bacteria to purify water. Bacteria are also used in making some drugs.
Bacterial cells resemble the cells of other living things in many ways, and so scientists study bacteria to learn about more complex organisms. For example, the study of bacteria has helped researchers understand how certain characteristics are inherited. Most types of bacteria reproduce quickly. This rapid reproduction enables scientists to grow large quantities for research.
Harmful bacteria. Some species of bacteria cause diseases in human beings. These diseases include cholera, gonorrhea, leprosy (Hansen's disease), pneumonia, syphilis, tuberculosis, typhoid fever, and whooping cough. The bacteria enter a human being's body through its natural openings, such as the nose or mouth, or through breaks in the skin. In addition, air, food, and water carry bacteria from one person to another. Harmful bacteria prevent the body from functioning properly by destroying healthy cells.
Certain bacteria produce toxins (poisons), which cause such diseases as diphtheria, scarlet fever, and tetanus. Some toxins are produced by living bacteria, but others are released only after a bacterium dies. A form of food poisoning called botulism is caused by toxins from bacteria in improperly canned foods.
Bacteria that usually live harmlessly in the body may cause infections when a person's resistance to disease is low. For example, if bacteria in the throat reproduce faster than the body can dispose of them, a person may get a sore throat.
Bacteria also cause diseases in other animals and in plants. Anthrax is a bacterial disease that infects many animals, especially cattle and sheep. Plant diseases caused by bacteria include fire blight, which occurs in apple and pear trees, and soft rot, which decays some fruits and vegetables. Bacteria also cause growths called crown galls, which attack various plants.
Protection against harmful bacteria. Many bacteria live on the skin and in the mouth, intestines, and breathing passages. But the rest of the body tissues are normally free of bacteria. The skin, and the membranes that line the digestive and respiratory systems, prevent most harmful bacteria from entering the rest of the body. When harmful bacteria do enter the body, white blood cells surround and attack them. Also, the blood produces antibodies, substances that kill or weaken the invaders. Toxins are neutralized by certain antibodies called antitoxins. Sometimes the body cannot make its own antitoxins fast enough. In such cases, a physician may inject an antitoxin from an animal, such as a horse or rabbit, or from another person.
Dead or weakened bacteria are used in making drugs called vaccines, which can prevent the diseases caused by those species of bacteria. Vaccines are injected into the body, causing the blood to produce antibodies that attack the bacteria. Some vaccines protect the body from infection for several years or longer. Drugs called antibiotics are made from microorganisms that inhabit the air, soil, and water. Antibiotics can kill or weaken disease-causing bacteria. However, extensive use of antibiotics may encourage the spread of bacteria resistant to the drugs. The drugs then become ineffective.
People use chemicals called antiseptics to prevent bacteria from growing on living tissues. Other chemicals, known as disinfectants, are used to destroy bacteria in water and on such items as clothing and utensils. Bacteria can also be killed by heat, and so heat is often used to sterilize food and utensils.
Nearly all kinds of bacteria are enclosed by a tough protective layer called a cell wall. The cell wall gives the bacterium its shape and enables it to live in a wide range of environments. Some species are further enclosed by a capsule, a slimy layer outside the cell wall. The capsule makes the cell resistant to destructive chemicals. All bacteria have a cell membrane, an elastic, baglike structure just inside the cell wall. Small molecules of food enter the cell through pores in this membrane, but large molecules cannot pass through. Inside the membrane is the cytoplasm, a soft, jellylike substance. The cytoplasm contains chemicals called enzymes, which help break down food and build cell parts.
Like the cells of all living things, bacterial cells contain DNA (deoxyribonucleic acid). DNA controls a cell's growth, reproduction, and all other activities. The DNA of a bacterial cell forms an area of the cytoplasm called the nucleoid. In all other organisms except cyanobacteria (blue-green algae), the DNA is in the nucleus, a part of the cell separated from the cytoplasm by a membrane.
Scientists generally divide bacteria into groups according to shape. Round bacteria are called cocci, and rod-shaped ones are bacilli. Bacteria that look like bent rods are vibrios. There are two types of spiral-shaped bacteria, spirilla and spirochetes. Two or more bacteria linked together may be described by the prefixes diplo- (pair), staphylo- (cluster), or strepto- (chain). For example, streptococci are a type of round bacteria linked together in chains.
Where bacteria live. Bacteria live almost everywhere, even in places where other forms of life cannot survive. The air, water, and upper layers of soil contain many bacteria. Bacteria are always present in the digestive and respiratory systems and on the skin of human beings and other animals.
Certain bacteria, called aerobes, require oxygen to live, but others, known as anaerobes, can survive without it. Some anaerobes can exist either with or without oxygen. Other anaerobes cannot live with even a trace of oxygen in their environment.
Some bacteria protect themselves against a lack of food, oxygen, or water by forming a new, thicker cell membrane inside the old one. The cell material surrounding the new membrane dies. The remaining organism becomes inactive and is called a bacterial spore. Bacterial spores may live for decades or even longer because they can resist extremely high or low temperatures and other harsh conditions. If food, oxygen, and water again become available, the spores change back into active bacteria.
How bacteria move. Bacteria are carried long distances by air and water currents. Clothing, utensils, and other objects also carry bacteria. Various kinds of bacteria have flagella (thin hairs) that enable them to swim. Some species that lack flagella move by wriggling.
How bacteria obtain food. Most kinds of bacteria, called heterotrophic bacteria, feed on other organisms. Some species, known as autotrophic bacteria, manufacture their own food. For example, photosynthetic bacteria make food from carbon dioxide, sunlight, and water. Certain bacteria may be autotrophic or heterotrophic, depending on the food available. The majority of heterotrophic bacteria feed on dead organisms. Others are parasites. Some parasitic bacteria cause little or no harm to the host organism, but others cause diseases.
How bacteria reproduce. Most bacteria reproduce asexually--that is, each cell simply divides into two identical cells by a process called binary fission. Most bacteria also reproduce quickly, and some species double their number every 20 minutes. If one of these cells were given enough food, over a billion bacteria would be produced in 10 hours. Industrial and laboratory processes often produce such enormous numbers of bacteria. But in nature, bacteria lack an adequate food supply to maintain such a high rate of reproduction.
