H-Evolution-007Animals

Evolution of Animals

Origin of Animal Diversity

Animal life began in Precambrian seas with the evolution of multiceluar creatures that ate other organisms. We are among their descendants.

What is an Animal?

Animals are eukaryotic, multicellular, heterotrophic organisms that obtain nutrients by ingestion -- ingestion means eating food. This mode of nutrition contrasts animals with fungi, which obtain nutrients by absorption after digesting the food outside the body. Animals digest their food within their bodies after ingesting other organisms, dead or alive, whole or by the piece (Figure 17.2).

Most animals reproduce sexually. The zygote (fertilized egg) develops into an early embryonic stage called a blastula, which is usually hollow ball of cells (Figure 17.3). The next embryonic stage in most animals is a gastrula, which has layers of cells that will eventually form the adult body parts. The gastrula also has a primitive gut, which will develop into the animal's digestive compartment. Continued development, growth, and maturation transform some animals directly from the embryo into an adult. However many animals include larval stages. A larva is a sexually immature form of an animal. It is anatomically distinct from the adult form, usually eats different foods, and may even have a different habitat. Think how different a frog is from its larval form, which we calla tadpole. A change of body form, called metamorphosis, eventually remodels the larva into the adult form.

Figure 17.2 Nutrition by ingestion, the animal way of life. Most animals ingest relatively large pieces of food, though rarely as large as the prey in this case. In this amazing scene, a rock python is beginning to ingest a gazelle. The snake will spend two weeks or more in a quiet place digesting its meal.

Figure 17.3 Life cycle of a sea star as an example of animal development. (1) Male and female adult animals produce haploid gametes (eggs and sperm) by meiosis. (2) An egg and a sperm fuse to produce a diploid zygote. (3) Early mitotic divisions lead to an embryonic stage called a blastula, common to all animals. Typically, the blastula consists of a ball of cells surrounding a hollow cavity. (4) Later, in the sea star and many other animals, one side of the blastula cups inward, forming an embryonic stage called a gastrula. (5) The gastrula develops into a saclike embryo with a two-layered wall and an opening at one end. Eventually, the outer layer (ectoderm) develops into the animal's epidermis (skin) and nervous system. The inner layer (endoderm) forms the digestive tract. Still later in development, in most animals, a third layer (mesoderm) forms between the other two and develops into most of the other internal organs (not shown in the figure). (6) Following the gastrula, many animals continue to develop and then mature directly into adults. But others, including the sea star, develop into one or more larval stages first. (7) The larva undergoes a major change of body form, called metamorphosis, in becoming an adult-a mature animal capable of reproducing sexually.

Most animals have muscle cells, as well as nerve cells that control the muscles. The evolution of this equipment for coordinated movement enhanced feeding, even enabling some animals to search for or chase their food. The most complex animals, of course, can use their muscular and nervous systems for many functions other than eating. Some species even use massive networks of nerve cells called brains to think.

Figure 17.4 One hypothesis for a sequence of stages in the origin of animals from a colonial protist. (1) The earliest colonies may have consisted of only a few cells, all of which were flagellated and basically identical. (2) Some of the later colonies may have been hollow spheres-floating aggregates of heterotrophic cells-that ingested organic nutrients from the water. (3) Eventually, cells in the colony may have specialized, with some cells adapted for reproduction and others for somatic (non-reproductive) functions, such as locomotion and feeding. (4) A simple multicellular organism with cell layers may have evolved from a hollow colony, with cells on one side of the colony cupping inward, the way they do in the gastrula of an animal embryo (see Figure 17.3). (5) A layered body plan would have enabled further division of labor among the cells. The outer flagellated cells would have provided locomotion and some protection, while the inner cells could have specialized in reproduction or feeding. With its specialized cells and a simple digestive compartment, the proto-animal shown here could have fed on organic matter on the sea floor.

Figure 17.5 A Cambrian seascape. drawing based on fossils collected at a site called Burgess Shale in British Columbia, Canada.

Early Animals and the Cambrian Explosion

Animals probably evolved from a colonial, flagellated protist that lived in Precambrian seas (Figure 17.4) .By the late Precambrian, about 600-700 million years ago, a diversity of animals had already evolved. Then came the Cambrian explosion. At the beginning of the Cambrian period, 545 million years ago, animals underwent a relatively rapid diversification. During a span of only about 10 million years, all the major animal body plans we see today evolved. It is an evolutionary episode so boldly marked in the fossil record that geologists use the dawn of the Cambrian period as the beginning of the Paleozoic era. Many of the Cambrian animals seem bizarre compared to the versions we see today, but most zoologists now agree that the Cambrian fossils can be classified as ancient representatives of contemporary animal phyla (Figure 17.5).

What ignited the Cambrian explosion? Hypotheses abound. Most researchers now believe that the Cambrian explosion simply extended animal diversification that was already well under way during the late Precambrian. But what caused the radiation of animal forms to accelerate so dramatically during the early Cambrian? One hypothesis emphasizes increasingly complex predator-prey relationships that led to diverse adaptations for feeding, motility, and protection. This would help explain why most Cambrian animals had shells or hard outer skeletons, in contrast to Precambrian animals, which were mostly soft-bodied. Another hypothesis focuses on the evolution of genes that control the development of animal form, such as the placement of body parts in embryos. At least some of these genes are common to diverse animal phyla. However, variation in how, when, and where these genes are expressed in an embryo can produce some of the major differences in body form that distinguish the phyla. Perhaps this developmental plasticity was partly responsible for the relatively rapid diversification of animals during the early Cambrian. In the last half billion years, animal evolution has mainly generated new variations of old "designs" that originated in the Cambrian seas.

Animal Phylogeny

Because animals diversified so rapidly on the scale of geologic time, it is difficult, using only the fossil record, to sort out the sequence of branching in animal phylogeny. To reconstruct the evolutionary history of animal phyla, researchers must depend mainly on clues from comparative anatomy and embryology. Molecular methods are now providing additional tools for testing hypotheses about animal phylogeny. Figure 17.6 represents one set of hypotheses about the evolutionary relationships among nine major animal phyla. The circled numbers on the tree highlight four key evolutionary branch points, and these numbers are keyed to the following discussion.

(1) The first branch point distinguishes sponges from all other animals based on structural complexity. Sponges, though multicellular, lack the true tissues, such as epithelial (skin) tissue, that characterize more complex animals.

(2) The second major evolutionary split is based partly on body symmetry: radial versus bilateral. To understand this difference, imagine a pail and shovel. The pail has radial symmetry. identical all around a central axis. The shovel has bilateral symmetry, which means there's only one way to split it into two equal halves-right down the midline. Figure 17.7 contrasts a radial animal with a bilateral one.

The symmetry of an animal generally fits its lifestyle. Many radial animals are sessile forms (attached to a substratum) or plankton (drifting or weakly swimming aquatic forms). Their symmetry equips them to meet the environment equally well from all sides. Most animals that move actively from place to place are bilateral. A bilateral animal has a definite "head end" that encounters food, danger, and other stimuli first when the animal is traveling. In most bilateral animals, a nerve center in the form of a brain is at the head end, near a concentration of sense organs such as eyes. Thus, bilateral symmetry is an adaptation for movement, such as crawling, burrowing, or swimming.

Figure 17.7 Body symmetry. (a) The parts of a radial animal, such as this sea anemone, radiate from the center. Any imaginary slice through the central axis would divide the animal into mirror images. (b) A bilateral animal, such as this lobster, has a left and right side. Only one imaginary cut would divide the animal into mirror-image halves.

Figure 17.8 Body plans of bilateral animals. The various organ systems of these animals develop from the three tissue layers that form in the embryo. (a) Flatworms are examples of animals that lack a body cavity. (b) Roundworms have a pseudocoelom, a body cavity only partially lined by mesoderm, the middle tissue layer. (c) Earthworms and other annelids are examples of animals with a true coelom. Acoelom is a body cavity completely lined by mesoderm. Mesenteries, also derived from mesoderm, suspend the organs in the fluid-filled coelom.

(3) The evolution of body cavities led to more complex animals. A body cavity is a fluid-filled space separating the digestive tract from the outer body wall. A body cavity has many functions. Its fluid cushions the suspended organs, helping to prevent internal injury. The cavity also enables the internal organs to grow and move independently of the outer body wall. If it were not for your body cavity, exercise would be very hard on your internal organs. And every beat of your heart or ripple of your intestine would deform your body surface. It would be a scary sight. In soft-bodied animals such as earth worms, the noncompressible fluid of the body cavity is under pressure and functions as a hydrostatic skeleton against which muscles can work. In fact, body cavities may have first evolved as adaptations for burrowing.

Among animals with a body cavity, there are differences in how the cavity develops. In all cases, the cavity is at least partly lined by a middle layer of tissue, called mesoderm, which develops between the inner ( endoderm) and outer ( ectoderm) layers of the gastrula embryo. If the body cavity is not completely lined by tissue derived from mesoderm, it is termed a pseudocoelom (Figure 17.8) .A true coelom. the type of body cavity humans and many other animals have, is completely lined by tissue derived from mesoderm.

