Animal Diversity

From sponges that filter seawater through their pores to whales, beetles and the person reading this sentence, animals display an astonishing range of form. Yet that variety is built on a handful of body plans, laid down more than 500 million years ago and modified ever since. This chapter gives you a map of the animal kingdom: the features biologists use to sort animals, the major invertebrate groups, and the vertebrate story from jawless fishes to the evolution of our own species.

Section 1Animal body plans and the animal tree of life

What is an animal?

Animals (kingdom Animalia, or Metazoa) are multicellular, heterotrophic eukaryotes that take in food by ingestion and digest it inside their bodies. Their cells lack cell walls; instead, proteins of the extracellular matrix, especially collagen, hold the body together, and specialized junctions (tight junctions, desmosomes and gap junctions) link neighboring cells. Two tissue types are unique to animals: nervous tissue, which carries impulses, and muscle tissue, which moves the body. Most animals reproduce sexually, and the diploid stage dominates the life cycle; the only haploid cells are eggs and sperm.

Animal development follows a common sequence. The zygote undergoes cleavage, rapid mitotic divisions without growth, forming a solid ball and then a hollow blastula. During gastrulation, one side of the blastula folds inward, forming a two-layered or three-layered gastrula with an inner pocket, the archenteron, that becomes the gut. The opening of the archenteron is the blastopore. Many animals pass through a sexually immature larva that later undergoes metamorphosis into the adult form. All animals share a family of developmental control genes, the Hox genes, that specify position along the head to tail axis. Variation in how Hox genes are deployed underlies much of the variety of animal body plans.

The closest living relatives of animals are the choanoflagellates, single-celled or colonial protists whose collar of microvilli around a flagellum is strikingly similar to the collar cells of sponges. The first animals probably appeared about 770 to 800 million years ago. Soft-bodied Ediacaran fossils date from about 635 to 541 million years ago, and during the Cambrian explosion, roughly 535 to 525 million years ago, most major animal phyla appear in the fossil record within a few million years, along with the first hard skeletons, predators and burrows.

Symmetry and tissues

Biologists sort animal body plans by a few features. Tissues: sponges lack true tissues, collections of specialized cells isolated from other tissues by membranous layers, while all other animals, the eumetazoans, have them. Symmetry: some animals are asymmetrical (most sponges). Radial symmetry means any cut through the central axis produces mirror halves, like a pie; radial animals (cnidarians, ctenophores) have a top and bottom but no front, back, left or right, suiting a sessile or drifting life in which food and danger can come from any direction. Bilateral symmetry means only one plane divides the body into mirror halves; bilateral animals have dorsal and ventral sides, left and right sides, and anterior and posterior ends. Bilateral symmetry goes with active, directional movement and with cephalization, the concentration of sense organs and nervous tissue in a head.

Germ layers and body cavities

Gastrulation produces the germ layers from which all tissues develop. Ectoderm, the outer layer, forms the epidermis and the nervous system. Endoderm, the inner layer, lines the digestive tract and forms organs such as the liver and lungs of vertebrates. Cnidarians and ctenophores have only these two layers and are diploblastic. Bilateral animals are triploblastic: they add a middle layer, mesoderm, which forms muscles, most organs, the circulatory system and, in vertebrates, the skeleton.

Triploblastic animals can be grouped by their body cavity, a fluid-filled space between the gut and the outer body wall. A body cavity cushions organs, lets the gut move independently of the body wall, and can act as a hydrostatic skeleton that muscles push against. An animal with a true coelom (a coelomate) has a cavity completely lined by mesoderm, with mesoderm-derived mesenteries suspending the organs. A pseudocoelomate has a cavity lined by mesoderm on the outside and endoderm on the inside. An acoelomate, such as a flatworm, has solid tissue between gut and body wall. Molecular evidence shows that coeloms and pseudocoeloms were gained and lost more than once, so body cavity type describes anatomy but is not a reliable guide to relationships.

Protostome and deuterostome development

Bilateral animals fall into two great developmental modes, named for the fate of the blastopore. In protostomes ("mouth first"), the blastopore usually becomes the mouth. In deuterostomes ("mouth second"), it becomes the anus, and the mouth forms later at the other end. The two modes also tend to differ in cleavage and in how the coelom forms.

