Pikaia was one of the first representatives of our own branch of the tree of life. The chordates include humans and all other vertebrates, but also lancetfish and the sac-like ascidians (sea squirts).

An eel-like animal with a flattened body, Pikaia had a notochord (a flexible skeletal rod, like a primitive backbone) supporting nerves and muscles, and a tailfin that enabled it to swim. In these ways Pikaia resembled the lancetfish, a group of primitive chordates that have survived to this day. But Pikaia differs in having a number of tentacles around its mouth, and its diet is unknown.