(4) Among animals with a true coelom. there are two main evolutionary branches. They differ in several details of embryonic development. Including the mechanism of coelom formation. One branch includes mollusks (such as clams, snails, and squids). annelids (such as earthworms), and arthropods (such as crustaceans, spiders. and insects). The two major phyla of the other branch are echInoderms (such as sea stars and sea urchIns) and chordates (including humans and other vertebrates).

Major Invertebrate Phyla

Living as we do on land, our sense of animal diversity is biased in favor of vertebrates, which are animals with a backbone. Vertebrates are well represented on land in the form of such animals as reptiles, birds, and mammals. However, vertebrates make up only one subphylum within the phylum Chordata, or less than 5% of all animal species. If we were to sample the animals in an aquatic habitat, such as a pond, tide pool, or coral reef, we would find ourselves in the realm of invertebrates. These are the animals without backbones. It is traditional to divide the animal kingdom into vertebrates and invertebrates, but this makes about as much zoological sense as sorting animals into flatworms and nonflatworms. We give special attention to the vertebrates only because we humans are among the backboned ones. However, by exploring the other 95% of the animal kingdom-the invertebrates-we'll discover an astonishing diversity of beautiful creatures that too often escape our notice.

Sponges

Sponges (phylum Porifera) are sessile animals that appear so sedate to the human eye that the ancient Greeks believed them to be plants (Figure 17.9). The simplest of all animals, sponges probably evolved very early from colonial protists. Sponges range in height from about 1 cm to 2 m. Sponges have no nerves or muscles, but the individual cells can sense and react to changes in the environment. The cell layers of sponges are loose federations of cells, not really tissues, because the cells are relatively unspecialized. Of the 9,000 or so species of sponges, only about 100 live in fresh water; the rest are marine.

Figure 17.9 Sponge

Figure 17.10 Anatomy of a sponge. Feeding cells called choanocytes have flagella that sweep water through the sponge's body. Choanocytes trap bacteria and other food particles, and amoebocytes distribute the food to other cells. To obtain enough food to grow by 100 g (about 3 ounces), a sponge must filter 1,000 kg (about 275 gallons) of seawater.

The body of a sponge resembles a sac perforated with holes (the name of the sponge phylum, Porifera, means "pore bearer"). Water is drawn through the pores into a central cavity, then flows out of the sponge through a larger opening (Figure 17.10). Most sponges feed by collecting bacteria from the water that streams through their porous bodies. Flagellated cells called choanocytes trap bacteria in mucus and then engulf the food by phagocytosis. Cells called arnoebocytes pick up food from the choanocytes, digest it, and carry the nutrients to other cells. Amoebocytes are the "do-all" cells of sponges. Moving about by means of pseudopodia, they digest and distribute food, transport oxygen, and dispose of wastes. Amoebocytes also manufacture the fibers that make up a sponge's skeleton. In some sponges, these fibers are sharp and spur-like. Other sponges have softer, more flexible skeletons; we use these pliant, honey-combed skeletons as bathroom sponges.

Cnidarians

Cnidarians (phylum Cnidaria) are characterized by radial symmetry and tentacles with stinging cells. Jellies, sea anemones, hydras, and coral animals are all cnidarians. Most of the 10,000 cnidarian species are marine.

The basic body plan of a cnidarian is a sac with a central digestive compartment, the gastrovascular cavity. A single opening to this cavity functions as both mouth and anus. This basic body plan has two variations: the sessile polyp and the floating medusa (Figure 17.11 ). Polyps adhere to the substratum and extend their tentacles, waiting for prey. Examples of the polyp form are hydras, sea anemones, and coral animals (Figure 17.12) .A medusa is a flattened, mouth-down version of the polyp. It moves freely in the water by a combination of passive drifting and contractions of its bell-shaped body. The animals we generally call jellies are medusas (jellyfish is another common name, though these animals are not really fishes, which are vertebrates). Some cnidarians exist only as polyps, others only as medusas, and still others pass sequentially through both a medusa stage and a polyp stage in their life cycle.

Figure 17.11 Polyp and medusa forms of cnidarians. Note that cnidarians have two tissue layers, distinguished in the diagrams by blue and yellow. The gastrovascular cavity is a digestive sac, meaning that it has only one opening, which functions as both mouth and anus. {a) Sea anemones are examples of the polyp form of the basic cnidarian body plan. {b) Jellies are examples of the medusa form.

Figure 17.12 Coral animals. Each polyp in this colony is about 3 mm in diameter. Coral animals secrete hard external skeletons of calcium carbonate (limestone). Each polyp builds on the skeletal remains of earlier generations to construct the "rocks" we call coral. Though individual coral animals are small, their collective construction accounts for such biological wonders as Australia's Great Barrier Reef, which Apollo astronauts were able to identify from the moon. Tropical coral reefs are home to an enormous variety of invertebrates and fishes.

Figure 17.13 Cnidocyte action. Each cnidocyte contains a fine thread coiled within a capsule. When a trigger is stimulated by touch, the thread shoots out. Some cnidocyte threads entangle prey, while others puncture the prey and inject a poison.

Cnidarians are carnivores that use tentacles arranged in a ring around the mouth to capture prey and push the food into the gastrovascular cavity, where digestion begins. The undigested remains are eliminated through the mouth/anus. The tentacles are armed with batteries of cnidocytes ("stinging cells"), unique structures that function in defense and in the capture of prey (Figure 17.13). The phylum Cnidaria is named for these stinging cells.

Flatworms

Flatworms (phylum Platyhelminthes) are [ the simplest bilateral animals. True to their name, these worms are leaf-like or ribbon-like, ranging from about 1 mm to about 20 m in length. There are about 20,000 species of flatworms living in marine, fresh water, and damp terrestrial habitats. Planarians are examples of free-living (nonparasitic) flatworms (Figure 17.14) .The phylum also includes many parasitic species, such as flukes and tapeworms.

Parasitic flatworms called blood flukes are a huge health problem in the tropics. These worms have suckers that attach to the inside of the blood vessels near the human host's intestines. This causes a long-lasting disease with such symptoms as severe abdominal pain, anemia, and dysentery. About 250 million people in 70 countries suffer from blood fluke disease. Flukes generally have complex life cycles that require more than one host species. People are most commonly exposed to blood flukes while working in irrigated fields contaminated with human feces. Blood flukes living in a human host reproduce sexually, and fertilized eggs pass out in the host's feces. If an egg lands in a pond or stream, a motile larva hatches. This larva can enter a snail, the next host. Asexual reproduction in the snail eventually produces other larvae that can infect humans. A person becomes infected when these larvae penetrate the skin.

Tapeworms parasitize many vertebrates, including humans. In contrast to planarians and flukes, most tapeworms have a very long, ribbon like body with repeated parts (Figure 17.15) .They also differ from other flatworms in not having any digestive tract at all. Living in partially digested food in the intestines of their hosts, tapeworms simply absorb nutrients across their body surface. Like parasitic flukes, tapeworms have a complex life cycle, usually involving more than one host. Humans can become infected with tapeworms by eating rare beef containing the worm's larvae. The larvae are microscopic, but the adults can reach lengths of 20 m in the human intestine. Such large tapeworms can cause intestinal blockage and rob enough nutrients from the human host to cause nutritional deficiencies. An orally administered drug called niclosamide kills the adult worms.

Roundworms

Roundworms (phylum Nematoda) get their common name from their cylindrical body, which is usually tapered at both ends (Figure 17.16a).Roundworms are among the most diverse (in species number) and widespread of all animals. About 90,000 species of roundworms are known, and perhaps ten times that number actually exist. Round worms range in length from about a millimeter to a meter. They are found in most aquatic habitats, in wet soil, and as parasites in the body fluids and tissues of plants and animals. Free living roundworms in the soil are important decomposers. Other species are major agricultural pests that attack the roots of plants, Humans host at least 50 parasitic roundworm species, including pinworms, hookworms, and the parasite that causes trichinosis (Figure 17.16b).

Figure 17.14 Anatomy of a planarian. This free-living flatworm has a head with two light-detecting eye-spots and a flap at each side that detects certain chemicals in the water. Dense clusters of nervous tissue form a simple brain. The digestive tract is highly branched, providing an extensive surface area for the absorption of nutrients. When the animal feeds, a muscular tube projects through the mouth and sucks food in. The digestive tract, like that of cnidarians, is a gastrovascular cavity (a single opening functions as both mouth and anus). Planarians live on the undersurfaces of rocks in freshwater ponds and streams.

Figure 17.15 Anatomy of a tapeworm. Humans acquire larvae of these parasites by eating undercooked meat that is infected. The head of a tapeworm is armed with suckers and menacing hooks that lock the worm to the intestinal lining of the host. Behind the head is a long ribbon of units that are little more than sacs of sex organs. At the back of the worm, mature units containing thousands of eggs break off and leave the host's body with the feces.

Roundworms exhibit two evolutionary innovations not found in flat-worms. First, roundworms have a complete digestive tract, which is a digestive tube with two openings, a mouth and an anus. This anatomy contrasts with the digestive sac, or gastrovascular cavity, of cnidarians and flatworms, which uses a single opening as both mouth and anus. All the remaining animals in our survey of animal phyla have a complete digestive tract. A complete digestive tract can process food and absorb nutrients as a meal moves in one direction from one specialized digestive organ to the next. In humans, for example, the mouth, stomach, and intestines are examples of digestive organs. A second evolutionary innovation we see for the first time in roundworms is a body cavity, which in this case is a pseudo coelom (see Figure 17.8).