The animal tree of life

Modern animal phylogeny rests mainly on DNA sequences, which have confirmed some traditional groups and overturned others. The broad picture: sponges (Porifera) branch near the base, together with the comb jellies (Ctenophora), whose exact position is still debated. Cnidarians are the sister group of the Bilateria. Bilaterians split into protostomes and deuterostomes, and protostomes split into two large clades. Lophotrochozoa includes animals with a lophophore (a crown of ciliated tentacles for feeding, as in ectoprocts and brachiopods) or a trochophore larva (as in many molluscs and annelids), along with flatworms and rotifers. Ecdysozoa includes animals that grow by molting a cuticle, a process called ecdysis: nematodes and arthropods. Ecdysozoans are the most species-rich animals on Earth. Deuterostomia includes echinoderms, hemichordates and chordates.

Section 2Invertebrates: sponges to arthropods

Invertebrates, animals without a backbone, make up about 95 percent of known animal species. "Invertebrate" is a convenient label, not a clade: it lumps together everything except the vertebrates. This section surveys the major phyla in roughly the order they branch on the tree.

Sponges

Sponges (phylum Porifera, "pore bearers") are sessile suspension feeders without true tissues, organs or a nervous system. Water enters through many small pores, flows into a central cavity, the spongocoel, and leaves through a larger opening, the osculum. Flagellated choanocytes (collar cells) line the interior; their beating flagella drive the current, and their collars trap bacteria and food particles, which are then engulfed by phagocytosis. In the gelatinous middle layer, the mesohyl, wandering amoebocytes digest and distribute food and build the skeleton of spicules (made of calcium carbonate or silica) or of flexible collagen fibers called spongin. Most sponges are hermaphrodites, each individual producing both eggs and sperm, usually at different times. Sponges also make unusual antibiotic and anticancer compounds that are studied as drug leads.

Cnidarians

Cnidarians (corals, sea anemones, hydras and jellyfish) are diploblastic and radially symmetrical. Their body plan is a sac with a central gastrovascular cavity and a single opening that serves as both mouth and anus, ringed by tentacles. The body wall has an outer epidermis and an inner gastrodermis separated by a jellylike mesoglea. Cnidarians come in two body forms: the cylindrical, usually attached polyp (mouth facing up) and the bell-shaped, free-swimming medusa (mouth facing down). Some species have only one form; others alternate between them. Their hallmark is the cnidocyte, a stinging cell containing a capsule called a cnida; the type that injects venom is a nematocyst. Cnidarians have a nerve net without a brain and move by contracting cells against the water in the gastrovascular cavity, which serves as a hydrostatic skeleton.

The four main cnidarian classes differ in which body form dominates. Hydrozoans (hydras, Portuguese man-of-war) usually alternate polyp and medusa, with the polyp often dominant. Scyphozoans (true jellyfish) spend most of their lives as medusae. Cubozoans (box jellies) have boxlike medusae, complex eyes and some of the most dangerous venoms known. Anthozoans (sea anemones and corals) exist only as polyps. Reef-building corals secrete calcium carbonate skeletons and host photosynthetic dinoflagellate symbionts; when stressed by high water temperature, corals expel these symbionts and turn white, a process called coral bleaching.

Lophotrochozoans: flatworms, rotifers, molluscs and annelids

Flatworms (phylum Platyhelminthes) are triploblastic acoelomates flattened from top to bottom. Their thinness lets every cell exchange gases by diffusion, so they lack respiratory and circulatory organs. The gastrovascular cavity has one opening. Excretion and water balance rely on protonephridia with ciliated flame cells. Free-living planarians have a simple brain, ventral nerve cords and light-sensing eyespots, and can regenerate a whole body from a fragment. Many flatworms are parasites: flukes (trematodes), such as the blood flukes that cause schistosomiasis in hundreds of millions of people, often cycle through a snail intermediate host; tapeworms (cestodes) attach to the intestine with a hooked scolex, lack a mouth or gut, absorb digested food across their body surface and shed egg-filled segments called proglottids.

