Animals with backbones, called vertebrates, form only a tiny fraction of all known creatures. In many environments, however, they exert an influence out of all proportion to their numbers, because they are in general larger than invertebrates and in many instances more mobile. Both size and mobility are made possible by a strong internal skeleton, jointed to give flexibility and held in place by tough connective tissue called ligaments.
Structure and development
Vertebrates are part of a larger group, the Chordates (Phylum Chordata). Chordata have a stiff, jellylike rod, called a notochord, running longitudinally through the back part of the body, and acting as an internal support to a series of muscles or muscle segments called myotomes. Above the notochord is a hollow nerve tube, which is usually folded at the front end to form the brain. Below the notochord lies the digestive tract. At some time in their lives all chordates have paired gill slits and a tail. Small, fishlike marine animals called lancelets (subphylum Cephalochordata) have a notochord throughout their life cycle.
In true vertebrates the notochord, present only in the embryonic stage, is replaced by cartilage or bone. Bone is a strong, hard substance, formed mainly of calcium phosphate plus collagen and other protein fibers. Bone has great strength for its weight and is well suited to act as the internal support for the body. It is possible that it originated as a waste product deposited in the skin of prehistoric fishlike marine creatures. When these animals began to live in brackish and fresh water, they faced a change in the balance of their mineral environment. This environment favored the production of bone, because the materials of which it was composed could be used by the body if necessary. This role as a reservoir of certain minerals is still an important function of bone. The early freshwater fishlike vertebrates developed abundant external bone— an enormous store of mineral wealth—which served the additional purpose of protective armor. Such a bony shell is obvious in some modern vertebrates, such as tortoises and armadillos. Though less evident, it also exists in all land vertebrates, in which the large and delicate brain is protected by the bony box of the skull.
The only direct descendants of the original bone-clad vertebrates still surviving are the lampreys (Petromyzonidae) and the hagfishes (Myxinidae), though these have lost all trace of hard skeletal structures. Lampreys and hagfish have a persistent notochord and a gristlelike skull. True fishes, however, have well developed skeletons and, in most cases, an armor of scales made of fine slips of bone set in the skin. The sharks and their relatives have retained bone-based external armor, but also have an internal skeleton formed of cartilage. In other vertebrates, skeletal cartilage is mainly a juvenile tissue, replaced by hard bone as the animals grow.
There are eight classes of vertebrates: the hagfish, or Myxini; the lampreys, or Cephalaspido-morphi; the sharks and other cartilaginous fish, or Chondrichthyes; the bony fish, or Oste-ichthyes; the frogs and other amphibians, or Amphibia; the reptiles, or Reptilia; the birds, or Aves; and the mammals, or Mammalia.
During the Devonian Period, which began about 408 million years ago, some fishes that were stranded in drought conditions struggled toward new pools using large, strong stiltlike fins. Since that time, vertebrate animals began to become less dependent upon a water environment.
The first creatures to move from the water onto the land were amphibians, which had to return to their ancient habitat to lay their eggs. Young amphibians generally resemble small fishes, but the adults live on land.
Next in evolutionary sequence came the reptiles. These developed shelled eggs, enclosing their embryos in the watery white of the egg, which served as their own private pool during the embryonic stage, which corresponds in some ways to the tadpole stage of amphibians. The egg also supplies the embryo’s nutrition, in the form of the yolk, which is the perfect, complete food needed for growth until the young reptile hatches. Hatched reptiles show no trace of gills, and their skin is hard and waterproof. Their metabolic pattern requires external warmth, however, so they are restricted to climatically favorable parts of the world.
During the Triassic Period, which began about 245 million years ago, mammals evolved from reptile species that are now extinct. Activity is the keynote of their being, and mammalian bodies are generally well adapted for easy movement. Such activity must be fueled by abundant food, and the relatively high-energy metabolism of mammals generally requires that the body is maintained at a steady temperature which in animals is aided by insulating hair or fur while regular breathing provides the large amounts of oxygen such a metabolism needs. The young of almost all species of mammals are born at a relatively advanced stage of development. A special maternal organ, the placenta, provides nourishment for the fetus while it grows within its mother’s body. After birth, the mother mammal feeds her young on milk from her body, which gives them all the nourishment they need at this stage. In many species, important maternal and social ties are formed and cemented during this period of suckling. Most mammals have relatively large and complex brains that enable the young to learn behavior, particularly from their parents.
During the Jurassic period, which began about 208 million years ago, birds evolved from reptile stock far removed from the ancestors of mammals. Birds have exploited the possibilities of flight more fully than other vertebrates. To achieve this, they have developed the most energy-intensive metabolic system of all. Birds show many modifications of the general vertebrate pattern, the most obvious being the transformation of the forelimbs into wings. Birds’ bodies are maintained at a high temperature and are insulated by feathers. Reproduction still depends on reptilelike eggs. Many birds care for their young for a prolonged period, which establishes social bonds comparable to those seen in mammals.