Mollusks

Snails and slugs, oysters and clams, and octopuses and squids are all mollusks (phylum Mollusca). Mollusks are soft-bodied animals, but most are protected by a hard shell. Slugs, squids, and octopuses have reduced shells, most of which are internal, or they have lost their she»s completely during their evolution. Many mollusks feed by using a strap-like rasping organ called a radula to scrape up food. Garden snails use their radulas like tiny saws to cut pieces out of leaves. Most of the 150,000 known species of mollusks are marine, though some inhabit fresh water, and there are land-dwelling mollusks in the form of snails and slugs.

Despite their apparent differences, all mollusks have a similar body plan (Figure 17.17) .The body has three main parts: a muscular foot, usually used for movement; a visceral mass containing most of the internal organs; and a fold of tissue called the mantle. The mantle drapes over the visceral mass and secretes the shell (if one is present).

The three major classes of mollusks are gastropods (including snails and slugs), bivalves (including clams and oysters), and cephalopods (including squids and octopuses). Most gastropods are protected by a single, spiraled shell into which the animal can retreat when threatened (Figure 17.18a). Many gastropods have a distinct head with eyes at the tips of tentacles (think of a garden snail). Bivalves, including numerous species of clams, oysters, mussels, and scallops, have shells divided into two halves hinged together (Figure 17.18b). Most bivalves are sedentary, living in sand or mud in marine and freshwater environments. They use their muscular foot for digging and anchoring. Cephalopods generally differ from gastropods and sedentary bivalves in being built for speed and agility. A few cephalopods have large, heavy shells, but in most the shell is small and internal (as in squids) or missing altogether (as in octopuses). Cephalopods are marine predators that use beaklike jaws and a radula to crush or rip prey apart. The cephalopod mouth is at the base of the foot, which is drawn out into several long tentacles for catching and holding prey (Figure 17.18c).

Figure 17.16 Roundworms. (a) This species has the classic roundworm shape: cylindrical with tapered ends. You can see the mouth at the end that is more blunt. Not visible is the anus at the other end of a complete digestive tract. This worm looks like it's wearing a corduroy coat, but the ridges actually indicate muscles that run the length of the body. (b) The disease called trichinosis is caused by these roundworms, encysted here in human muscle tissue. Humans acquire the parasite by eating undercooked pork or other meat that is infected. The worms then burrow into the human intestine and eventually travel to other parts of the body, encysting in muscles and other organs.

Annelids

Annelids (phylum Annelida) are worms with body segmentation, which is the division of the body along its length into a series of repeated segments that look like a set of fused rings. Look closely at an earthworm, an annelid you have all encountered, and you'll see why these creatures are also called segmented worms (Figure 17.19). In all, there are about 15,000 annelid species, ranging in length from less than 1 mm to the 3-m giant Australian earthworm. Annelids live in the sea, most freshwater habitats, and damp soil. The three main classes of annelids are the earthworms and their relatives, the polychaetes, and the leeches.

Figure 17.17 The general body plan of a mollusk. Note the body cavity (a true coelom, though a small one) and the complete digestive tract, with both mouth and anus.

Earthworms eat their way through the soil, extracting nutrients as the soil passes through the digestive tube (Figure 17.208). Undigested material, mixed with mucus secreted into the digestive tract, is eliminated as castings through the anus. Farmers value earthworms because the animals till the earth, and the castings improve the texture of the soil. Charles Darwin estimated that each acre of British farmland had about 50,000 earthworms that produced 18 tons of castings per year.

In contrast to earthworms, most polychaetes are marine, mainly crawling or burrowing in the seafloor. Segmental appendages with hard bristles help the worm wriggle about in search of small invertebrates to eat. The appendages als-o increase the animal's surface area for taking up oxygen and disposing of metabolic wastes, including carbon dioxide (Figure 17.20b ).

The third group of annelids, leeches, are notorious for the bloodsucking habits of some species. However, most species are free-living carnivores that eat small invertebrates such as snails and ins~cts. The majority of leeches live in fresh water, but a few terrestrial species inhabit moist vegetation in the tropics. Until the twentieth century, bloodsucking leeches were frequently used by physicians for bloodletting, the removal of what was considered "bad blood" from sick patients. Some leeches have razorlike jaws that cut through the skin, and they secrete saliva containing a strong anesthetic and an anticoagulant into the wound. The anesthetic makes the bite virtually painless, and the anticoagulant keeps the blood from clotting. Leech anticoagulant is now being produced commercially by genetic engineering and may find wide use in human medicine. Tests show that it prevents blood clots that can cause heart attacks. Leeches are also still occasionally used to remove blood from bruised tissues and to help relieve swelling in fingers or toes that have been sewn back on after accidents (Figure 17.20c). Blood tends to accumulate and cause swelling in a reattached finger or toe until small veins have a chance to grow back int9 it. Leeches are applied to remove the excess blood.

Arthropods

Arthropods (phylum Arthropoda) are named for their Jointed appendages. {such as crabs and lobsters), arachnids (such as spiders and scorpions), and insects are all examples of arthropods. Zoologists estimate that the total arthropod population of Earth numbers about a billion billion (10¹⁸) individuals. Researchers have identified about a million arthropod species, mostly insects. In fact, two out of every three species of life that have been described are arthropods. And arthropods are represented in nearlyall habitats of the biosphere. On the criteria of species diversity, distribution, and sheer numbers, arthropods must be regarded as the most successful of all animal phyla.

Figure 17.18 Mollusks. (a) Shell collectors are delighted by the variety of gastropods. (b) This scallop, a bivalve, has many eyes peering out between the two halves of the hinged shell. (c) An octopus is a cephalopod without a shell. All cephalopods have large brains and sophisticated sense organs, which contribute to the success of these animals as mobile predators. This octopus lives on the seafloor, where it scurries about in search of crabs and other food. Its brain is larger and more complex, proportionate to body size, than that of any other invertebrate. Octopuses are very intelligent and have shown remarkable learning abilities in laboratory experiments.

General Characteristics of Arthropods

Arthopods are segmented animals. In contrast to the matching segments of annelids, however, arthropod segments and their appendages have become specialized for a great variety of functions. This evolutionary flexibility contributed to the great diversification of arthropods. Specialization of segments, or fused groups of segments, also provides for an efficient division of labor among body regions. For example, the appendages of different segments are variously modified (for walking, feeding, sensory reception, copulation, and defense (Figure 17.21).

Figure 17.19 Segmented anatomy of an earthworm. Annelids are segmented both externally and internally. Many of the internal structures are repeated, segment by segment. The coelom (body cavity) is partitioned by walls (only two segment walls are fully shown here). The nervous system (yellow) includes a nerve cord with a cluster of nerve cells in each segment. Excretory organs (green), which dispose of fluid wastes, are also repeated in each segment. The digestive tract, however, is not segmented; it passes through the segment walls from the mouth to the anus. Segmental blood vessels connect continuous vessels that run along the top (dorsallocation) and bottom (ventral location) of the worm. The segmental vessels include five pairs of accessory hearts. The main heart is simply an enlarged region of the dorsal blood vessel near the head end of the worm.

Figure 17.21 Arthropod characteristics of a lobster. The whole body, including the appendages, is covered by an exoskeleton. The two distinct regions of the body are the cephalothorax (consisting of the head and thorax) and the abdomen. The head bears a pair of eyes, each situated on a movable stalk. The body is segmented, but this characteristic is only obvious in the abdomen. The animal has a tool kit of specialized appendages, including pincers, walking legs, swimming appendages, and two pairs of sensory antennae. Even the multiple mouthparts are modified legs, which is why they work form side to side rather than up and down (as our jaws do).

The body of an arthropod is completely covered by an exoskeleton (external skeleton). This coat is constructed from layers of protein and a polysaccharide called chitin. The exoskeleton can be a thick, hard armor over some parts, of the body yet paper-thin and flexible in other locations, such as the joints. The exoskeleton protects the animal and provides points of attachment for the muscles that move the appendages. There are, of course, advantages to wearing hard parts on the outside. Our own skeleton is interior to most of our soft tissues, an arrangement that doesn't provide much protection from injury. But our skeleton does offer the advantage of being able to grow along with the rest of our body. In contrast, a growing arthropod must occasionally shed its old exoskeleton and secrete a larger one. This process, called molting, leaves the animal temporarily vulnerable to predators and other dangers.

Figure 17.20 Annelids. (a) Giant Australian earthworms are bigger than most snakes. Perhaps you've slipped on slimy worms, but imagine actually tripping over one. (b) Polychaetes have segmental appendages that function in movement and as gills. On the left is a sandworm. The beautiful polychaete on the right is an example of a fan worm, which lives in a tube it constructs by mixing mucus with bits of sand and broken shells. Fan worms use their feathery head-dresses as gills and to extract food particles from the seawater. This species is called a Christmas tree worm. (c) A nurse applied this medicinal leech (Hirudo medicinalisl to a patient's sore thumb to drain blood from a hematoma (abnormal accumulation of blood around an internal injury).