Rotifers are tiny pseudocoelomates with a crown of cilia that sweeps food into a complete digestive tract with mouth and anus. Some groups reproduce entirely by parthenogenesis, development of unfertilized eggs. Lophophorates, including ectoprocts (moss animals) and brachiopods (lamp shells, which resemble clams but are unrelated), feed with a lophophore.

Molluscs (snails, slugs, clams, oysters, octopuses and squids) are soft-bodied coelomates, most protected by a calcium carbonate shell. All share a body plan with three main parts: a muscular foot for movement, a visceral mass holding most internal organs, and a mantle, a fold of tissue that drapes over the visceral mass and secretes the shell. The mantle encloses a mantle cavity housing the gills, anus and excretory pores. Many molluscs feed with a radula, a rasping, tooth-covered strap. Gastropods (snails and slugs, the largest class) undergo torsion during development, a 180-degree twist that brings the mantle cavity and anus above the head. Bivalves (clams, mussels, oysters, scallops) have a hinged two-part shell, no distinct head and no radula, and filter feed with their gills. Cephalopods (squids, octopuses, cuttlefishes, nautiluses) are fast predators whose foot is modified into tentacles and a siphon for jet propulsion. They have a beak, a closed circulatory system, camera-type eyes and the largest brains of any invertebrate.

Annelids ("little rings") are segmented worms with a true coelom divided by partitions. Segmentation lets each segment's muscles act on its own fluid-filled compartment, allowing precise, wavelike movement. Earthworms have a closed circulatory system with five pairs of muscular vessels that act as hearts, paired excretory metanephridia in each segment, a ventral nerve cord with segmental ganglia, and a digestive tract with a crop and gizzard. Earthworms are hermaphrodites that exchange sperm, and their burrowing aerates and enriches soil. Traditional annelid groups include the mostly marine polychaetes, with paddle-like parapodia bearing bristles, and the leeches, some of which feed on blood and secrete the anticoagulant hirudin.

Ecdysozoans: nematodes and arthropods

Nematodes (roundworms) are unsegmented, cylindrical pseudocoelomates covered by a tough cuticle that they shed as they grow. They have a complete digestive tract and only longitudinal muscles, which produce a thrashing motion. Nematodes are found almost everywhere, in soil, water and inside other organisms, often in enormous numbers. Caenorhabditis elegans, a free-living soil nematode with exactly 959 somatic cells in the adult hermaphrodite, is a key model organism for development and aging. Parasitic nematodes include hookworms, pinworms and Trichinella spiralis, which encysts in muscle and is acquired from undercooked pork or game.

Arthropods ("jointed feet") are the most successful animals on Earth, with over a million described species. Their success rests on a segmented body with specialized regions (tagmata, such as head, thorax and abdomen), a hard exoskeleton made of protein and the polysaccharide chitin, and jointed appendages modified for walking, feeding, sensing, swimming and mating. Because the exoskeleton cannot grow, arthropods must molt, shedding it and secreting a larger one, a vulnerable process controlled by the hormone ecdysone. Arthropods have an open circulatory system in which a heart pumps hemolymph into sinuses that bathe the organs (the cavity is called a hemocoel). Aquatic species breathe with gills; insects use tracheal systems, branched air tubes that deliver oxygen directly to cells.

Insects owe much of their diversity to flight, which opened new habitats and escape routes, and to metamorphosis. In incomplete metamorphosis (grasshoppers, cockroaches), young nymphs look like small adults and change gradually through a series of molts. In complete metamorphosis (beetles, flies, butterflies, bees), a wormlike larva specialized for eating and growing transforms during a pupa stage into an adult specialized for dispersal and reproduction. Because larva and adult often eat different foods, they do not compete with each other. Insects pollinate most flowering plants, recycle dead matter and transmit diseases such as malaria (Anopheles mosquitoes).

Section 3Echinoderms and chordates

Echinoderms

Echinoderms ("spiny skin") are slow-moving or sessile marine deuterostomes: sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers and sea lilies. Adults are usually pentaradial, with body parts arranged in fives around a central axis, but their larvae are bilaterally symmetrical, which shows that echinoderms descend from bilateral ancestors and became radial secondarily. Beneath a thin skin lies an endoskeleton of hard calcium carbonate plates called ossicles, often bearing spines.

