What did all chordates evolve from?

All chordates evolved from a common ancestor that was a simple, aquatic organism resembling a tunicate or lancelet. This ancestor possessed key characteristics such as a notochord, a dorsal nerve cord, pharyngeal slits, and a post-anal tail, which are defining features of the phylum Chordata. Over time, these traits were modified and diversified, leading to the wide variety of chordate species we see today, including fish, amphibians, reptiles, birds, and mammals.