Pikaia.

Pikaia is a genus of animal from the Cambrian Period. It was about an inch and a half long, and looked like a worm flattened sideways, with an arrow-like tail which could have helped it swim. Scientists have discovered only 60 specimens of Pikaia, so that could mean that they were rare.

Click picture to see Pikaia swim

Pikaia was first described as a polychaete worm by Walcott, but scientists now think Pikaia was an early chordate, resembling the modern day lancelet. It probably swam like an eel, near the bottom of the ocean.

Pikaia may have been the ancestor of all vertebrates, including humans. Although Pikaia had a notochord, it was not a vertebrate because it did not have a backbone.