The defining echinoderm feature is the water vascular system, a network of hydraulic canals that branch into extensions called tube feet. Water enters through a sieve plate on the upper surface, the madreporite, and flows through a ring canal into radial canals in each arm. Each tube foot has a muscular bulb, the ampulla, at its base; when the ampulla contracts, water extends the foot, which grips the substrate by suction and adhesive chemicals. Thousands of coordinated tube feet let a sea star creep along and pry open a clam. A sea star then everts its stomach through its mouth into the clam's shell and digests the prey in place. Tube feet also function in gas exchange. Echinoderms lack a centralized brain, and many can regenerate lost arms.

Echinoderms are the closest relatives of the chordates apart from the small group of hemichordates (acorn worms), which share pharyngeal slits with chordates. Together these groups form the deuterostomes.

The four chordate traits

Chordates are bilaterian deuterostomes defined by four features that appear at some stage of development, even if they disappear in the adult:

1. A notochord, a flexible rod of fluid-filled cells wrapped in fibrous tissue, lying between the gut and the nerve cord. It supports the body and gives muscles something to pull against. In most vertebrates a jointed skeleton replaces it, and in humans remnants persist as the nucleus pulposus of the intervertebral discs.
2. A dorsal, hollow nerve cord, formed when a plate of ectoderm rolls into a tube above the notochord. It becomes the brain and spinal cord. Most other bilaterians have solid nerve cords running along the ventral side.
3. Pharyngeal slits or clefts, openings or grooves in the pharynx (the region just behind the mouth). In lancelets and tunicates the slits filter food from water; in fishes they became gill slits supported by gill arches; in tetrapods the clefts and arches develop into parts of the ear, the tonsils, the thymus, the parathyroid glands and structures of the head and neck.
4. A muscular post-anal tail, extending beyond the anus and providing propulsion in many aquatic chordates. Humans have one as embryos; it regresses, leaving the coccyx.

Chordates also have an endostyle, a groove in the floor of the pharynx that secretes mucus to trap food in lancelets and tunicates; in vertebrates it develops into the iodine-binding thyroid gland.

Invertebrate chordates

Two groups of chordates lack backbones. Lancelets (cephalochordates) are small, bladelike animals that burrow in sand with only the head sticking out, filtering food through their pharyngeal slits. They keep all four chordate traits as adults and swim with segmented blocks of muscle, the same arrangement seen in vertebrate embryos. Tunicates (urochordates, or sea squirts) are, surprisingly, the closest living relatives of vertebrates. Their tadpole-like larvae have all four chordate traits, but most adults lose the notochord, nerve cord and tail and become sessile filter feeders that draw water in through one siphon and out through another, squirting when touched.

Craniates and vertebrates

Craniates are chordates with a head, including a skull (cranium) that protects a brain and paired sense organs. Craniate development depends on the neural crest, a band of cells that forms along the edges of the closing neural tube and migrates throughout the embryo to form parts of the skull, teeth, peripheral nerves, the adrenal medulla and pigment cells. Craniates also have a heart with at least two chambers, red blood cells with hemoglobin, and kidneys.

Hagfishes are jawless craniates with a skull of cartilage but only rudimentary vertebrae; they are slimy scavengers that can release enormous volumes of slime when attacked. Vertebrates add a vertebral column, a series of vertebrae that encloses the spinal cord and takes over the notochord's support role, plus a more elaborate brain and skeleton. Lampreys are jawless vertebrates; many adults are parasites that attach to fish with a round, toothed mouth and feed on blood. Molecular data group hagfishes and lampreys together as the cyclostomes, the jawless vertebrates, and suggest that hagfishes lost vertebral elements. Many extinct jawless fishes, the armored ostracoderms, also existed in the early Paleozoic.