Arthropod Diversity The four main groups of arthropods are the arachnids, the crustaceans, the millipedes and centipedes, and the insects. Most arachnids live on land. Scorpions, spiders, ticks, and mites are examples (Figure 17.22). Arachnids usually have four pairs of walking legs and a specialized pair of feeding appendages. In spiders, these feeding appendages are fanglike and equipped with poison glands. As a spider uses these appendages to immobilize and chew its prey, it spills digestive juices onto the torn tissues and sucks up its liquid meal.

Crustaceans are nearly all aquatic. Crabs, lobsters, crayfish, shrimps, and barnacles are all crustaceans (Figure 17.23). They all exhibit the crustacean hallmark of multiple pairs of specialized appendages (see Figure 17.21). One group of crustaceans, the isopods, is represented on land by pill bugs, which you have probably found on the undersides of moist leaves and other organic debris.

Figure 17.22 Arachnids. (a) Scorpions are nocturnal hunters. Their ancestors were among the first terrestrial carnivores, preying on herbivorous arthropods that fed on the early land plants. Scorpions have a pair of appendages modified as large pincers that function in defense and food capture. The tip of the tail bears a poisonous stinger. Scorpions eat mainly insects and spiders. They will sting people only when prodded or stepped on. (If you camp in the desert and leave your shoes on the ground when you go to bed, make sure there are no scorpions in those shoes before putting them on in the morning.) (b) Spiders are usually most active during the daytime, hunting insects or trapping them in webs. Spiders spin their webs of liquid silk, which solidifies as it comes out of specialized glands. Each spider engineers a style of web that is characteristic of its species, getting the web right on the very first try. Besides building their webs of silk, spiders use the fibers in many other ways: as droplines for rapid escape; as cloth that covers eggs; and even as "gift wrapping" for food that certain male spiders offer to seduce females. (c) This magnified house dust mite is a ubiquitous scavenger in our homes. Each square inch of carpet and every one of those dust balls under a bed s are like cities to thousands of dust mites. Unlike some mites that carry pathogenic bacteria, dust mites are harmless except to people who are allergic to the mites' feces.

(a) A shrimp (b) Barnacles Figure 17.24 A millipede

Figure 17.23 Crustaceans. (a) A grass shrimp. (b) Easily confused with bivalve mollusks, barnacles are actually sessile crustaceans with exoskeletons hardened into shells by calcium carbonate (lime). The jointed appendages projecting from the shell capture small plankton.

Millipedes and centipedes have similar segments over most of the body and superficially resemble annelids, but their jointed legs give them away as arthropods. Millipedes are landlubbers that eat decaying plant matter (Figure 17.24) .They have two pairs of short legs per body segment. Centipedes are terrestrial carnivores, with a pair of poison claws used in defense and to paralyze prey, such as cockroaches and flies. Each of their body segments bears a single pair of long legs.

In species diversity, insects outnumber all other forms of life combined. They live in almost every terrestrial habitat and in fresh water, and flying insects fill the air. Insects are rare, though not absent, in the seas, where crustaceans are the dominant arthropods. There is a whole big branch of biology, called entomology. that specializes in the study of insects.

The oldest insect fossils date back to about 400 million years ago, during the Paleozoic era (see Table 14.1). Later, the evolution of flight sparked an explosion in insect variety (Figure 17.25). Like the grasshopper in Figure 17.25, most insects have a three-part body: head, thorax, and abdomen. The head usually bears a pair of sensory antennae and a pair of eyes. Several pairs of mouthparts are adapted for particular kinds of eating-for example, for biting and chewing-plant material in grasshoppers; for lapping up fluids in houseflies; and for piercing skin and sucking blood in ! mosquitoes and other biting flies. Most adult insects have three pairs of legs and one or two pairs of wings, all borne on the thorax.

Flight is obviously one key to the great success of insects. An animal that can fly can escape many predators, find food and mates, and disperse to new habitats much faster than an animal that must crawl about on the ground. Because their wings are extensions of the exoskeleton and not true appendages, insects can fly without sacrificing any walking legs. By contrast, the flying vertebrates-birds and bats-have one of their two pairs of wa1king legs modified for wings, which explains why these vertebrates are generally not very swift on the ground.

Figure 17.25 A small sample of insect diversity

Figure 17.26 Metamorphosis of a monarch butterfly. (a) The larva (caterpillar) spends its time eating and growing, molting as it grows. (b) After several molts, the larva encases itself in a cocoon and becomes a pupa. (c) Within the pupa, the larval organs break down and adult organs develop from cells that were dormant in the larva. (d) Finally, the adult emerges from the cocoon. (e) The butterfly flies off and reproduces, nourished mainly from the calories it stored when it was a caterpillar.

Many insects undergo metamorphosis in their development. In the case of grasshoppers and some other insect groups, the young resemble adults but are smaller and have different body proportions. The animal goes through a series of molts, each time looking more like an adult, until it reaches full size. In other cases, insects have distinctive larval stages specialized for eating and growing that are known by such names as maggots, grubs, or caterpillars. The larval stage looks entirely different from the adult stage, which is specialized for dispersal and reproduction. Metamorphosis from the larva to the adult occurs during a pupal stage (Figure 17.26).

Animals so numerous, diverse, and widespread as insects are bound to affect the lives of all other terrestrial organisms, including humans. On one hand, we depend on bees, flies, and many other insects to pollinate our crops and orchards. On the other hand, insects are carriers of the micro-organisms that cause many diseases, including malaria and African sleeping sickness. Insects also compete with humans for food. In parts of Africa, for instance, insects claim about 75% of the crops. Trying to minimize their losses, farmers in the United States spend billions of dollars each year on pesticides, spraying crops with massive doses of some of the deadliest poisons ever invented. Try as they may, not even humans have challenged the preeminence of insects and their arthropod kin. As Cornell University's Thomas Eisner puts it: "Bugs are not going to inherit the Earth. They own it now. So we might as well make peace with the landlord:

Echinoderms

The echinoderms .(phylum Echinodermata) are named for their spiny surfaces. Sea urchins, the porcupines of the invertebrates, are certainly echinoderms that live up to the phylum name. Among the other echmoderms are sea stars, sand dollars, and sea cucumbers (Figure 17.27). Echinoderms are all marine. Most are sessile or slow moving. Echinoderms lack body segments, and most have radial symmetry as adults. Both the external and the internal parts of a sea star, for instance, radiate from the center like spokes of a wheel. In contrast to the adult, the larval stage of echinoderms is bilaterally symmetrical. This supports other evidence that echinoderms are not closely related to cnidarians or other radial animals that never show bilateral symmetry. Most echinoderms have an endoskeleton (interior skeleton) constructed from hard plates just beneath the skin. Bumps and spines of this endoskeleton account for the animal's rough or prickly surface. Unique to echinoderms is the water vascular system, a network of water-filled canals that circulate water throughout the echinoderm's body, facilitating gas exchange and waste disposal. The water vascular system also branches into extensions called tube feet. A sea star or sea urchin pulls itself slowly over the seafloor using its suction- cup-like tube feet. Sea stars also use their tube feet to grip prey during feeding (see Figure 17.27a).

Looking at sea stars and other adult echinoderms, you may think they .have little in common with humans and other vertebrates. But if you return to Figure 17.6, you'll see that echinoderms share an evolutionary branch with chordates, the phylum that includes vertebrates. Analysis of embryonic development reveals this relationship. The mechanism of coelom formation and many other details of embryology differentiate the echinoderms and chordates from the evolutionary branch that includes mollusks, annelids, and arthropods. With this phylogenetic context, we're now ready to make the transition from invertebrates to vertebrates.

Figure 17.27 Echinoderms. (a) The mouth of a sea star, not visible here, is located in the center of the undersurface. The inset shows how the tube feet function in feeding. When a sea star encounters an oyster or clam, its favorite foods, it grips the mollusk's shell with its tube feet and positions its mouth next to the narrow opening between the two halves of the prey's shell. The sea star then pushes its stomach out through its mouth and through the crack in the mollusk's shell. The predator then digests the soft tissue of its prey. (b) In contrast to sea stars, sea urchins are spherical and have no arms. If you look closely, you can see the long tube feet projecting among the spines. Unlike sea stars, which are mostly carnivorous, sea urchins mainly graze on seaweed and other algae. (c) On casual inspection, sea cucumbers do not look much like other echinoderms. Sea cucumbers lack spines, and the hard endoskeleton is much reduced. However, closer inspection reveals many echinoderm traits, including five rows of tube feet.

The vertebrate Genealogy

Most of us are curious about our genealogy. On the personal level, we wonder about our family ancestry. As biology students, we are interested in tracing human ancestry within the broader scope of the entire animal kingdom. In this quest, we ask three questions: What were our ancestors like? How are we related to other animals? and What are our closest relatives?

Figure 17.28 Backbone, extra long. Vertebrates are named for their backbone, which consists of a series of vertebrae. The vertebrate hallmark is apparent in this snake skeleton. You can also see the skull, the bony case protecting the brain. The backbone and skull are parts of an endoskeleton, a skeleton inside the animal rather than covering it.

