Among them were the chordates, to which vertebrates (animals with backbones) such as humans belong. What sparked this biological bonanza isn't clear. It may be that oxygen in the atmosphere ...

Accordingly, a vertebrate camera-like eye must have been ... jawless, eel-shaped marine chordates. This developmental progression is explained most parsimoniously if we assume that lampreys ...

All chordates, such as vertebrates, eel-like lancelets, and tunicates, or sea squirts, at some point in their lives have a flexible, rod-shaped nerve structure called a notochord in their backs.