When bacteria reproduce by binary fission, the DNA in each of the two resulting cells is identical to the DNA in the original bacterium. Some bacteria can exchange DNA by a kind of simple sexual process called conjugation. Conjugation involves the direct transfer of DNA from one type of bacterial cell, called a male, to another type, called a female. DNA also may be transferred by viruses. Bacteria also may pick up fragments of DNA from dead bacterial cells. By transferring DNA, bacterial cells transfer individual traits. For example, bacterial cells that are resistant to certain antibiotics may transfer this characteristic to nonresistant bacterial cells.
Scientists have developed techniques that allow them to isolate fragments of DNA responsible for particular traits. Inserting these fragments into different bacteria, called recombinant DNA technology, produces useful new kinds of bacteria. For example, some of these bacteria chemically break down oil and also help clean up oil spills. Others are used to make substances with medical applications, such as insulin.
The first living things on the earth probably included simple forms of bacteria. The oldest known fossils are those of bacteria that lived about 31/2 billion years ago. Some scientists believe certain bacteria gradually developed into multicelled organisms that were the ancestors of the more complex plants and animals of today.
Bacteria were first described in the mid-1670's by Anton van Leeuwenhoek, a Dutch amateur scientist. For many years, scientists believed that bacteria came from nonliving matter. But in the late 1800's, the French chemist Louis Pasteur showed that only living things can produce living things. Pasteur and Robert Koch, a German physician, helped develop the science of bacteriology (the study of bacteria).
Protista consists chiefly of microscopic organisms, such as diatoms and protozoans, that often live in colonies. Most protists, like prokaryotes, consist of a single cell. All other living things are multicellular--that is, they are made up of many cells.
Unlike the monerans, protists have a well-defined nucleus in their cells. Many also have specialized body parts that they use to gather food and to move in their environment. Scientists divide protists into groups based on how these organisms move about. For example, ciliates move by beating hairlike structures called cilia. Zooflagellates use a long tail called a flagellum to move.
Fungi are multicellular organisms that obtain their food from dead or living organic matter. Organisms in this kingdom include molds, mushrooms, and yeasts. Fungi secrete certain proteins called enzymes into the plants, animals, and decaying organisms on which they live. The enzymes help to break organic molecules into nutrients that the fungus can absorb.
All algae contain chlorophyll. They help purify the air and water by the process of photosynthesis. Algae also serve as food for fish and other animals that live in the water.
Some algae multiply rapidly in polluted lakes and rivers. Thick layers of algae, called algal blooms, may form when waste materials, such as sewage and fertilizers, are dumped in the water. The increased algal population sometimes upsets the natural balance of life in water. The water eventually may become extremely low in oxygen and unfit for use by people.
Most botanists classify the blue-green algae, also called cyanobacteria, with bacteria as members of the kingdom Prokaryotae. They classify all other algae in the kingdom Protista.
Blue-green algae
Some kinds of blue-green algae form slippery, dark coatings on rocks along the shores of rivers, lakes, and oceans. Others occur in soil, forming a slimy layer on wet ground. Lakes with large numbers of blue-green algae look greenish or bluish-green. A few species of blue-green algae may poison fish or cattle and other animals that drink water containing these organisms.
Most blue-green algae can be seen only with a microscope. Some species have only one cell. In others, the cells form strands. The cells of blue-green algae lack a distinct nucleus. Besides chlorophyll, they contain blue or red pigments (coloring matter). The combination of pigments causes some to appear pinkish, brownish, or black. Many species can take nitrogen from the air, convert it to compounds called nitrates, and so help to fertilize soil or water. Most blue-green algae reproduce only by cell division.
Other kinds of algae
All other algae have cells with at least one nucleus. The cells contain chlorophyll and other pigments in specialized cell parts called chloroplasts. These algae are generally grouped according to color--brown, green, or red. They grow and reproduce by cell division. Most kinds can also reproduce sexually.
This broad group of algae includes diatoms and dinoflagellates, most of which have only one cell. Many of these algae occur with marine animals in drifting masses called plankton. Dinoflagellate cells swim by means of two hairlike structures called flagella. Diatoms have cell walls made up of silica. These "skeletons" resist decay and may accumulate on the ocean floor. In some places, they form a whitish material called diatomite, which has many industrial uses.
Brown algae are plentiful along many seashores of temperate zones. Some kinds, called kelps, grow as much as 200 feet (60 meters) long. Algin, a gummy substance obtained from kelp, is used to thicken cosmetics, ice cream, mayonnaise, and other products. Some brown algae are used as fertilizer.
Green algae occur in both fresh and salt water. Most species are microscopic and live in lakes, ponds, and streams. Large quantities of such algae may color an entire lake. Other species are larger and grow along seashores. Many coral beaches of the tropics consist of pieces of green seaweeds filled with lime. Some scientists are experimenting with growing green algae for food.
Red algae are found mostly in subtropical seas, where they sometimes grow with corals. A few species of red algae live in fresh water. Some red algae have blue as well as green and red pigments. Certain red algae are the source of agar, a gelatinlike substance used in laboratories to grow bacteria. In Japan, people eat a red alga that is called nori. It is usually sold dried as papery sheets.
Scientific classification. Blue-green algae belong to the kingdom Prokaryotae. All other algae belong to the kingdom Protista.
Bacteria are simple organisms that consist of one cell. They are among the smallest living things. Most bacteria measure from 0.3 to 2.0 microns in diameter and can be seen only through a microscope. (One micron equals 0.001 millimeter or 1/25,400 inch.) Scientists classify bacteria as prokaryotes.
Bacteria exist almost everywhere. There are thousands of kinds of bacteria, most of which are harmless to human beings. Large numbers of bacteria live in the human body but cause no harm. Some species cause diseases, but many others are helpful.
The importance of bacteria
Helpful bacteria. Certain kinds of bacteria live in the intestines of human beings and other animals. These bacteria help in digestion and in destroying harmful organisms. Intestinal bacteria also produce some vitamins needed by the body.
Bacteria in soil and water play a vital role in recycling carbon, nitrogen, sulfur, and other chemical elements used by living things. Many bacteria help decompose (break down) dead organisms and animal wastes into chemical elements. Other bacteria help change chemical elements into forms that can be used by plants and animals. For example, certain kinds of bacteria convert nitrogen in the air and soil into nitrogen compounds that can be used by plants.
A chemical process called fermentation, used in making alcoholic beverages and cheese and many other foods, is caused by various bacteria. Sewage treatment plants use bacteria to purify water. Bacteria are also used in making some drugs.
