Our backbones helped us and other vertebrate animals conquer the oceans, and move onto land and into skies. Until now, the early history of the group have been a bit of a mystery, but a new analysis suggests vertebrate animals evolved in shallow waters.

All vertebrates have backbones or spinal columns. They are thought to have begun to diversify around 480 million years ago, splitting into groups that would become jawless fish (like lampreys), cartilaginous fish (including sharks), and a lineage that includes bony fish (such as salmon). It’s this last group that ultimately gave rise to amphibians, reptiles, birds and mammals.

However, our understanding of this diversification event is hampered by the fact that most of our well-preserved fish fossils are only 360 million years old or younger.

Now Lauren Sallan at the University of Pennsylvania, and colleagues, have brought together records of nearly 3000 early fish fossils from before this time into a single database. Comparing the features of these fossils and what kinds of environments they were preserved in reveals new clues about exactly where they diversified.

“We found that all vertebrates, from the first jawless forms to sharks and bony fishes, originated in very restricted shallow waters hugging the coast line,” says Sallan.

This is a bit of a surprise – big evolutionary steps are usually associated with biodiversity hotspots, such as coral reefs. But the data suggests that the evolutionary events that helped fill the seas with fishes occurred in shallow, salt-water environments like tidal areas and lagoons.

Those environments may have encouraged the evolution of vertebrates because their bones helped them withstand swirling or crashing waves in shallow water environments, suggests Sallan.

The finding could explain why we have relatively few fossils of early vertebrates. If these animals first lived on the edges of the ocean, forceful waves may have broken up most bodily remains before they could fossilise.