Deuterostomes are a superphylum of animals including chordates, echinoderms, hemichordates, and Xenoturbellida. They have indeterminate cleavage and a more prominent coelom, allowing for the development of complex organs. The first simple deuterostomes evolved into the first vertebrates in 20 million years. Tunicates shed their nerve cord in adulthood, raising the possibility that vertebrates evolved from the larval form of some invertebrates.
Deuterostomes are a large superphylum of animals that includes chordates (vertebrates), echinoderms, hemichordates (acorn worms), and a small phyla, Xenoturbellida, which consists of two marine worm-like species. “Deuterostome” means “second organism mouth”, a reference to the fact that during embryological development in deuterostomes, the first opening of the body becomes the anus and the second the mouth, unlike in protostomes, where it is the opposite. Another difference is in the cleavage of the embryo as it grows: deuterostomes exhibit indeterminate cleavage, where there is more room for variation in where the cells ultimately end up, in contrast to the determinate cleavage of protostomes, where the position end of each cell is fixed.
The smallest superphylum compared to protostomes, deuterostomes have the advantage of containing the most complex animals. All animals above some arbitrarily defined level of complexity are likely to be deuterostomes. For example, probably the most complex phyla, the Chordates, are all deuterostomes. Unlike protostomes, which often have very little or no body cavity (coelom) at all, deuterostomes have more prominent coeloms, which help provide a structure for muscles to pull against, as well as allow for the development of more complex organs whose structural evolution is independent of that of the body wall.
Many of the simplest deuterostomes have three simple characteristics: gill slits, a hollow nerve cord, and a segmented body. It seems likely that the common ancestor of all deuterostomes had these characteristics. There is some uncertainty about when the deuterostomes branched off from the protostomes, but many paleontologists consider 558 million years ago as a likely lower limit. Ernettia, a late Ediacaran fossil, appears to be a probable deuterostome, while the mysterious extinct Early Cambrian phylum, Vetulicolia, has features that suggest they were probably deuterostomes.
In just about 20 million years, the first simple deuterostomes evolved into the first vertebrates, especially jawless fish, which are the ancestors of all fish, sharks, and land vertebrates. Many of the spin-off groups still exist today: Xenoturbellida, Acorn Worms, Tunicates, Lancets, etc. Tunicates are one of the more unusual of these groups as their larvae display a nerve cord, but shed it in adulthood. This raises the possibility that vertebrates evolved from the larval form of some invertebrate animals.
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