Bacterial cells resemble the cells of other living things in many ways, and so scientists study bacteria to learn about more complex organisms. For example, the study of bacteria has helped researchers understand how certain characteristics are inherited. Most types of bacteria reproduce quickly. This rapid reproduction enables scientists to grow large quantities for research.
Harmful bacteria. Some species of bacteria cause diseases in human beings. These diseases include cholera, gonorrhea, leprosy (Hansen's disease), pneumonia, syphilis, tuberculosis, typhoid fever, and whooping cough. The bacteria enter a human being's body through its natural openings, such as the nose or mouth, or through breaks in the skin. In addition, air, food, and water carry bacteria from one person to another. Harmful bacteria prevent the body from functioning properly by destroying healthy cells.
Certain bacteria produce toxins (poisons), which cause such diseases as diphtheria, scarlet fever, and tetanus. Some toxins are produced by living bacteria, but others are released only after a bacterium dies. A form of food poisoning called botulism is caused by toxins from bacteria in improperly canned foods.
Bacteria that usually live harmlessly in the body may cause infections when a person's resistance to disease is low. For example, if bacteria in the throat reproduce faster than the body can dispose of them, a person may get a sore throat.
Bacteria also cause diseases in other animals and in plants. Anthrax is a bacterial disease that infects many animals, especially cattle and sheep. Plant diseases caused by bacteria include fire blight, which occurs in apple and pear trees, and soft rot, which decays some fruits and vegetables. Bacteria also cause growths called crown galls, which attack various plants.
Protection against harmful bacteria. Many bacteria live on the skin and in the mouth, intestines, and breathing passages. But the rest of the body tissues are normally free of bacteria. The skin, and the membranes that line the digestive and respiratory systems, prevent most harmful bacteria from entering the rest of the body. When harmful bacteria do enter the body, white blood cells surround and attack them. Also, the blood produces antibodies, substances that kill or weaken the invaders. Toxins are neutralized by certain antibodies called antitoxins. Sometimes the body cannot make its own antitoxins fast enough. In such cases, a physician may inject an antitoxin from an animal, such as a horse or rabbit, or from another person.
Dead or weakened bacteria are used in making drugs called vaccines, which can prevent the diseases caused by those species of bacteria. Vaccines are injected into the body, causing the blood to produce antibodies that attack the bacteria. Some vaccines protect the body from infection for several years or longer. Drugs called antibiotics are made from microorganisms that inhabit the air, soil, and water. Antibiotics can kill or weaken disease-causing bacteria. However, extensive use of antibiotics may encourage the spread of bacteria resistant to the drugs. The drugs then become ineffective.
People use chemicals called antiseptics to prevent bacteria from growing on living tissues. Other chemicals, known as disinfectants, are used to destroy bacteria in water and on such items as clothing and utensils. Bacteria can also be killed by heat, and so heat is often used to sterilize food and utensils.
The structure of bacteria
Nearly all kinds of bacteria are enclosed by a tough protective layer called a cell wall. The cell wall gives the bacterium its shape and enables it to live in a wide range of environments. Some species are further enclosed by a capsule, a slimy layer outside the cell wall. The capsule makes the cell resistant to destructive chemicals. All bacteria have a cell membrane, an elastic, baglike structure just inside the cell wall. Small molecules of food enter the cell through pores in this membrane, but large molecules cannot pass through. Inside the membrane is the cytoplasm, a soft, jellylike substance. The cytoplasm contains chemicals called enzymes, which help break down food and build cell parts.
Like the cells of all living things, bacterial cells contain DNA (deoxyribonucleic acid). DNA controls a cell's growth, reproduction, and all other activities. The DNA of a bacterial cell forms an area of the cytoplasm called the nucleoid. In all other organisms except cyanobacteria (blue-green algae), the DNA is in the nucleus, a part of the cell separated from the cytoplasm by a membrane.
Scientists generally divide bacteria into groups according to shape. Round bacteria are called cocci, and rod-shaped ones are bacilli. Bacteria that look like bent rods are vibrios. There are two types of spiral-shaped bacteria, spirilla and spirochetes. Two or more bacteria linked together may be described by the prefixes diplo- (pair), staphylo- (cluster), or strepto- (chain). For example, streptococci are a type of round bacteria linked together in chains.
The life of bacteria
Where bacteria live. Bacteria live almost everywhere, even in places where other forms of life cannot survive. The air, water, and upper layers of soil contain many bacteria. Bacteria are always present in the digestive and respiratory systems and on the skin of human beings and other animals.
Certain bacteria, called aerobes, require oxygen to live, but others, known as anaerobes, can survive without it. Some anaerobes can exist either with or without oxygen. Other anaerobes cannot live with even a trace of oxygen in their environment.
Some bacteria protect themselves against a lack of food, oxygen, or water by forming a new, thicker cell membrane inside the old one. The cell material surrounding the new membrane dies. The remaining organism becomes inactive and is called a bacterial spore. Bacterial spores may live for decades or even longer because they can resist extremely high or low temperatures and other harsh conditions. If food, oxygen, and water again become available, the spores change back into active bacteria.
How bacteria move. Bacteria are carried long distances by air and water currents. Clothing, utensils, and other objects also carry bacteria. Various kinds of bacteria have flagella (thin hairs) that enable them to swim. Some species that lack flagella move by wriggling.
How bacteria obtain food. Most kinds of bacteria, called heterotrophic bacteria, feed on other organisms. Some species, known as autotrophic bacteria, manufacture their own food. For example, photosynthetic bacteria make food from carbon dioxide, sunlight, and water. Certain bacteria may be autotrophic or heterotrophic, depending on the food available. The majority of heterotrophic bacteria feed on dead organisms. Others are parasites. Some parasitic bacteria cause little or no harm to the host organism, but others cause diseases.
How bacteria reproduce. Most bacteria reproduce asexually--that is, each cell simply divides into two identical cells by a process called binary fission. Most bacteria also reproduce quickly, and some species double their number every 20 minutes. If one of these cells were given enough food, over a billion bacteria would be produced in 10 hours. Industrial and laboratory processes often produce such enormous numbers of bacteria. But in nature, bacteria lack an adequate food supply to maintain such a high rate of reproduction.