Figure 17.29 Invertebrate chordates. (a) lancelets owe their name to their bladelike shape. Marine animals only a few centimeters long, lancelets wiggle backward into sand, leaving only their head exposed. The animal filters tiny food particles from the seawater. (b) This adult tunicate, or sea squirt, is a sessile filter feeder that bears little resemblance to other chordates. However, a tunicate goes through a larval stage that is unmistakably chordate.

Figure 17.30 Cordate characteristics

In this section, we trace the evolution of the vertebrates, the group that includes humans and their closest relatives. Mammals, birds, reptiles, amphibians, and the various classes of fishes are all classified as vertebrates. Among the unique vertebrate features are the cranium and backbone, a series of vertebrae for which the group is named (Figure 17.28). Our first step in tracing the vertebrate genealogy is to determine where vertebrates fit in the animal kingdom.

Characteristics of Chordates

Vertebrates make up one subphylum of the phylum Chordata. Our phylum also includes two subphyla of invertebrates, animals lacking a backbone: lancelets and tunicates (Figure 17.29). These invertebrate chordates and vertebrates all share four key features that appear in the embryo and sometimes in the adult. These four chordate hallmarks are ( 1) a dorsal, hollow nerve cord ( the chordate brain and spinal cord); (2) a notochord, which is a flexible, longitudinal rod located between the digestive tract and the nerve cord; ( 3) pharyngeal slits, which are gill structures in the pharynx, the region of the digestive tube just behind the mouth; and ( 4) a post -anal tail, which is a tail to the rear of the anus (Figure 17.30). Though these chordate characteristics are often difficult to recognize in the adult animal, they are always present in chordate embryos. For example, the notochord, for which our phylum is named, persists in adult humans only in the form of the cartilage disks that function as cushions between the vertebrae. (Back injuries described as "ruptured disks" or "slipped disks" refer to these structures. )

Body segmentation is another chordate characteristic, though not a unique one. The chordate version of segmentation probably evolved independently of the segmentation we observe in annelids and arthropods. Chordate segmentation is apparent in the backbone of vertebrates (see Figure 17.28) and in the segmental muscles of all chordates (see the chevron-shaped- »» -muscles in the lancelet of Figure 17.29a). Segmental musculature is not so obvious in adult humans unless one is motivated enough to sculpture those washboard "abs of steel."

Vertebrates retain the basic chordate characteristics, but have additional features that are unique, including, of course, the backbone (see Figure 17.28). Figure 17.31 is an overview of chordate and vertebrate evolution that will provide a context for our survey of the vertebrate classes.

Fishes

The first vertebrates probably evolved during the early Cambrian period about 540 million years ago. These early vertebrates, the agnathans, lacked jaws. Agnathans are represented today by vertebrates called lampreys (see Figure 17.31). Some lampreys are parasites that use their jawless mouths as suckers to attach to the sides of large fishes and draw blood. In contrast, most vertebrates have jaws, which are hinged skeletons that work the mouth. We know from the fossil record that the first jawed vertebrates

were fishes that replaced most agnathans by about 400 million years ago. In addition to jaws, these fishes had two pairs of fins, which made them maneuverable swimmers. Some of those fishes were more than 10 m long. Sporting jaws and fins, some of the early fishes were active predators that could chase prey and bite off chunks of flesh. Even today, most fishes are carnivores. The two major groups of living fishes are the class Chondrichthyes ( cartilaginous fishes-the sharks and rays) and the class Osteichthyes (the bony fishes, including such familiar groups as tuna, trout, and goldfish).

Cartilaginous fishes have a flexible skeleton made of cartilage. Most sharks are adept predators-fast swimmers with streamlined bodies, acute senses, and powerful jaws (Figure 17.]2a). A shark does not have keen eyesight, but its sense of smell is very sharp. In addition, special electrosensors on the head can detect minute electrical fields produced by muscle contractions in nearby animals. Sharks also have a lateral line system. a row of sensory organs running along each side of the body. Sensitive to changes in water pressure, the lateral line system enables a shark to detect minor vibrations caused by animals swimming in its neighborhood. There are fewer than 1,000 living species of cartilaginous fishes, nearly all of them marine.

Bony fishes (Figure 17.]2b) have a skeleton reinforced by hard calcium salts. They also have a lateral line system, a keen sense of smell, and excellent eyesight. On each side of the head, a protective flap called the operculwn (plural, opercula) covers a chamber housing the gills. Movement of the operculum allows the fish to breathe without swimming. (By contrast, sharks lack opercula and must swim to pass water over their gills. ) Bony fishes also have a specialized organ that helps keep them buoyant-the swim bladder. a gasfilled sac. Thus, many bony fishes can conserve energy by remaining almost motionless, in contrast to sharks, which sink to the bottom if they stop swimming. Some bony fishes have a connection between the swim bladder and the digestive tract that enables them to gulp air and extract oxygen from it when the dissolved oxygen level in the water gets too low. In fact, swim bladders evolved from simple lungs that augmented gills in absorbing oxygen from the water of stagnant swamps, where the first bony fishes lived.

Figure 17.32 Two classes of fishes. (a) A member of the class Chondrichthyes, the cartilaginous (b) A member of the class Osteichthyes, the bony fishes.

The largest class of vertebrates { about 30,000 species ), bony fishes are common in the seas and in freshwater habitats. Most bony fishes, including trout, bass, perch, and tuna, are ray-finned fishes. Their fins are supported by thin, flexible skeletal rays (see Figure 17 .32b). A second evolutionary branch of bony fishes includes the lungfishes and lobe-finned fishes. Lung-fishes live today in the Southern Hemisphere. They inhabit stagnant ponds and swamps, surfacing to gulp air into their lungs. The lobe-fins are named for their muscular fins supported by stout bones. Lobe-fins are extinct except for one species, a deep-sea dweller that may use its fins to waddle along the seafloor. Ancient freshwater lobe-finned fishes with lungs played a key role in the evolution of amphibians, the first terrestrial vertebrates.

Amphibians

In Greek, the, word amphibios means "living a double life. Most members of the class Amphibia exhibit a mixture of aquatic and terrestrial adaptations. Most species are tied to water because their eggs, lacking shells, dry out quickly in the air; The frog in Figure 17.33 spends much of its time on land, but it lays its eggs in water. An egg develops into a larva called a tadpole, a legless, aquatic algae-eater with gills, a lateral line system resembling that of fishes, and a long finned tail. In changing into a frog, the tadpole undergoes a radical metamorphosis. When a young frog crawls onto shore and begins life as a terrestrial insect -eater, it has four legs, air-breathing lungs instead of gills, a pair of external eardrums, and no lateral line system. Because of metamorphosis, many amphibians truly live a double life. But even as adults, amphibians are most abundant in damp habitats, such as swamps and rain forests. This is partly because amphibians depend on their moist skin to supplement lung function in exchanging gases with the environment. Thus, even those frogs that are adapted to relatively dry habitats spend much of their time in humid burrows or under piles of moist leaves. The amphibians of today, including frogs and salamanders, account for only about 8% of all living vertebrates, or about 4,000 species.

Figure 17.33 The uduallifeu of an amphibian. Though not all amphibians have aquatic larval stages. this class of vertebrates is named for the familiar tadpole-to-frog metamorphosis of many species

Figure 17.34 The origin of tetrapods. Fossils of some lobe-finned fishes have skeletal supports extending into their fins Early amphibians left fossilized limb skeletons that probably functioned in movement on land

Amphibians were the first vertebrates to colonize land. They descended from fishes that liad lungs and fins with muscles and skeletal supports strong enough to enable some movement, however clumsy, on land (Figure 17.34). The fossil record chronicles the evolution of four-limbed amphibians from fishlike ancestors. Terrestrial vertebrates-amphibians, reptiles, birds, and mammals-are collectively called tetrapods, which means "four legs:' Had our amphibian ancestors had three pairs of legs on their undersides instead of just two, we might be hexapods. This image seems silly, but serves to reinforce the point that evolution, as descent with modification, is constrained by history.

Reptiles

Class Reptilia includes snakes, lizards, turtles, crocodiles, and alligators. The evolution of reptiles from an amphibian ancestor paralleled many additional adaptations for living on land. Scales containing a protein called keratin waterproof the skin of a reptile, helping to prevent dehydration in dry air. Reptiles cannot breathe through their dry skin and obtain most of their oxygen with their lungs. Another breakthrough for living on land that evolved in reptiles is the amniotic egg, a water-containing egg enclosed in a shell (Figure 17.35). The amniotic egg functions as a "self -contained pond" that enables reptiles to complete their life cycle on land. With adaptations such as waterproof skin and amniotic eggs, reptiles broke their ancestral ties to aquatic habitats. There are about 6,500 species of reptiles alive today.

Reptiles are sometimes labeled "cold-blooded" animals because they do not use their metabolism extensively to control body temperature. But reptiles do regulate body temperature through behavioral adaptations. For example, many lizards regulate their internal temperature by basking in the sun when the air is cool and seeking shade when the air is too warm. Because they absorb external heat rather than generating much of their own, reptiles are said to be ectotherms, a term more accurate than cold-blooded. By heating directly with solar energy rather than through the metabolic breakdown of food, a reptile can survive on less than 10% of the calories required by a mammal of equivalent size.