The evolution of jaws

All other vertebrates are gnathostomes ("jaw mouths"). Jaws evolved by modification of skeletal rods, the pharyngeal arches, that supported the anterior pharyngeal slits. Hinged jaws, with teeth, let vertebrates grip and slice prey, opening up an enormous range of feeding niches and active predation. Gnathostomes also have paired fins (pectoral and pelvic), a larger forebrain, enhanced smell and vision, and a lateral line system of sensory organs along the body that detect vibrations in water. Gnathostomes appear in the fossil record about 440 million years ago. The placoderms, armored jawed fishes, dominated the seas for a time and then went extinct; living gnathostomes descend from two other lineages, the cartilaginous fishes and the bony fishes.

Section 4Fishes, amphibians and reptiles including birds

Cartilaginous fishes

Chondrichthyans (sharks, rays, skates and ratfishes) have skeletons made mostly of cartilage, often strengthened with calcium. Their ancestors had bony skeletons, so cartilage here is a derived condition, not a primitive one. Sharks are streamlined swimmers with powerful jaws and rows of teeth that are continually replaced. They lack a swim bladder; an oily liver provides some buoyancy, and many must keep swimming to stay up and to pass water over their gills. Sharks have keen senses: smell, vision, the lateral line, and electroreceptors that detect the weak electric fields of prey muscles. Fertilization is internal, and depending on the species, sharks lay eggs (oviparous), retain eggs that hatch inside the mother (ovoviviparous), or nourish embryos through a placenta (viviparous). Rays and skates are flattened bottom dwellers that feed mostly on mollusks and crustaceans.

Bony fishes: ray-finned and lobe-finned

Osteichthyans have skeletons of bone, a protective bony flap called the operculum over the gills (letting them breathe while stationary by pumping water across the gills), and, in most species, a swim bladder, an air sac that adjusts buoyancy. The swim bladder evolved from lungs in early bony fishes. This group includes the tetrapods, because tetrapods descend from bony fishes. The aquatic members fall into two lineages.

Ray-finned fishes (actinopterygians) have fins supported by thin, bony rays. They include nearly all familiar fishes (trout, tuna, goldfish, seahorses, eels) and more than 30,000 species, the most species-rich vertebrate group. Lobe-finned fishes (sarcopterygians) have rod-shaped bones surrounded by muscle in their pectoral and pelvic fins. Only three lineages survive: coelacanths, thought to be extinct until one was caught off southern Africa in 1938; lungfishes, which gulp air and can survive drought buried in mud; and the tetrapods. Lungfishes are the closest living fish relatives of tetrapods.

Tetrapods and the move onto land

Tetrapods are gnathostomes with four limbs bearing digits. Their limbs evolved from the lobed fins of sarcopterygians around 365 million years ago, probably in shallow, oxygen-poor waters where support and air breathing were already useful. Transitional fossils document the change. Tiktaalik, about 375 million years old, was a fish with scales, fins and gills, but also lungs, a flattened head with eyes on top, a neck and fin bones that could prop up its body like an elbow and wrist. Acanthostega had legs with eight digits yet still lived in water. Tetrapods also gained a neck, ribs able to support the body and pelvic girdle bones fused to the backbone so the hind limbs could carry weight.

Amphibians

Amphibians ("both ways of life") include about 8,000 living species in three orders: salamanders (Urodela, "tailed ones"), frogs and toads (Anura, "without tails") and the legless, burrowing caecilians (Apoda). Many amphibians live a double life. A frog begins as an aquatic, herbivorous tadpole with gills, a lateral line and a tail, then undergoes metamorphosis, driven by thyroid hormone, into a carnivorous adult with lungs and legs. Amphibian eggs lack a shell and dry out quickly, so most species lay eggs in water or damp places, and external fertilization is common in frogs. Adults exchange gases through moist, thin skin as well as lungs and the lining of the mouth, which keeps them dependent on humid habitats. Many species secrete toxic or distasteful compounds from skin glands, sometimes advertised with bright warning colors.

Amphibians have declined faster than any other vertebrate group in recent decades, with hundreds of species lost or severely reduced. Causes include habitat loss, climate change, pollution and especially chytridiomycosis, a skin disease caused by a chytrid fungus (Batrachochytrium) that disrupts the electrolyte balance across the skin.