When bacteria reproduce by binary fission, the DNA in each of the two resulting cells is identical to the DNA in the original bacterium. Some bacteria can exchange DNA by a kind of simple sexual process called conjugation. Conjugation involves the direct transfer of DNA from one type of bacterial cell, called a male, to another type, called a female. DNA also may be transferred by viruses. Bacteria also may pick up fragments of DNA from dead bacterial cells. By transferring DNA, bacterial cells transfer individual traits. For example, bacterial cells that are resistant to certain antibiotics may transfer this characteristic to nonresistant bacterial cells.
Scientists have developed techniques that allow them to isolate fragments of DNA responsible for particular traits. Inserting these fragments into different bacteria, called recombinant DNA technology, produces useful new kinds of bacteria. For example, some of these bacteria chemically break down oil and also help clean up oil spills. Others are used to make substances with medical applications, such as insulin.
History
The first living things on the earth probably included simple forms of bacteria. The oldest known fossils are those of bacteria that lived about 31/2 billion years ago. Some scientists believe certain bacteria gradually developed into multicelled organisms that were the ancestors of the more complex plants and animals of today.
Bacteria were first described in the mid-1670's by Anton van Leeuwenhoek, a Dutch amateur scientist. For many years, scientists believed that bacteria came from nonliving matter. But in the late 1800's, the French chemist Louis Pasteur showed that only living things can produce living things. Pasteur and Robert Koch, a German physician, helped develop the science of bacteriology (the study of bacteria).
Protista consists chiefly of microscopic organisms, such as diatoms and protozoans, that often live in colonies. Most protists, like prokaryotes, consist of a single cell. All other living things are multicellular--that is, they are made up of many cells.
Unlike the monerans, protists have a well-defined nucleus in their cells. Many also have specialized body parts that they use to gather food and to move in their environment. Scientists divide protists into groups based on how these organisms move about. For example, ciliates move by beating hairlike structures called cilia. Zooflagellates use a long tail called a flagellum to move.
Fungi are multicellular organisms that obtain their food from dead or living organic matter. Organisms in this kingdom include molds, mushrooms, and yeasts. Fungi secrete certain proteins called enzymes into the plants, animals, and decaying organisms on which they live. The enzymes help to break organic molecules into nutrients that the fungus can absorb.
Prokaryote, pronounced proh KAR ee oht, also called moneran (pronounced muh NIHR uhn), is the name of a group of primitive one-celled organisms. Prokaryotes make up the kingdom Prokaryotae. This kingdom consists of blue-green algae, also called cyanobacteria, and bacteria. Prokaryotes live alone or in clusters called colonies. The individual organisms can be seen only with a microscope, but some colonies are visible with the unaided eye. Prokaryotae is one of the five kingdoms of living things recognized by most scientists. The other kingdoms are Animalia (animals), Fungi (fungi), Plantae (plants), and Protista (protists). Some scientists classify prokaryotes as part of either the protist or plant kingdom.
Most biologists believe prokaryotes are among the oldest types of organisms. Unlike all other living cells, prokaryotes do not have a nucleus surrounded by a membrane. But they do have a nuclear area that contains DNA, the substance that controls heredity. Prokaryotes also lack typical organelles, structures that perform functions in other cells.
Prokaryotes live throughout the world, even where no other life can survive. For example, blue-green algae live in the water of hot springs as well as in frozen wastelands. Free-living bacteria dwell throughout the soil and water, and parasitic species live within nearly all multicelled plants and animals.
Most biologists believe prokaryotes are among the oldest types of organisms. Unlike all other living cells, prokaryotes do not have a nucleus surrounded by a membrane. But they do have a nuclear area that contains DNA, the substance that controls heredity. Prokaryotes also lack typical organelles, structures that perform functions in other cells.
Prokaryotes live throughout the world, even where no other life can survive. For example, blue-green algae live in the water of hot springs as well as in frozen wastelands. Free-living bacteria dwell throughout the soil and water, and parasitic species live within nearly all multicelled plants and animals.
Friday, January 19, 2007
Evolution
Evolution
It is a process of change over a long period. The word evolution may refer to various types of change. For example, scientists generally describe the formation of the universe as having occurred through evolution. Many astronomers think that the stars and planets evolved from a huge cloud of hot gases. Anthropologists study the evolution of human culture from hunting and gathering societies to complex, industrialized societies.
Most commonly, however, evolution refers to the formation and development of life on earth. The idea that all living things evolved from simple organisms and changed through the ages to produce millions of species is known as the theory of organic evolution. Most people call it simply the theory of evolution.
The French naturalist Chevalier de Lamarck proposed a theory of evolution in 1809. But evolution did not receive widespread scientific consideration until 1858, when the British naturalist Charles R. Darwin presented his theory of evolution. Since then, advances in various scientific fields have resulted in refinements of the theory. The main ideas of evolution, however, have remained largely unchanged.
This article discusses the main ideas of evolutionary theory and the scientific evidence that supports the theory. For information about other types of evolution, see the World Book articles on UNIVERSE (Changing views of the universe) and EARTH (How the earth began).
Main ideas of evolutionary theory
The theory of evolution is actually a set of several interrelated ideas. The basic idea is that species undergo changes in their inherited characteristics over time. These changes transformed some of the species that lived long ago into the species that are alive today. In the last few million years--a relatively brief period in the history of the earth--thousands of species have become extinct and thousands of other species have evolved.
Evolutionary theory holds that all species probably evolved from a single form of life which lived about 31/2 billion years ago. Over time, the basic life form evolved into two or more species. These species, in turn, developed into many other species. This branching process, called speciation, produced the more than 10 million species that inhabit the earth today.
Related to speciation is the idea of common ancestry. Because all organisms evolved from one basic life form, any two species once had a common ancestor. Closely related species share a more recent common ancestor, but distantly related species must trace their ancestry far into the past to find a common ancestor. For example, human beings, chimpanzees, and gorillas evolved from a common ancestor that lived between 4 million and 10 million years ago, while the common ancestor of human beings and reptiles lived about 300 million years ago.
Another idea related to evolution is gradualism. Gradualism is the idea that evolutionary changes do not occur suddenly but over large stretches of time, ranging from decades to millions of years. Scientists think that evolution continues today at rates comparable to those of the past.