As successful as reptiles are today, they were far more widespread, numerous, and diverse during the Mesozoic era, which ,is sometimes called the "age of reptiles:' Reptiles diversified extensively during that era, producing a dynasty that lasted until the end of the Mesozoic, about 65 million years ago. Dinosaurs, the most diverse group, included the largest animals ever to inhabit land. Some were gentle giants that lumbered about while browsing vegetation. Others were voracious carnivores that chased their larger prey on two legs (Figure 17.36).

Figure 17.35 Terrestrial equipment of reptiles. This bull snake displays two reptilian adaptations to living on land: a waterproof skin with keratinized scales; and amniotic eggs, with shells that protect a watery, nutritious internal environment where the embryo can develop on land. Snakes evolved from lizards that adapted to a burrowing lifestyle.

Figure 17.36 A Mesozoic feeding frenzy. Hunting in packs, Deinonychus (meaning "terrible claw") probably used its sickle-shaped claws to slash at larger prey.

Figure 17.37 A bald eagle in flight. Bird wings are airfoils, which have shapes that create lift by altering air currents. Air passing over a wing must travel farther in the same amount of time than air passing under the wing. This expands the air above the wing relative to the-air below the wing. And this makes the air pressure pushing upward against the lower wing surface greater than the pressure of the expanded air pushing downward on the wing. The wings of birds and airplanes owe their "lift" to this pressure differential.

The age of reptiles began to wane about 70 million years ago. During the Cretaceous, the last period of the Mesozoic era, global climate became cooler and more variable. This was a period of mass extinctions that claimed all the dinosaurs by about 65 million years ago, except for one lineage. That lone surviving lineage is represented today by birds.

Birds

Birds (class Aves)evolved during the great reptilian radiation of the Mesozoic era. Amniotic eggs and scales on the legs are just two of the reptilian features we see in birds. But modern birds look quite different from modern reptiles because of their feathers and other distinctive flight equipment. Almost all of the 8,600 living bird species are airborne. The few flightless species, including the ostrich, evolved from flying ancestors. Appreciating the avian world is all about understanding flight.

Almost every element of bird anatomy is modified in some way that enhances flight. The bones have a honeycombed structure that makes them strong but light ( the wings of airplanes have the same basic construction). For example, a huge seagoing species called the frigate bird has a wingspan of more than 2 m, but its whole skeleton weighs only about 113 g (4 ounces). Another adaptation that reduces the weight of birds is the absence of some internal organs found in other vertebrates. Female birds, for instance, have only one ovary instead of a pair. Also, modern birds are toothless, an adaptation that trims the weight of the head (no uncontrolled nosedives). Birds do not chew food in the mouth but grind it in the gizzard, a chamber of the digestive tract near the stomach.

Flying requires a great expenditure of energy and an active metabolism. In contrast to the ectothermic reptiles, birds are endotherms. That means they use their own metabolic heat to maintain a warm, constant body temperature. A bird's most obvious flight equipment is its wings. Bird wings are air-foils that illustrate the same principles of aerodynamics as the wings of an airplane (Figure 17.37). A bird's flight motors are its powerful breast muscles, which are anchored to a keel-like breastbone. It is mainly these flight muscles that we call "white meat" on a turkey or chicken. Some birds, such as eagles and hawks, have wings adapted for soaring on air currents and flap their wings only occasionally. Other birds, including hummingbirds, excel at maneuvering but must flap continuously to stay aloft. In either case, it is the shape and arrangement of the feathers that form the wing into an air-foil. Feathers are made of keratin, the same protein that forms our hair and fingernails as well as the scales of reptiles. Feathers may have functioned first as insulation, helping birds retain body heat, only later being co-opted as flight gear.

In tracing the ancestry of birds back to the Mesozoic era, we must search for the oldest fossils with feathered wings that could have the functioned in flight. Fossils of an ancient bird named Archaeopteryx have been found in Bavarian limestone in Germany and date back some 150 million years into the dinosaurs, birds, and Jurassic period (see Figure 14.15). Archaeopteryx is not considered the ancestor of modern birds, and paleontologists place it on a side branch of the avian linage Nonetheless, Archaeopteryx probably was derived from ancestral forms that also gave rise to modern birds. Its skeletal anatomy indicates that it was a weak flyer, perhaps mainly a tree-dwelling glider. A combination of gliding downward and jumping into the air from the ground may have been the earliest mode of flying in the bird lineage.

Figure 17.38 The three major groups of mammals. (a) Monotremes, such as this echidna, are the only mammals that lay eggs (inset). (b) The young of marsupials, such as this brushtail opossum, are born very early in their development. They finish their growth while nursing from a nipple in their mother's pouch. (c) In eutherians (placentals), such as these zebras, young develop within the uterus of the mother. There they are nurtured by the flow of blood though the dense network of vessels in the placenta. The reddish portion of the afterbirth clinging to the newborn zebra in this photograph is the placenta. (see prior)

Mammals

Mammals (class Mammalia) evolved from reptiles about 225 million years ago, long before there were any dinosaurs. During the peak of the age of reptiles, there were a number of mouse-sized, nocturnal mammals that lived on a diet of insects. Mammals became much more diverse after the downfall of the dinosaurs. Most mammals are terrestrial. However, there are nearly 1,000 species of winged mammals, the bats. And about 80 species of dolphins, porpoises, and whales are totally aquatic. The blue whale, an endangered species that grows to lengths of nearly 30 m, is the largest animal that has ever existed.

Two features-hair and mammary glands that produce milk that nourishes the young-are mammalian hallmarks. The main function of hair is to insulate the body and help maintain a warm, constant internal temperature; mammals, like birds, are endotherms. There are three major groups of mammals: the monotremes, the marsupials, and theeutherians. The duck-billed platypus is one of only threeexisting species of monotremes, the egg-laying mammals. The platypus lives along rivers in eastern Australia and on the nearby island of Tasmania. It eats mainly small shrimps and aquatic insects. The female usually lays two eggs and incubates them in a leaf nest. After hatching, the young nurse by licking up milk secreted onto the mother's fur. The animals called echidnas are also monotremes (Figure 17.38a).

Most mammals are born rather than hatched. During gestation in marsupials and eutherians, the embryos are nurtured inside the mother by an organ called the placenta. Consisting of both embryonic and maternal tissues, the placenta joins the embryo to the mother within the uterus. The embryo is nurtured by maternal blood that flows close to the embryonic blood system in the placenta. The embryo is bathed in fluid contained by an amniotic sac, which is homologous to the fluid compartment within the amniotic eggs of reptiles.

Marsupials are the so-called pouched mammals, including kangaroos and koalas. These mammals have a brief gestation and give birth to tiny, embryonic offspring that complete development while attached to the mother's nipples. The nursing young are usually housed in an external pouch, called the marsupium, on the mother's abdomen (Figure 17.38b ). Nearly all marsupials live in Australia, New Zealand, and Central and South America. Australia has been a marsupial sanctuary for much of the past 60 million years. Australian marsupials have diversified extensively, filling terrestrial habitats that on other continents are occupied by eutherian mammals.

Eutherians are also called placental mammals because their placentas provide more intimate and long-lasting association between the mother and her developing young than do marsupial placentas (Figure 17.38c ) .Eutherians make up almost 95% of the 4,500 species of living mammals. Dogs, cats, cows, rodents, rabbits, bats, and whales are all examples of eutherian mammals. One of the eutherian groups is the order Primates, includes monkeys, apes, and humans.

Figure 17.39 The arboreal athleticism of primates. This orangutan displays primate adaptations for living in the trees: limber shoulder joints, manual dexterity, and stereo vision due to eyes on the front of the face.

The Human Ancestry

We have now traced the animal genealogy to the mammalian group that includes Homo sapiens and its closest kin. We are primates. To understand .what that means, we must trace our ancestry back to the trees, where some of our most treasured traits originated as arboreal adaptations.

The Evolution of Primates

Primate evolution provides a context for understanding human origins. The fossil record supports the hypothesis that primates evolved from insect-eating mammals during the late Cretaceous period, about 65 million years ago. Those early primates were small, arboreal { tree-dwelling) mammals. Thus, the order Primates was first distinguished by characteristics that were shaped, through natural selection, by the demands of living in the trees. For example, primates have limber shoulder joints, which make it possible to brachiate {swing from one branch to another). The dexterous hands of primates can hang on to branches and manipulate food. Nails have replaced claws in many primate species, and the fingers are very sensitive. The eyes of primates are close together on the front of the face. The overlapping fields of vision of the two eyes enhance depth perception, an obvious advantage when brachiating. Excellent eye-hand coordination is also important for arboreal maneuvering {Figure 17.39). Parental care is essential for young animals in the trees. Mammals devote more energy to caring for their young than most other vertebrates, and primates are among the most attentive parents of all mammals. Most primates have single births and nurture their offspring for a long time. Though humans do not live in trees, we retain in modified form many of the traits that originated there.

