Vertebrates

The Vertebrata are a subphylum of the phylum Chordata. The six vertebrate classes are lampreys, true fish, frogs and toads, reptiles, birds, and mammals. A vertebrate animal has a spinal column (backbone) made of bone or cartilage, and a brain case (skull). Vertebrates also have two pairs of limbs (though some have lost limbs through evolution), and a bilaterally paired muscular system. The backbone that gives the subphylum its name is a group of small bones or cartilage pieces with articulating surfaces (vertebrae). Ribs and bones that support the limbs are attached to the backbone. The ribs protect the heart, lungs, and other internal organs, and can expand and contract. The earliest vertebrate fossils occur in rock from the Paleozoic era. The vertebrae serve to encase and protect the spinal cord, a major part of the vertebrate nervous system. The central nervous system also has an enlarged, highly differentiated upper portion, the brain. The bony skull of a vertebrate also serves to encase and protect that brain, as well as providing a base for the vulnerable sensory organs of eyes, ears, nose, and mouth, which are thus efficiently located close to the brain. The more primitive members of the phylum Chordata have notochords, solid or segmented columns that cover the nervous system. Vertebrates resemble chordates in having notochords in their embryonic state. The vertebrae develop around the notochord. The trunk of a vertebrate is a hollow cavity in which the heart, lungs, and digestive tract are suspended. The central nervous system branches out from the spinal column to reach all internal organs, muscles, and skin. The support offered to the brain and nervous system by the vertebrae has allowed vertebrates to evolve increasingly large brains, which in turn has allowed vertebrates to become increasingly intelligent and responsive to their environment.

Bone and Bones
Bone, the material that form the vertebrae, first evolved 500,000 years ago. Bone is the hard supportive framework of all vertebrates. The framework, a skeleton, has hundreds of separate parts; for instance, there are 206 bones in a human skeleton. Bones protect delicate organs, such as the brain and lungs. Muscles, attached to bones, enable walking, flying, swimming, and all other means of motion. Bones provide body calcium needs and contain sites for making blood cells. Muchbone in adult vertebrates arises fromcartilage, an elastic, fibrous connective tissue, and the main component of fetal vertebrate skeletons. Such bone is cartilage bone. Cartilage is an extracellular matrix made by chondrocyte cells. It is firm and elastic, due to its collagen fibrils. The fibrils provide mechanical stability and high tensile strength, while allowing nutrients to enter chondrocytes. Blood vessels around cartilage supply nutrients and remove wastes. Cartilage-containing skeletons of newborn vertebrates become bone by a process of calcification, chondrocyte destruction, and replacement by bone cells. In young vertebrates, cartilage is the site of growth and calcification that lengthens bone to attain adult size.

Vertebrae
In higher vertebrates, each vertebra consists of a lower part, called the centrum, and an upper, Yshaped part, called the neural arch. The arch has a downward and backward projection, which can be felt as the bumps along a vertebrate's back, and two sideways projections, where muscles and ligaments can attach. The space between the arms of the Yon the neural arch and the centrumcreate an opening called the vertebral foramen, through which the spinal cord passes. Intervertebral disks, made of cartilage, separate the centrums and serve as shock absorbers. The vertebral column has five regions: the cervical region (neck), thoracic (chest), lumbar (lower back), sacral (pelvic girdle), and caudal (tail). The top two cervical vertebrae, called the atlas and axis vertebrae, make a joint to attach with the skull. The number of vertebrae varies by species.

Vertebrate Evolution
Vertebrates, all designed on the same general plan, flourish on land, in the air, and in both fresh and salt water. Vertebrates first evolved in the Silurian period, around 438 to 408 million years ago. In the intervening millennia, according to need, evolution produced valuable morphological changes to optimize vertebrate biofunctions. For example, whales, once land dwellers, evolved into ocean dwellers with lungs and sonar to help them navigate and find food. Birds developed wings to ride the air, and mammals proliferated in forms that fit varied habitats worldwide. Fish were the first vertebrates to appear, around 480 million years ago. Amphibians and reptiles appeared around 360 million years ago, while birds and mammals begin to appear around 205 million years ago. Morphological change led to a balance of nature, where herbivores ate plants, preventing their overgrowth, and carnivores ate herbivores, preventing their superabundance. Then the ultimate vertebrates, humans, developed civilization. Humans domesticated animals for food, clothing, transportation, and pets. In so doing, many species have been eradicated, endangered, or put at risk. For example, blue whales were endangered because their blubber was useful. Wolves and tigers have been eradicated or endangered by the quest for hunting trophies and to keep them from eating livestock. Mustelids were treated similarly because their pelts made attractive fur garments. Fortunately, human tolerance for this treatment of animals is decreasing, lest many species be seen only in zoos or faded photographs.