Still another idea is natural selection, a process by which the organisms best suited to their environment tend to leave the most descendants. All living things must compete for a limited supply of food, water, space, and other necessities. The individual plants and animals whose variations are best adapted to conditions have an advantage in this struggle. These organisms, on average, tend to leave a larger number of offspring than other members of their group. As a result, the proportion of the group sharing the traits of the best-adapted organisms increases from generation to generation. Scientists use the term fitness to refer to the ability of an organism to reproduce. For this reason, natural selection is often called the "survival of the fittest."
Although evolution is called a "theory," this does not mean that evolutionary biology is guesswork or is not supported by evidence. In science, a theory is a set of ideas based on observations about nature that explains many related facts. The theory of evolution is supported by evidence from many scientific fields. When a theory is supported by so much evidence, it becomes accepted as a scientific fact. Almost all scientists consider the theory of evolution to be a scientific fact.
Many people, however, reject the theory of evolution because of their religious beliefs. They believe the theory conflicts with the Biblical account of the Creation, which states that all forms of life were created essentially as they exist today.
Causes of evolutionary change
Most evolutionary change is caused by the interaction of two processes: (1) mutation and (2) natural selection. Mutation produces random (chance) variation in the genetic makeup of a species or a population--that is, individuals of the same species living in the same area. Natural selection sorts out these random changes according to their value in enhancing the individual's reproduction and survival. Such selection ensures that variations that make a species better adapted to its environment will pass on to future generations. At the same time, natural selection eliminates variations that make a species less able to survive and reproduce.
Another process that causes evolutionary change in populations is called genetic drift. Some scientists think it is much less important than natural selection.
Mutation is a permanent change in the hereditary material of an organism. Mutations may produce changes in the inherited characteristics of an organism. To understand how mutations produce these changes, it is necessary to understand how characteristics are inherited.
How characteristics are inherited. Hereditary characteristics of organisms are carried by threadlike structures called chromosomes in cells. Chromosomes carry large numbers of genes, the basic units of heredity. Genes consist of a substance called DNA (deoxyribonucleic acid). DNA contains the coded information that determines hereditary characteristics.
Among most animals and plants, each body cell has a full set of paired chromosomes. Human body cells, for example, have 46 chromosomes, arranged in 23 pairs. Offspring inherit half a set of chromosomes from each parent. Parents pass on their chromosomes to their offspring during sexual reproduction. Egg cells and sperm cells form in a special process that gives them one chromosome at random from each pair of the parent's set. As a result, egg and sperm cells have half the number of chromosomes found in all other cells in the body. During reproduction, a sperm and an egg unite in the process called fertilization, and the fertilized egg then has the full number of chromosomes.
Sometimes, the genes from one of a pair of chromosomes change places with genes on the other chromosome as a sperm or egg cell is formed. This change in the arrangement of genes, called recombination, can result in new combinations of inherited traits.
As the fertilized egg cell begins to grow, each chromosome in the nucleus of the cell duplicates itself. The chromosome and its duplicate lie next to each other in pairs. During normal cell division, one of each pair of chromosomes goes into each of two new cells. Thus, the new cells contain chromosomes that are identical with those in the original cell. This process of growth through cell division continues until it has produced all the cells that make up an organism.
How mutations change a species. Mutations may be caused by environmental factors, such as chemicals and radiation, which alter the DNA in genes, or by errors in the copying of DNA during cell division. After a gene has changed, it duplicates itself in its changed form. If these mutant genes are present in the egg or sperm cells of an organism, they may alter some inherited characteristics. Only this mutation can introduce new hereditary characteristics. For this reason, mutations are the building blocks of evolutionary change and of the development of new species.
Mutations occur regularly but infrequently, and most of them produce unfavorable traits. Albinism is one such mutation. Albino animals have mutant genes that lack the ability to produce normal skin pigment. These animals do not survive and reproduce as well as nonmutant animals. In most cases, such mutant genes are eliminated by natural selection because most of the organisms that have them die before producing any offspring. Some mutations, however, help organisms adapt better to their environment. A plant in a dry area might have a mutant gene that causes it to grow longer roots. The plant would have a better chance of survival than others of its species because its roots could reach deeper for water. This type of beneficial mutation provides the raw material for evolutionary change.
Natural selection can involve any feature that affects an individual's ability to leave offspring. These features include appearance, body chemistry, and physiology (how an organism functions), as well as behavior.
For natural selection to operate, two biological conditions must be met. First, the individuals of a population must differ in their hereditary characteristics. Humans, for example, vary in almost every aspect of their appearance, including height, weight, and eye color. People also differ in not-so-obvious features, such as brain size, thickness of bones, and amount of fat in the blood. Many of these differences have some genetic basis.
The second requirement for natural selection is that some of the inherited differences must affect chances for survival and reproduction. When this occurs, the fittest individuals will pass on more copies of their genes to future generations than will other individuals. Over time, a species accumulates genes that increase its ability to survive and reproduce in its environment.
Natural selection is a group process. It causes the evolution of a population or a species as a whole--not the evolution of an individual--by gradually shifting the average characteristics of the group over time.
Natural selection can be illustrated by a cactus called the prickly pear, which normally grows close to the ground and has soft spines. On the Galapagos Islands, in the Pacific Ocean off South America, prickly pears are the chief food of giant tortoises. A tortoise is more likely to eat an ordinary prickly pear than a tall one with tough spines. As a result, tall, tough-spined prickly pears have evolved from their short, soft-spined ancestors and reproduced in greater numbers over the years. Today, they are the most common form of prickly pears on the islands. But on the islands with no tortoises, almost all the prickly pears are short and have soft spines.
There are several types of natural selection. They include (1) directional selection, (2) stabilizing selection, and (3) sexual selection.
Directional selection produces new features that help a species adapt to its environment. This type of selection is what most people think of as natural selection. The evolution of the prickly pear is an example of directional selection. The individuals that differ most from the population average of a characteristic--in this case, the spiniest individuals--leave the most offspring. This causes a continual change in the species toward the more extreme characteristic.
Stabilizing selection occurs if a species is already well adapted to its environment. In such cases, the individuals with average characteristics leave the most offspring, and individuals that differ most from the average leave fewest. One example of stabilizing selection is the survival rate of human babies according to birth weight. Babies of average weight tend to survive better than those who are either heavier or lighter. Unlike directional selection, stabilizing selection eliminates extreme characteristics, reducing the amount of variation in a population. Stabilizing selection may actually be the most common type of natural selection.
