Vertebrates are animals that have vertebrae (or "backbones" as they are commonly called). They are members of the phylum Chordata, which includes all animals that possess an organ called a notochord. In vertebrates, however, the notochord is surrounded by a series of bony growths that develop into vertebrae. In higher vertebrates, such as humans, the notochord is most readily seen in embryos and becomes almost wholly replaced by vertebrae as the animals mature. Primitive vertebrates, first known to have appeared in the Cambrian Period of the Paleozoic Era (about 525 million years ago), apparently were adapted to grazing algae in shallow ocean waters and moving about from place to place. These two early adaptations were made possible by three important vertebrate characteristics, a vertebral column (specialized for waving back and forth to allow active swimming), a brain and complex nervous system (which allowed an active animal to maneuver and keep track of its ever changing environment), and a gill system (which makes lower vertebrates roving vacuum cleaners). The vacuum action of the gills not only helped to suck in food, but it also provided an active ventilation system that brought air into the body and aided in breathing.
Early vertebrates (called Ostracoderms) were quite different from any living today because they lacked fins, a lower jaw, and they were heavily armored for protection from invertebrate predators. Early in the Silurian Period (about 425 million years ago), fishes with jaws first appear in the fossil record. The jaws were apparently derived from an anterior pair of gill support arches, which originally were used to keep the gills open so that water could pass through them efficiently. Primitive vertebrates that developed jaws for catching food also brought the edge of the scaled skin, that lay around the mouth, into the mouth and over the newly invented jaws. These scales, once they developed specializations for holding and cutting food, became teeth, which are the hardest and most frequently preserved parts of vertebrates.
By the beginning of the Devonian Period (about 410 million years ago), vertebrates with jaws had diversified into four main groups, acanthodians, Placoderms, cartilaginous fishes, and bony fishes. By the end of the Devonian (about 360 million years ago), they had replaced entirely the armored jawless Ostracoderms. Acanthodian and Placoderms both died out before the end of the Paleozoic Era, and all living vertebrates (except lampreys and hagfishes, which are jawless and thus descended from ostracoderms) are either descended from primitive cartilaginous fishes or bony fishes. Cartilaginous fishes today include the sharks, rays, chimeras, and sawfishes, which are mostly found in marine waters. Bony fishes include most of the economically important marine fishes (such as tuna, herring, salmon, cod, mackerel, and swordfish) and nearly all the common freshwater fishes.
Additionally, bony fishes include lungfishes and the coelacanth, which today is found only off the coast of east Africa. The coelacanth is the closest living relative to the ancestry of land-living vertebrates, which first appeared toward the close of the Devonian Period (about 360 million years ago). At that time, in fresh water deposits, coelacanth-like fish appeared with fins that had become stout and strong enough to support the weight of the body out of water. Although primitive forms also retained a prominent tail fin, they possessed new features such as lungs and toes, which helped them to get around on land. These animals became the earliest known amphibians, and they underwent a major radiation into diverse damp land environments during the Coal Age, or Carboniferous Period (from about 360 to 286 million years ago). Living descendants of these animals include salamanders, frogs, and caecilians, all of which more or less still resemble their amphibian ancestors in having an aquatic larval stage, scaleless skins, and toes that lack toenail.
From within the amphibian group there also arose a lineage that evolved scales, toenails, and eggs with hard shells that could withstand drying. This new group, which appeared somewhat later in the Carboniferous, was the ancestral stock of reptiles. Soon after their appearance, still within the Carboniferous, reptiles split into two main lines of descent. One group remained in wetter, lowland climates, but became adept at surviving under cool to cold conditions. This group began to develop hair and to carry its eggs in pouches on the mother's stomach. This line, called mammal-like reptiles, gave rise to modern mammals. The other line, although it did not develop hair or brood pouches to carry its young, became adept at surviving in hot dry climates by developing a number of traits that helped to store and conserve water. This line gave rise to modern reptiles (turtles, lizards, snakes, crocodiles), dinosaurs, and birds. Although this basic split occurred in the Carboniferous, it was not until the end of the Triassic Period in the Mesozoic Era (about 213 million years ago), that the ancestors of modern types of amphibians, reptiles, birds, and mammals appear in the fossil record. During the late Paleozoic, mammal-like reptiles dominated. However, with the drier climates of the Mesozoic Era (248 to 65 million years ago), the reptile lineage reigned supreme, and the mammal lineage waned. After the great extinction at the end of the Mesozoic, the reptile line was largely wiped out, and the mammals again came to the forefront. With the exception of the great extinction at the end of the Mesozoic, which resulted in the extinction of the dinosaurs and a number of more primitive vertebrate groups, the history of vertebrates since the beginning of the Mesozoic has been largely one of steady diversification and advancement. Although not the most diverse or abundant group of animals in the modern world, they consistently occupy the top of the food chain both on land and in the sea, they exist from the deepest ocean basins to the highest mountain ranges, and they survive successfully everywhere from frigid polar regions to hot desolate deserts. For all of these reasons, vertebrates certainly are contenders for the position of the single most successful group of animals in the world.
