Carl Zimmer in his always excellent blog, The Loom:
Go back far enough in our history–maybe about 650 million years–and you come to a time when our ancestors were still invertebrates. That is, they had no skulls, teeth, or other bones. They didn’t even have a brain.
How invertebrates became vertebrates is a fascinating question, made all the more fascinating because the answer tells us something about how we got to be the way we are. In order to reconstruct what happened, scientists can study several different kinds of evidence. They can look at the bodies of invertebrates to find the ones that share traits with vertebrates not found in other invertebrates. Those common traits may be signs of common ancestry. Scientists can look for signs of this ancestry by studying the DNA of vertebrates and invertebrates. They can also examine the fossil record, to discover transitional forms that offer clues to the transitions that can’t be found in living species.