Sexual selection occurs primarily among animals. Adults of many species prefer mates who display certain behaviors or have certain external features. Over time, this process can lead to the evolution of complicated courtship rituals, bright coloring to attract a mate, and other features. Sexual selection explains, for example, why males of many bird species have more-colorful feathers than the females.
Genetic drift is a random change in the frequencies of genes in populations. It is caused by the random way that egg and sperm cells receive some chromosomes from each parent as they form. Because these reproductive cells contain only half a set of chromosomes, only half of a parent's genes are present in an egg or sperm. If the parents produce a limited number of offspring, some of their genes may not be passed on.
Genetic drift does not enable species to evolve to adapt to their environment because it causes random changes in the frequencies of traits. Over time, however, genetic drift can gradually change the genetic makeup of a population.
Evolution of new species
Among sexually reproducing plants and animals, a species is a group of plants or animals whose members can breed with one another. Members of different species cannot produce fertile offspring together.
Various devices in nature that keep species distinct are called reproductive isolating factors. They include factors that prevent different species living in the same area from mating with each other. For example, many species of birds have unique courtship rituals, and females of one species will not respond to the courtship of males from other species. Although courtship and mating may occur between different species, other reproductive isolating factors make any offspring of such matings unable to survive or to reproduce. A well-known example is the mule. A mule is the offspring of a female horse and a male donkey, and is sterile.
A group of plants or animals that develop reproductive isolating factors, preventing them from breeding outside the group, have evolved into a new species. Most biologists believe that this process, called speciation, usually begins when a species is separated into two or more groups that are geographically isolated.
The geographic isolation of land species may result from the movement of continents over millions of years or from the division of habitats by such features as glaciers and rivers. The rise of land bridges, such as the Isthmus of Panama, may separate marine species.
Over time, isolated populations evolve in different ways because their environments differ and because different mutations occur in each population. If the geographic isolation lasts long enough, the populations may become so dissimilar that members of one population cannot breed with members of another. Reproductive isolating factors would then have evolved, and the populations would have become distinct species.
Speciation is normally extremely slow, taking millions of years. But in certain cases it can occur more rapidly. Rapid speciation is especially likely after a species settles in a new habitat, such as an unpopulated island. The species then becomes subject to strong, new forces of natural selection, such as a different climate or food supply. There is also a unique form of rapid speciation in plants called allopolyploidy in which major increases in the number of chromosomes can give rise to new species within two generations.
Evidence of evolution
Most evolutionary change occurs too slowly to be observed directly. But direct observation is not the only way to determine whether a process has taken place. Scientists have accumulated a tremendous amount of evidence documenting the occurrence of evolution. This evidence comes from six principal sources: (1) the fossil record, (2) the geographic distribution of species, (3) embryology, (4) vestigial organs, (5) direct observation of evolution, and (6) artificial selection.
The fossil record provides some of the strongest evidence for evolution. Most organisms preserved as fossils were buried under layers of mud or sand that later turned into rock. Scientists determine the age of fossils by means of radiocarbon dating and other methods of dating.
The fossil record has many gaps because relatively few species were preserved. Nevertheless, paleontologists (scientists who study prehistoric life) have found enough fossils to form a fairly complete record that documents much of the history of life on earth.
The fossil record shows a progression from the earliest types of one-celled life to the first simple, multicelled organisms, and from these organisms to the many simple and complex organisms living today. The fossils found in ancient layers of rock include the simplest forms of life and differ greatly from many organisms that exist today, while fossils in recently formed layers of rock include complex forms of life and are more similar to living plants and animals. Thus, the fossil record shows that many species became extinct and that the species alive today have not always lived on earth.
The fossil record also documents many examples of continuous evolutionary change and speciation. A famous example is the evolution of mammals from reptiles. The fossil record contains no mammals before 250 million years ago but has many species of reptiles from that period. Mammals first appear as fossils about 200 million years ago. Between these two periods, scientists have found many remains of mammallike reptiles. The skeletons of the first mammallike reptiles are nearly identical to those of reptiles, but later skeletons resemble those of mammals. In between occur creatures with a mixture of skeletal characteristics of reptiles and mammals. The transition from reptiles to mammals is so gradual that it is impossible to fix a point when reptiles became mammals. These fossils clearly document the evolution of mammals from reptile ancestors.
Another example of continuous evolution found in the fossil record is that of the horse.
Other fossil evidence for evolution includes transitional forms--that is, organisms that show common ancestry between groups of animals living today. Many transitional forms have a combination of the characteristics of living species. For example, Archaeopteryx is a fossil that may be closely related to the common ancestor of birds and reptiles. This fossil had a skeleton nearly identical to that of a small dinosaur but had such birdlike features as feathers, a beak, and an early form of wings. For an illustration of an Archaeopteryx.
Other transitional forms include the early ancestors of human beings. Since the 1920's, paleontologists have assembled a collection of fossils showing the evolution of modern human beings from apelike ancestors called australopithecines.
Geographic distribution of species, also known as biogeography, provides important evidence for the theory of evolution. Certain island groups, called oceanic islands, arose from the sea floor and have never been connected to continents. They include Hawaii, Tahiti, and the Galapagos Islands. The species found on oceanic islands are those that can travel easily, particularly over large stretches of water. These islands are rich in flying insects, bats, birds, and certain types of plants that floated to the islands as seeds. But oceanic islands lack many major types of animals and plants that live on continents. For example, the Galapagos Islands have no native land mammals. The islands also have no amphibians--animals that live part of their life in water and part on land, such as frogs and toads. Mammals and amphibians cannot easily migrate from continents to islands.
In addition, the majority of species found on oceanic islands are most similar to those on the nearest mainland, even if the environment and climate are different. The Galapagos Islands, for example, lie off the coast of mainland Ecuador in South America. The islands are much drier and rockier than the coast, which has a humid climate and lush tropical forests. But the plants and birds on the Galapagos Islands are more similar to those of the wet, tropical coast than to the plants and birds of other arid islands. This suggests that the first species to inhabit the Galapagos Islands came from South America rather than originating on the islands.
The species on oceanic islands also make up a much larger part of the plant or animal population than similar species do on continents, and some species are found nowhere else. The Galapagos Islands, for example, have 21 native species of land birds. Of these, 13 species are finches--a much higher proportion of finches than is found on any continent. These finches all belong to species found only on the Galapagos Islands. Thus, the distribution of species supports the idea that a limited number of species came to the islands from the nearest mainland and then evolved into new species.
