The first vertebrates on Earth were fish, and experts consider they first appeared around 480 million years ago. But fossil records from this time are blotchy, with only small fragments recognized. By 420 million years ago, though, the fossil record blooms, with a vast diversity of fish species existing all together.
New research from the University of Pennsylvania shows that nearly every major vertebrate separation, from the initial armored jaw-less fish all the way up through sharks and our own ancestors, all started out right near the beach, far inshore of the reef. Even as older groups expanded, newer groups were also appearing at the shoreline.
Precisely where vertebrates originated and spread has been a fiercely debated subject in paleontology. Certain groups of fossils from this crucial period in the middle Paleozoic Era told one story that perhaps a freshwater site of origin i.e. while other groups may point to a birthplace in the open ocean, and still others popped up in other habitat types.
Further obfuscating matters, the origin story of invertebrate biodiversity seem established: They diversified around coral reefs, their descendants consequently striking out to dwell in shallower or deeper waters.
A similar deviation has been seen in modern fish, such as sticklebacks, which progressed a bottom-dwelling and a free-swimming form from common ancestors in more modern times.