Taxonomists divide the primates into twp main groups: prosimians and anthropoids. The oldest primate fossils are prosimians. Modern prosimians include the lemurs of Madagascar and the lorises, pottos, and tarsiers that live in tropical Africa and southern Asia (Figure 17.40a). Anthropoids include monkeys, apes, and humans. All monkeys in the New World ( the Americas) are arboreal and are distinguished by prehensile tails that function as an extra appendage for brachiating (Figure 17.40b). (If you see a monkey in a zoo swinging by its tail, you know it's from the New World. ) Although some Old World monkeys are also arboreal, their tails are not prehensile. And many Old World monkeys, including baboons, macaques, and mandrills, are mainly ground-dwellers (Figure 17.40c).

Figure 17.40 Primate diversity (a) A prosimian. (b) – (c) Monkeys (d) – (g) Apes (h) human

Our closest anthropoid relatives are the apes: gibbons, orangutans, gorillas, and chimpanzees (Figure 17.40d-g). Modern apes live only in tropical regions of the Old World. With the exception of some gibbons, apes are larger than monkeys, with relatively long arms and short legs and no tail. Although all the apes are capable of brachiation, only gibbons and orangutans are primarily arboreal. Gorillas and chimpanzees are highly social. Apes have larger brains proportionate to body size than monkeys, an their behavior is consequently more adaptable.

The Emergence of Humankind

Humanity is one very young twig on the vertebrate branch, just one of many branches on the tree of life. In the continuum of life spanning 3.5 billion years, humans and apes have shared a common ancestry for all but the last 5-7 million years (Figure 17.41 ). Put another way, if we compressed the history of life to a year, humans and chimpanzees diverged from a common ancestor less than 18 hours ago. The fossil record and molecular systematics concur in that vintage for the human lineage. Paleoanthropology, the study of human evolution, focuses on this very thin slice of biological history. Some Common Misconceptions Certain misconceptions about human evolution that were generated during the early part of the twentieth century still persist today in the minds of many, long after these myths have been debunked by the fossil evidence.

Figure 17.41 Primary Phylogeny

Let's first dispose of the myth that our ancestors were chimpanzees or any other modern apes. Chimpanzees and humans represent two divergent branches of the anthropoid tree that evolved from a common, less specialized ancestor. Chimps are not our parent species, but more like our phylogenetic siblings or cousins.

Another misconception envisions human evolution as a ladder with a series of steps leading directly from an ancestral anthropoid to Homo sapiens. This is often illustrated as a parade of fossil hominids (members of the human family) becoming progressively more modern as they march across the page. If human evolution is a parade, then it is a disorderly one, with many splinter groups having traveled down dead ends. At times in hominid history, several different human species coexisted (Figure 17.42). Human phylogeny is more like a multibranched bush than a ladder, with our species being the tip of the only twig that still lives.

One more myth we must bury is the notion that various human characteristics, such as upright posture and an enlarged brain, evolved in unison. A popular image is of early humans as half-stooped, half-witted cave-dwellers. In fact, we know from the fossil record that different human features evolved at different rates, with erect posture, or bipedalism, leading the way. Our pedigree includes ancestors who walked upright but had ape-sized brains. After dismissing some of the folklore on human evolution, however, we must admit that many questions about our ancestry have not yet been

resolved.

Figure 17.42 A timeline of human evolution. Notice that there have been times when two or more hominid species coexisted. The skulls are all drawn to the same scale enable you to compare the sizes of craniums and hence brain

Figure 17.43 The antiquity of upright posture. (a) Lucy, a 3.18-million-year-old skeleton, represents the hominid species Australopithecus afarensis. Fragments of the pelvis and skull put A. afarensis on two feet. (b) Some 3.7 million years ago, several bipedal (upright-walking) hominids left footprints in damp volcanic ash in what is now Tanzania in East Africa. The prints fossilized and were discovered by British anthropologist Mary Leakey in 1978. The footprints are part of the strong evidence that bipedalism is a vefry old human trait. (c) This A afarensis skull, 3.9 million years old, articulated with a vertical back bone. The upright posture of humans is a least that old.

Australopithecus and the Antiquity of Bipedalism Before there was Homo, several hominid species of the genus Australopithecus walked the African savanna (grasslands with clumps of trees ). Paleoanthropologists have focused much of their attention on A. afarensis, an early species of Australopithecus. Fossil evidence now pushes bipedalism in A. afarensis back to at least 4 million years ago (Figure 17.43) .Doubt remains whether an even older hominid, Ardipithecus ramidus, which dates back at least 4.4 million years, was bipedal.

One of the most complete fossil skeletons of A. afarensis dates to about 3.2 million years ago in East Africa. Nicknamed Lucy by her discoverers, the individual was a female, only about 3 feet tall, with a head about the size of a softball (see Figure 17.43a). Lucy and her kind lived in savanna areas and may have subsisted on nuts and seeds, bird eggs, and whatever animals they could catch or scavenge from kills made by more efficient predators such as large cats and dogs.

All Australopithecus species were extinct by about 1.4 million years ago. Some of the later species overlapped in time with early species of our own genus, Homo ( see Figure 17.42 ) .Much debate centers on the evolutionary relationships of the Australopithecus species to each other and to Homo. Were the Australopithecus species all evolutionary side branches? Or were some of them ancestors to later humans? Either way, these early hominids show us that th~ fundamental human trait ofbipedalism evolved millions of years before the other major human trait-an enlarged brain. As evolutionary biologist Stephen Jay Gould put it, "Mankind stood up first and got smart later”.

Homo habilis and the Evolution of Inventive Minds Enlargement of the human brain is first evident in fossils from East Africa dating to the latter part of the era of Australopithecus, about 2.5 million years ago. Anthropologists have found skulls with brain capacities intermediate in size between those of the latest Australopithecus species and those of Homo sapiens. Simple handmade stone tools are sometimes found with the larger-brained fossils, which have been dubbed Homo habilis ("handy man"). After walking upright for about 2 million years, humans were finally beginning to use their manual dexterity and big brains to invent tools that enhanced their hunting, gathering, and scavenging on the African savanna.

Homo erectus and the Global Dispersal of Humanity The first species to extend humanity's range from its birthplace in Africa to other continents was Homo erectus, perhaps a descendant of H. habilis. The global dispersal began about 1.8 million years ago. But don't picture this migration as a mad dash for new territory or even as a casual stroll. If H. erectus simply expanded its range from Africa by about a mile per year, it would take only about 12,000 years to populate many regions of Asia and Europe. The gradual spread of humanity may have been associated with a change in diet to include a larger proportion of meat. In general, animals that hunt require more geographic territory than animals that feed mainly on vegetation.

Homo erectus was taller than H. habilis and had a larger brain capacity. During the 1.5 million years the species existed, the H. erectus brain increased to as large as 1,200 cubic centimeters ( cm3), a brain capacity that overlaps the normal range for modern humans. Intelligence enabled humans to continue succeeding in Africa and also to survive in the colder climates of the north. Homo erectus resided in huts or caves, built fires, made clothes from animal skins, and designed stone tools that were more refined than the tools of H. habilis. In anatomical and physiological adaptations, H. erectus was poorly equipped for life outside the tropics but made up for the deficiencies with cleverness and social cooperation.

Some African, Asian, European, and Australasian ( from Indonesia, New Guinea, and Australia) populations of H. erectus gave rise to regionally diverse descendants that had even larger brains. Among these descendants of H. erectus were the Neanderthals, who lived in Europe, the Middle East, and parts of Asia from about 130,000 years ago to about 35,000 years ago. (They are named Neanderthals because their fossils were first found in the Neander Valley of Germany. ) Compared with us, Neanderthals had slightly heavier browridges and less pronounced chins, but their brains, on average, were slightly larger than ours. Neanderthals were skilled toolmakers, and they participated in burials and other rituals that required abstract thought. Much current research on the Neanderthal skull addresses an intriguing question: Did Neanderthals have the anatomical equipment necessary for speech?

The Origin of Homo sapiens The oldest known post-H. erectus fossils, dating back over 300,000 years, are found in Africa. Many paleoanthropologists group these African fossils along with Neanderthals and various Asian and Australasian fossils as the earliest forms of our species, Homo sapiens. These regionally diverse descendants of H. erectus are sometimes referred to as "archaic Homo sapiens." The oldest fully modern fossils of H. sapiens-skulls and human evolution. other bones that look essentially like those of today's humans-are about 100,000 years old and are located in Africa. Similar fossils almost as ancient have also been discovered in caves in Israel. The famous fossils of modern humans from the Cro-Magnon caves of France date back about 35,000 years.

The relationship of the Cro-Magnons and other modern humans to archaic Homo sapiens is a question that will continue to engage paleoanthropologists for decades. What was the fate of the various descendants of Homo erectus who populated different parts of the world! In the view of some anthropologists, these archaic H. sapiens populations gave rise to modern humans. According to this hypothesis, called the multiregional hypothesis, modern humans evolved simultaneously in different parts of the world (Figure 17.44a). If this view is correct, then the geographic diversity of humans originated relatively early, when H. erectus spread from Africa into the other continents between 1 and 2 million years ago. This hypothesis accounts for the great genetic similarity of all modern people by pointing out that interbreeding among neighboring populations has always provided corridors for gene flow throughout the entire geographic range of humanity.