Embryology is the study of the way organisms develop during the earliest stages of life. The embryonic development of many organisms includes peculiar events that can only be explained by the evolution of the organism from another species. For example, a mammal embryo forms three types of kidneys in succession during its development. The first two kidneys perform no function and break down and disappear shortly after they form. The third type of kidney then takes shape and develops into the mature, functioning kidney of the mammal. In the embryos of fishes, amphibians, and reptiles, however, one of the first two types of kidneys becomes the mature kidneys of these animals. These events suggest that mammals have retained some of the developmental features of their evolutionary ancestors. The repetition of embryonic features of evolutionary ancestors during the early development of an organism is called recapitulation.
Another example of recapitulation occurs during the development of human beings. A human fetus (unborn child) grows a coat of fine hair called the lanugo, which is shed before or shortly after birth. The lanugo is almost certainly a developmental feature that has remained from the time of the apelike ancestors of human beings, because monkey and ape fetuses also develop a coat of hair but do not shed it.
Vestigial organs are the useless remains of organs that were once useful in an evolutionary ancestor. For example, many species of fish, amphibians, and other animals that live in caves are blind but still have eyes. Some species have nearly complete eyes but lack an optic nerve, while others have tiny, malformed eyes. Some cave-dwelling crayfish have eyestalks but no eyes. These species evolved from ancestors with functioning eyes. Because eyes are useless in a dark habitat, mutations that damaged the vision of these species did not decrease their fitness, and these species gradually lost their sight. Similarly, many whales have tiny vestigial leg bones, the useless evolutionary remains of their land-dwelling ancestors.
One of the best-known vestigial organs is the human appendix, a narrow tube attached to the large intestine. In orangutans and other apes, the appendix is a fully formed intestinal sac that helps digest plant material. In human beings, it serves no known purpose and is, in fact, often harmful to them.
Direct observation of evolution. Evolutionary change is normally extremely slow. But in some cases it is rapid and can be seen as it occurs in living species. This happens most often when a species undergoes genetic change in response to a disturbance of its environment by human beings.
The peppered moth is an example of an organism that evolved rapidly in response to environmental change. At one time, nearly all peppered moths in industrial areas of the United Kingdom were white with black spots. Only a few mutant ones were black. The light-colored moths blended in with the light-colored lichens that covered the trunks of trees, but the black ones could easily be seen--and then eaten--by birds. In the mid-1800's, soot from newly built factories began to kill the lichens and blacken many trees. As a result of the change in tree color, the light-colored moths became easier for the birds to spot than the black ones. The number of light-colored moths in industrial areas soon declined, and the mutation for black coloring became widespread.
Other examples of rapid, observable evolutionary change have occurred among certain insects and disease-causing bacteria. In areas where DDT and other insecticides were used, some insects developed immunity to the chemicals within a few years. Some disease-causing bacteria have become resistant to antibiotics in a similar way. Such resistance may develop so quickly that researchers must continually come up with new antibiotics to replace those no longer effective.
Artificial selection. In the mid-1800's, Darwin noted that animal and plant breeders use a method similar to natural selection to produce new varieties. Breeders commonly breed only the individuals in a species that show desired characteristics. This process, called artificial selection, eventually leads to large changes in a species. For example, the various breeds of dogs differ widely in size, appearance, and behavior. They probably descended, however, from one or a few dog species that were bred to develop various traits. Many of these traits helped the dog perform a specific job, such as hunting badgers or herding sheep.
Plant breeders developed most food crops from wild ancestors by the same process. For example, cabbage, broccoli, kohlrabi, cauliflower, and Brussels sprouts all belong to a single species that was selectively bred to develop different characteristics.
Artificial selection differs from natural selection only because human beings--instead of the natural environment--determine which characteristics give individuals an advantage in reproduction. The ability of artificial selection to cause dramatic changes in a short time leaves little doubt that natural selection could cause larger changes over the vast spans of earth's history.
History of the theory of evolution
Early theories. Some of the first scientific investigations of evolution were conducted in the 1700's by two French naturalists, Comte de Buffon and Baron Cuvier. They concluded from their studies of fossils and comparative anatomy that life on earth had undergone a series of changes. But neither Buffon nor Cuvier had any idea how long ago these changes had occurred because they knew little about the earth's history.
In 1809, the French naturalist Lamarck formulated the first comprehensive theory of evolution. He observed that an animal's body parts could change during its lifetime, depending on the extent to which it used them. Organs and muscles that were frequently used became larger and stronger, but those that were rarely used tended to shrink. According to Lamarck, such acquired traits became hereditary. His theory of the inheritance of acquired characteristics influenced many scientists. Later discoveries in genetics disproved his theory.
Darwin's theory. In 1858, Darwin introduced a theory of evolution that, in modified form, is accepted by almost all scientists today. Darwin's theory states that all species evolved from a few common ancestors by means of natural selection. He set forth his theory in The Origin of Species (1859). Another British naturalist, Alfred R. Wallace, proposed an identical theory at about the same time. However, Darwin's ideas were developed much more thoroughly in a best-selling book, and his work has become better known.
Darwin used three principal sources in developing his theory. These were (1) his personal observations, (2) the geological theory of the British scientist Sir Charles Lyell, and (3) the population theory of the British economist Thomas Robert Malthus. Darwin made many of his observations as a member of a scientific expedition aboard the H.M.S. Beagle from 1831 to 1836. The ship made stops along the coast of South America, and Darwin collected many specimens of plants and animals and wrote detailed notes.
Darwin was particularly impressed by the variety of species on the Galapagos Islands. He found striking differences not only between species on the islands and those on the mainland, but also among those on each island. Darwin's findings led him to reject the idea of divine creation and to search for another explanation for the origin of species.
The theories of Lyell and Malthus influenced Darwin's ideas about the earth's history and the relationship between living things and their environment. Lyell's Principles of Geology, published in the early 1830's, stated that the earth had been formed by natural processes over long periods of time. Darwin wondered whether life on earth had also developed gradually as a result of natural processes. In 1798, Malthus wrote that the growth of the human population would someday exceed the food supply unless checked by such factors as war and disease. Darwin assumed that some environmental factor also regulated the population of all other living things. He concluded that only the individuals most fit for their environment would tend to survive and pass on their characteristics to their offspring.
