Vertebrate

Mammals are the only vertebrates that raise their young by feeding them on milk produced by the mother’s body, and the only ones that have teeth that are individually specialized for particular functions. Mammal species number about 4,600, and they include the largest animals on land and in the sea. Dogs, bears, monkeys, whales, and humans are all mammals.

Biologists believe that vertebrates evolved over millions of years from animals similar to today’s lancelets, which burrow in sand on the seabed and filter food from the water. Lancelets possess certain traits similar to vertebrates, including a reinforcing structure called a notochord that runs the length of the body. In a lancelet the notochord is the only hard part of the body, and it allows the animal to wriggle without losing its shape. In most vertebrates, the notochord is lost during early development, and its role is taken over by bone. The characteristics shared by lancelets and vertebrates cause scientists to classify them together in the chordate phylum.

Scientists do not know exactly how the transition from lancelet to vertebrate occurred. Fossils of fishlike animals found in China indicate that vertebrates evolved at the start of the Cambrian Period, an interval of geologic time that began about 570 million years ago. These fish lacked a bony skeleton and teeth (scientists propose that their skeletal structures were made of cartilage), but they did have gill slits and a muscle arrangement similar to today’s fish. Once vertebrates evolved hard body parts, they began to leave more fossilized remains. Fish called ostracoderms, which had bony plates covering their bodies, first appeared in the late Cambrian Period, about 500 million years ago. Like present-day lampreys and hagfish, ostracoderms had no jaws. They probably fed by sucking water into their mouths and then swallowing any food it contained.

With the evolution of jaws, vertebrates acquired a valuable new asset in the struggle for survival, one that enabled them to collect food in a variety of different ways. Jaws first appeared in fish about 420 million years ago, during the mid-Silurian Period. Unlike earlier vertebrates, jawed fish developed complex internal skeletons and paired fins, which helped them maneuver as they pursued their food or escaped from their enemies.

Over time, evolution has produced vertebrates with many different body types and behaviors. As a result, vertebrates can now be found in almost every part of the world.