In sharp contrast to the multiregional hypothesis is a hypothesis that modern H. sapiens arose from a single ardlaic group in Africa. According to this "Out of Africa" hypothesis ( also called the replacement hypothesis) the Neanderthals and other archaic peoples outside Africa were evolutionary dead ends (Figure 17.44b). Proponents of this hypothesis argue that modern humans spread out of Africa about 100,000 years ago and completely replaced the archaic H. sapiens in other regions. So far, genetic evidence mostly; supports the replacement hypothesis. Based on comparisons of mitochondrial DNA (mtDNA) and of Y chromosomes between samples from various human populations, today's global human population seems to be very genetically uniform. Supporters of the replacement hypothesis argue that such uniformity could only stem from a single ancestral stock, not from diverse, geographically isolated populations. Proponents of the multiregional model counter that interbreeding between populations can explain our great genetic similarity. Debate about which hypothesis is correct-a multiregional or an "Out of Africa" origin of modem humans--centers mainly on differences in interpreting the fossil record and molecular data. As research continues, perhaps anthropologists will come closer to a consensus on our origins.

Figure 17.44 "Cut of Africa"-but when? Two hypotheses for the origin of modern Homo

sapiens. There is no question that Africa is the cradle of humanity; we can all trace our ancestry back to that continent. But how recently was the ancestor common to all the world's modern humans still in Africa? (a) According to the multiregional hypothesis, modern humans throughout the world evolved from the Homo erectus populations that spread to different regions beginning almost 2 million years ago. Interbreeding between populations (dashed line in the diagram) kept humans very similar genetically. (b) In contrast, the "Out of Africa" hypothesis postulates that all regional descendants of H. erectus became extinct except in Africa. And according to this model, it was a second dispersal out of Africa, just 100,000 years ago, that populated the world with modern humans, which had evolved from H. erectus in Africa.

Cultural Evolution An erect stance was the most radical anatomical change in our evolution; it required major remodeling of the foot, pelvis, and vertebral column. Enlargement of the brain was a secondary alteration made possible by prolonging the growth period of the skull and its contents (see Figure 14.17). The primate brain continues to grow after birth, and the period of growth is longer for a human than for any other primate. The extended period of human development also lengthens the time parents care for their offspring, which contributes to the child's ability to benefit from the experiences of earlier generations. This is the basis of culture—the transmission of accumulated knowledge over generations. The major means of this transmission is language, written and spoken.

Cultural evolution is continuous, but there have been three major stages. The first stage began with nomads who hunted and gathered food on the African grasslands 2 million years ago. They made tools, organized communal activities, and divided labor. Beautiful ancient art is just one example of our cultural roots in these early societies (Figure 17.45) .The second main stage of cultural evolution came with the development of agriculture in Africa, Eurasia, and the Americas about 10,000 to 15,000 years ago. Along with agriculture came permanent settlements and the first cities. The third major stage in our cultural evolution was the Industrial Revolution, which began in the eighteenth century. Since then, new technology has escalated exponentially; a single generation spanned the flight of the Wright brothers and Neil Armstrong's walk on the moon. It took less than a decade for the World-Wide Web to transform commerce, communication, and education. Through all this cultural evolution, from simple hunter-gatherers to high-tech societies, we have not changed biologically in any significant way. We are probably no more intelligent than our cave-dwelling ancestors. The same toolmaker who chipped away at stones now designs microchips and software. The know-how to build. skyscrapers, computers, and spaceships is stored not in our genes but in the cumulative product of hundreds of generations of human experience, passed along by parents, teachers, books, and electronic media.

European Bison Rhinocerous Human hand Horses Owl

Figure 17.45 Art History goes way back, and so does our fascination with and dependence on animal diversity. Cro-Magnon wildlife artists created these remarkable paintings beginning about 30,000 years ago. Three cave explorers found this prehistoric art gallery on Christmas eve 1994, when they ventured into a cavern near Vllon –Pont d’Arc in southern France.

Summary

Early Animals and the Cambrian Explosion Animals probably evolved from a colonial, flagellated protist more than 700 million years ago. At the beginning of the Cambrian period, 545 million years ago, animal diversity exploded.

.Animal Phylogeny Major branches of animal evolution are defined by four key evolutionary differences: the presence or absence of true tissues; radial versus bilateral body symmetry; presence or absence of a body cavity at least partly lined by mesoderm; and details of embryonic development.

.Sponges Sponges (phylum Porifera) are sessile animals with porous bodies and choanocytes but no true tissues. They filter-feed by drawing water through pores in the sides of the body.

.Cnidarians Cnidarians (phylum Cnidaria) have radial symmetry, a gas-trovascular cavity, and tentacles with cnidocytes. The body is either a sessile polyp or floating medusa.

.Flatworms Flatworms (phylum Platyhelminthes) are the simplest bilatera! animals. They may be free-living or parasitic in or on plants and animals.

.Roundworms Roundworms (phylum Nematoda) are unsegmented and cylindrical with tapered ends. They have a complete digestive tract and a pseudocoelom. They may be free-living or parasitic in plants and animals.

.Mollusks Mollusks (phylum Mollusca) are soft-bodied animals often protected by a hard shell. The body has three main parts: a muscular foot, a visceral mass, and a fold of tissue called the mantle-

.Annelids Annelids (phylum Annelida) are segmented worms. They may be free-living or parasitic on other animals.

.Arthropods Arthropods (phylum Arthropoda) are segmented animals with an exoskeleton and specialized, jointed appendages. Arthropods consist of four main groups: Arachnids, crustaceans, millipedes and centipedes, and insects. In species diversity, insects outnumber all other forms of life combined.

.Echinoderms Echinoderms (phylum Echinodermata) are sessile or slow-moving marine animals that lack body segments and possess a unique water vascular system. Bilaterally symmetrical larvae usually change to radially symmetrical adults. .

.Characteristics of Chordates Chordates (phylum Chordata) are defined by a dorsal, hollow nerve cord; a flexible notochord; pharyngeal slits; and a post-anal tail. Tunicates and lancelets are invertebrate chordates. All other chordates are vertebrates, possessing a cranium and backbone.

.Fishes Agnathans are jawless vertebrates. Cartilaginous fishes ( class Chondrichthyes), such as sharks, are mostly predators with powerful jaws and a flexible skeleton made of cartilage. Bony fishes ( class Osteichthyes) have a stiff skeleton reinforced by hard calcium salts. Bony fishes are further classified into ray-finned fishes, lungfishes, and lobe-finned fishes-

.Amphibians Amphibians ( class Amphibia) are tetrapod vertebrates that usually deposit their eggs (lacking shells) in water. Aquatic larvae typically undergo a radical metamorphosis into the adult stage. Their moist skin requires that amphibians spend much of their adult life in moist environments.

.Reptiles Reptiles (class Reptilia) are terrestrial ectotherms with lungs and waterproof skin covered by scales. Their amniotic eggs enhanced reproduction on land.

.Birds Birds ( class Aves ) are endothermic vertebrates with amniotic eggs, wings, feathers, and other adaptations for flight.

.Mammals Mammals ( class Mammalia) are endothermic vertebrates with hair and mammary glands. There are three major groups: Monotremes lay eggs; marsupials use a placenta but give birth to tiny offspring that usually complete development while attached to nipples inside the mother's pouch; and eutherians, or placental mammals, use their placenta in a longer-lasting association between the mother and her developing young.

Homo Sapiens (modern humans) originated in Africa about 250,000 and emerged from Africa about 100,000. By 30,000 they are in Siberia, by 14,000 in Americas and by 1,600 in remote Pacific islands.

Earth’s New Crisis

Of the many crises in the history of life, the impact of one species, Homo sapiens, is the latest and potentially the most devastating.

Evolution of the human brain may have been anatomically simpler than acquiring an upright stance, but the global consequences of cerebral expansion have been enormous. Cultural evolution made Homo sapiens a new force in the history of life-a species that could defy its physical limitations and shortcut biological evolution. We do not have to wait to adapt to an environment through natural selection; we simply change the environment to meet our needs. We are the most numerous and widespread of all large animals, and everywhere we go, we bring environmental change. There is nothing new about environmental change. The history of life is the story of biological evolution on a changing planet. But it is unlikely that change has ever been as rapid as in the age of humans. Cultural evolution outpaces biological evolution by orders of magnitude. We are changing the world faster than many species can adapt; the rate of extinctions in the twentieth century was 50 times greater than the average for the past 100,000 years.

This rapid rate of extinction is mainly a result of habitat destruction, which is a function of human cultural changes and overpopulation. Feeding, clothing, and housing 6 billion people imposes an enormous strain on Earth's capacity to sustain life. If all these people suddenly assumed the high standard of living enjoyed by many people in developed nations, It is likely that Earths support systems would be overwhelmed. Already, for example, current rates of fossil-fuel consumption, mainly by developed nations, are so great that waste carbon dioxide may be causing the temperature of the atmosphere to increase enough to alter world climates. Today, it is not just individual species that are endangered, but entire ecosystems, the global atmosphere, and the oceans. Tropical rain forests, which playa vital role in moderating global weather, are being cut down at a startling rate. Scientists have hardly begun to study these ecosystems, and many species in them may become extinct before they are even discovered.