The discovery of new fossil footprints indicates that our ancestors left the sea for the land at least 18 million years earlier than thought.
"These results force us to reconsider our whole picture of the transition from fish to land animals," says Per Ahlberg of Uppsala University.
A Polish-Swedish team found the prints at Zachelmie Quarry in Poland. They show that large tetrapods, up to three metres in length, inhabited the marine intertidal zone during the early Middle Devonian, some 395 million years ago.
"This means not that not only tetrapods but also elpistostegids originated much earlier than we thought, because the position of elpistostegids as evolutionary precursors of tetrapods is not in doubt, and so they must have existed at least as long," says Ahlberg.
The elpistostegids, it seems, weren't, as previously thought, a short-lived transitional stage, but must have existed alongside their tetrapod descendants for at least 10 million years.
The location is also a surprise. Almost all previous theories for the origin of tetrapods have placed this event in a freshwater setting, and have associated it with the development of land vegetation and a terrestrial ecosystem.
Instead, our distant ancestors may first have left the water in order to feed on stranded marine life left behind by the receding tide, says Ahlberg.
The results are published in Nature.