Amniotes and the amniotic egg

Amniotes, the reptiles (including birds) and mammals, are tetrapods named for their key adaptation to land, the amniotic egg. It contains four extraembryonic membranes, made by the embryo but not part of its body, that let the embryo develop on land inside its own private pond. In reptiles and monotremes, the egg also has a shell, leathery or calcium-rich, that slows water loss while allowing gas exchange. Most mammals no longer lay eggs but retain the four membranes, which in placental mammals contribute to the placenta and umbilical cord.

Amniotes also have relatively impermeable skin, waterproofed with the protein keratin, and they ventilate their lungs by expanding the rib cage rather than by gulping air as amphibians do. Early in their history, about 310 million years ago, amniotes split into two lineages: the synapsids, which led to mammals, and the sauropsids (reptiles), which led to lizards, snakes, turtles, crocodilians and birds.

Reptiles

Reptiles have scales made of keratin, lay shelled eggs on land (or retain them internally), and have internal fertilization. Most nonavian reptiles are ectotherms: they gain body heat mainly from the environment and regulate their temperature by behavior, such as basking in the sun or retreating to shade. Ectothermy is not a weakness; it lets a reptile live on a small fraction of the food a same-sized mammal needs. Living reptiles include the lepidosaurs (tuataras, lizards and snakes; snakes are legless lizards that smell with a forked tongue and many detect infrared heat with pit organs), the turtles (with a shell fused to the ribs and backbone), and the archosaurs, which include crocodilians, the extinct pterosaurs and nonavian dinosaurs, and birds. Crocodilians are the closest living relatives of birds, and like birds they have a four-chambered heart.

Birds

Birds are feathered archosaurs descended from theropod dinosaurs, the group that includes Tyrannosaurus and Velociraptor. Many theropods already had feathers, hollow bones, nesting behavior and a wishbone (furcula). Archaeopteryx, about 150 million years old, had feathered wings but also teeth, clawed fingers and a long bony tail. Living birds are built for flight. Feathers, made of keratin, form the airfoil of the wing and insulate the body. Weight is reduced by hollow bones, the absence of teeth (a light keratin beak instead), a single ovary in most females and gonads that shrink outside the breeding season. A keeled sternum anchors huge flight muscles. Flight demands a great deal of energy, which birds supply through endothermy (generating heat by metabolism), a four-chambered heart, and a highly efficient respiratory system in which air sacs keep air flowing in one direction through the lungs. Most birds also have excellent vision. Some, such as ostriches and penguins, have lost flight, but they carry the same features.

Section 5Mammals and human evolution

What makes a mammal

Mammals are amniotes descended from the synapsid lineage, which arose more than 300 million years ago. Early synapsids, sometimes misleadingly called "mammal-like reptiles," gradually acquired mammalian traits over about 100 million years, and the first true mammals appeared around 200 million years ago, small and probably nocturnal, living alongside the dinosaurs. After the mass extinction 66 million years ago removed the nonavian dinosaurs, mammals diversified rapidly into the ecological roles the dinosaurs had left open.

Living mammals share a distinctive set of traits:

• Mammary glands that produce milk to nourish the young, the trait that gives the group its name.
• Hair, made of keratin, which insulates the body and supports endothermy.
• A four-chambered heart that fully separates oxygenated and deoxygenated blood, and a muscular diaphragm that ventilates the lungs.
• A lower jaw made of a single bone, the dentary, and three middle ear bones (malleus, incus and stapes). Two of these, the malleus and incus, evolved from jaw bones of synapsid ancestors, a transition beautifully documented in the fossil record and repeated in mammalian embryos.
• Differentiated teeth (incisors, canines, premolars and molars) specialized for cutting, tearing and grinding, usually replaced only once (milk teeth, then adult teeth).
• A relatively large brain, and in most species extended parental care and learning.

Marsupials and eutherians often show convergent evolution: unrelated species in similar environments evolve similar forms. Australian marsupial "moles," "mice," "flying squirrels" (sugar gliders) and the extinct thylacine (a wolflike predator) parallel the eutherian moles, mice, flying squirrels and wolves of other continents.