The synthetic theory was formulated during the 1930's and 1940's by a number of scientists, including two American biologists, Russian-born Theodosius Dobzhansky and German-born Ernst Mayr, and the British geneticist and statistician Ronald A. Fisher. The theory is called synthetic because it synthesizes (combines) Darwin's theory of natural selection with the principles of genetics and of certain other sciences. Darwin had observed that the characteristics of organisms may change during the process of being passed on to offspring. However, he could not explain how or why these changes took place because the principles of genetics were not yet known.
The genetic principles of variation and mutation filled this gap in Darwin's theory. Gregor Mendel, an Austrian monk, had discovered the principles of genetics in the 1860's. Mendel's findings remained unnoticed until the early 1900's, when the science of genetics was established. About 1910, the American biologist Thomas Hunt Morgan discovered that genes are carried by chromosomes. Morgan also described the process of recombination, in which genes are exchanged from one chromosome to another, producing new combinations of hereditary traits.
Recent developments. The theory of evolution has not changed substantially since the 1940's. But evolutionary biologists have continued to discover more about how this process works, particularly through discoveries in other fields. Some of the most important contributions have come from molecular biology, which deals with the genetic processes involved in evolution. In the 1940's, biologists identified DNA as the substance in chromosomes that carries hereditary information. In the 1950's and 1960's, studies of DNA revealed much about its structure and its role in evolutionary changes. These studies have led scientists to believe that, at the molecular level, evolution occurs through the substitution of one nucleotide (a small bit of DNA) or one amino acid for another.
In the 1970's, molecular biologists developed methods to determine the complete sequences of DNA molecules. This discovery enabled scientists to directly measure the amount of genetic variation among individuals of a species. Biologists also developed methods to estimate the amount of genetic similarity between species and thus to measure the evolutionary relatedness of one species to another. This measurement makes it possible to reconstruct the evolutionary history of organisms by comparing the DNA of existing species. For example, scientists did not know whether the giant panda was more closely related to the raccoon or to bears. But DNA analysis has led most scientists to think that the panda is actually more closely related to bears.
Similar analysis of DNA has led scientists to revise the timeline of earth's evolutionary history. For example, the explosive growth of multicellular life once thought to have occurred at the beginning of the Cambrian Period (544 million years ago) is now believed to have taken place at least 1 billion years ago, during Precambrian time.
In the 1990's, scientists studying animal embryos at the molecular level found that a common set of genes controls basic aspects of development in different animals. These aspects include the formation of body segments in animals as diverse as worms and human beings. Such findings imply that parts of the developmental plan were present long ago in the common ancestors of most species, and have remained unchanged.
Recent discoveries in molecular biology and paleontology have led to controversy about human evolution. Many scientists believe that modern human beings evolved in Africa and then migrated to other continents, displacing earlier hominids (humanlike creatures) who lived in these places. Others think that earlier hominids evolved into modern human beings at the same time in different places. Resolving such conflicting views will require the discovery of more fossils.
Scientists also remain uncertain about the relative importance of genetic drift and natural selection in explaining evolutionary change. Many species have characteristics that have no obvious value in adapting to the environment. Scientists disagree whether these characteristics, such as differences in the shape of leaves among various species of trees, affect an individual's survival and reproduction or are merely random variations caused by genetic drift.
Acceptance of evolution
Today, the theory of evolution is considered the most important fundamental concept in the biological sciences. Nearly all scientists accept it. However, large numbers of people opposed the theory when it was introduced. Many people still do not accept it today.
In Darwin's time, the theory of evolution was attacked by many scientists, religious leaders, and other groups. Biologists argued that the evolutionary concept of hereditary variations within species contradicted the theory of blending inheritance. According to this theory, which was popular during the 1800's, hereditary characteristics became mixed and diluted as the blood carried them from one generation to another.
By the early 1900's, discoveries in genetics and other fields had resolved virtually all the original scientific objections to evolution. But other philosophical or religious objections remained. Many Christian leaders denounced the idea of evolution because it conflicted with the Biblical account of the Creation and suggested that human beings had evolved from apelike ancestors.
In the United States, much of the controversy centered on whether evolution should be taught in schools. In the 1920's, some states passed laws that banned such teaching in the public schools. In 1925, John T. Scopes, a Tennessee high-school teacher, was convicted in the famous "monkey trial" of teaching Darwin's theory. Although Scopes's conviction was later overturned because of a legal error, few public schools included evolution in the biology curriculum for many years after the trial.
In 1968, the Supreme Court of the United States ruled that laws banning the teaching of evolution were unconstitutional. The ruling stated that such laws made religious considerations part of the curriculum and thus violated the First Amendment to the Constitution. During the 1970's and 1980's, many religious groups proposed legislation that would require evolution to be taught along with an opposing view called creationism. Creationists believe that each species has remained relatively unchanged since the Creation and that no species has evolved from any other. Strict creationists accept the Bible's account of the Creation as literal truth. They believe the earth is only thousands of years old. They also hold that all species were created simultaneously and that much of early life was destroyed by a worldwide flood.
In 1981, Arkansas became the first state to enact a law requiring public schools to teach creationism whenever evolution is taught. However, a federal court declared this law unconstitutional before it went into effect. The court ruled that there was no scientific evidence for creationism and that these views constituted a religious and not a scientific explanation of life. Therefore, the court held that the Arkansas law violated the separation of church and state guaranteed by the First Amendment.
Because of these court decisions, opponents of evolution have largely abandoned their efforts to promote laws banning its teaching. However, such opponents have turned their attentions toward influencing local school boards to reduce or eliminate the teaching of evolution in biology classes.
Evolution and religion
Many people--including some Christians, Muslims, and Orthodox Jews--do not accept the theory of evolution because it conflicts with their religious beliefs. For example, the Biblical account of the Creation states that God created all living things, including human beings, within a short time. A literal reading of this account contradicts the idea that organisms evolved over millions of years. The Bible also states that human beings were created in the image of God and thus were elevated above all other forms of life. Some people find it difficult to reconcile this view with the idea that human beings evolved through natural processes.
Many other people, however, accept the basic principles of evolution within the framework of their religious beliefs. For example, some people interpret the story of the Creation as a symbolic and not a literal account of the origin of life. They find this symbolic interpretation compatible with the discoveries of evolutionary biology. For many people, the idea that human beings evolved from other forms of life does not diminish the uniqueness of human capabilities and the accomplishments of human civilization.
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