Vertebrates: what are they?

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Vertebrates have a backbone and make up most of the Chordata phylum. They evolved for structural support and a nervous system superhighway. Fish were the first to evolve limbs and walk on land, followed by amphibians and reptiles. Vertebrates are effective for animals longer than a few cm and make up the largest animals on land and sea. They get their name from their segmented spine, which provides flexibility and strength. A broken spinal column usually results in death.

Vertebrates are animals with a backbone, also known as a vertebral column. They make up most of the phylum Chordata, one of 38 animal phyla. All other phyla are exclusively invertebrates. A couple of members of the phylum Chordata, lancelets and tunicates, are invertebrates. Lancelets have only a notochord rather than a true vertebral column, and tunicates have a notochord only during early development. Fish, sharks, rays, amphibians, reptiles, birds and mammals (including humans) are vertebrates.

The major evolutionary reason for the evolution of vertebrates is the need for a strong support structure for durability and structural integrity. The spine also acts as a superhighway for the nervous system, supporting greater complexity. The earliest vertebrates are known from fossils in the Maotianshan shale in China, dated to between 525 and 520 million years ago, during an adaptive radiation episode called the Cambrian explosion. They are simple jawless fish like Myllokunmingia, which bear a superficial resemblance to modern-day hagfishes. These fish predate other vertebrate fossils by about 30 million years.

Among vertebrates, fish were the first to evolve limbs and began to walk on land, becoming amphibious. A line of amphibians developed scaly skin and amniotic eggs, becoming the reptiles, the first vertebrate line to colonize the continental interior. On land, the evolutionary strategy of vertebrates is particularly effective for all animals longer than a few cm, which must have sufficient structural integrity to support their weight. For this reason the largest land animals have been vertebrates for hundreds of millions of years. More recently, cetaceans (whales, which evolved just 50 million years ago from land animals) were the largest animals in the seas, so vertebrates make up the largest animals on both land and sea.

Vertebrates get their name from their vertebrae, segmented sections of the spine. These segmented sections give the spine some flexibility while maintaining its strength. The vertebrae are surrounded by nerve fibers that send commands, via electrical signals, from the brain to all limbs. If a vertebrate has a broken spinal column, it usually dies, although humans have developed forms of surgery to repair broken spinal columns as long as the damage is not too great.

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