Primates

Primates include lemurs, lorises, tarsiers, monkeys and apes, including humans. Many primate traits are adaptations to life in trees: grasping hands and feet with flat nails instead of claws; sensitive fingertip pads; an opposable thumb (in many species) that can touch the other fingers for precise grip; a large brain relative to body size; forward-facing eyes close together on the face, giving overlapping visual fields and excellent depth perception; color vision in many species; and a high degree of parental care, usually for single offspring.

Primates divide into two groups. Strepsirrhines include the lemurs of Madagascar and the lorises and pottos of Africa and southern Asia; they often have a moist nose and rely more on smell. Haplorhines include the tarsiers and the anthropoids (monkeys and apes). New World monkeys of Central and South America have flat noses with nostrils facing sideways, and many have prehensile tails that grasp branches. Old World monkeys of Africa and Asia (baboons, macaques) have nostrils facing downward and never have prehensile tails. Apes (hominoids: gibbons, orangutans, gorillas, chimpanzees, bonobos and humans) have no tail, relatively long arms, and the largest brains among primates. The closest living relatives of humans are chimpanzees and bonobos, which share about 98.8 percent of our DNA sequence.

Hominin evolution

Hominins are the species on the human branch after it split from the lineage leading to chimpanzees. More than 20 extinct hominin species are known, and they did not form a single straight line of ancestors and descendants; several species often lived at the same time. The fossil record shows mosaic evolution: different traits changed at different times and rates. Most importantly, bipedalism, upright walking on two legs, came first, millions of years before large brains or stone tools. Skeletal clues to bipedalism include a foramen magnum (the opening for the spinal cord) located directly under the skull rather than at the back, an S-shaped spine, a short, bowl-shaped pelvis, knees angled inward under the body, and a nongrasping big toe aligned with the other toes.

The earliest proposed hominins include Sahelanthropus (about 7 to 6 million years ago, with a forward-placed foramen magnum) and Ardipithecus (about 5.8 to 4.4 million years ago), which walked upright on the ground but kept a grasping big toe for climbing. The australopiths followed. Australopithecus afarensis, known from the partial skeleton nicknamed "Lucy" (about 3.2 million years old), was fully bipedal but had a chimpanzee-sized brain of about 400 to 500 cubic centimeters and long arms suited to climbing. Fossil footprints preserved in volcanic ash at Laetoli, Tanzania, about 3.6 million years old, show a humanlike upright stride. The robust Paranthropus species had massive jaws and molars for chewing tough plant foods and were a side branch that left no descendants.

The genus Homo appears about 2.4 million years ago with Homo habilis ("handy man"), whose brain was somewhat larger (about 600 cubic centimeters) and who is associated with simple stone tools. Homo erectus, appearing about 1.9 million years ago, had a brain of about 900 cubic centimeters, a tall body with long legs suited to long-distance walking and running, more advanced tools (hand axes), likely control of fire, and was the first hominin to spread out of Africa into Asia and Europe. Homo neanderthalensis (Neanderthals) lived in Europe and western Asia from about 400,000 to 40,000 years ago. They had brains as large as or larger than ours, made sophisticated tools, buried their dead and survived ice-age climates. Ancient DNA shows that they interbred with Homo sapiens: people of non-African ancestry carry about 1 to 2 percent Neanderthal DNA. Another archaic group known mainly from DNA, the Denisovans, contributed genes to some Asian and Oceanian populations. The small-bodied Homo floresiensis of Indonesia and Homo naledi of South Africa show that hominin diversity persisted until relatively recently.

The origin of Homo sapiens

Fossils from Jebel Irhoud in Morocco show that early Homo sapiens lived in Africa by about 300,000 years ago, and fossil and genetic evidence agree that modern humans originated in Africa. Africa holds the greatest human genetic diversity, as expected if other populations descend from a subset of African ancestors. Small groups of H. sapiens dispersed out of Africa, most successfully around 60,000 to 70,000 years ago, reaching Australia by at least 50,000 years ago and the Americas by at least 15,000 to 20,000 years ago. As they spread, they encountered and occasionally interbred with Neanderthals and Denisovans, which later disappeared. Distinctive modern behaviors, such as symbolic art, personal ornaments and complex language, became widespread over this period. All living humans belong to one species with very little genetic variation compared with other great apes; two chimpanzees from neighboring African populations often differ more genetically than two humans from different continents.

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