The sea-dwelling ancestors of modern-day mammals, reptiles and birds emerged from the water millions of years earlier than previously believed, according to new research published Wednesday.



A set of fossilised footprints pictured in the scientific journal Nature show the first tetrapods - a term applied to any four-footed animal with a spine - were waddling ashore 397 million years ago, well before scientists thought they existed.



An expert unconnected with the research said the find would force experts to reconsider a critical period in evolution when sea-based vertebrates took their first steps toward becoming dinosaurs, mammals, and - eventually - human beings.



"It blows the whole story out of the water, so to speak," said Jenny Clack, a paleontologist at Cambridge University.



Until now, scientists through they had the evolution from fin to foot fairly well understood. The earliest tetrapods had been traced to 385 million years ago. Experts theorized that they had split from their close relatives, a fleshy-finned family of fish known as Elpistostealians, a few million years earlier and then gone on to conquer land.



But the new fossil footprints - uncovered between 2002 and 2007 in a disused quarry in central Poland - push the timing back by several million years, according to Grzegorz Pienkowski, the scientific director of the Polish Geological Institute in Warsaw, where most of the article's authors are based. He said the fossils had been securely dated from the deposits they were found with.



Pienkowski said that the footprints were first created in what was probably a lagoon-like environment at the time - arguing against the commonly accepted notion that fish came ashore from lakes or rivers.



Pienkowski said such a coastal location made sense because shifting tides could strand small marine animals, giving our fishy forebears an incentive to explore open land.



Clack said the new fossils would force scientists - herself included - to reconsider what it was that originally turned fish into land-lovers.



She said some theorised that tetrapods originally went ashore to lay their eggs out of reach of water-going predators or that their ancestors grew legs to scurry from pool to pool. She said she had personally favored the notion that fish emerged from oxygen-deprived waters in order, quite literally, to catch their breath.



None of those theories was supported by the Polish find, she said.



It wouldn't be logical for fish to lay their eggs in a place where the tide would wash right over them, for example, and the pool-hopping behavior wouldn't make sense in a coastal environment. As for her oxygen hypothesis, Clack said "that's probably out the window." The fossils suggested that tetrapods evolved well before marine oxygen levels started to drop, she said.



Nevertheless, Clack warned against relying exclusively on the marks left by a small animal on the bottom of a muddy surface hundreds of millions of years ago. She said it would be critical to see fossil evidence of the creature that made the footprints before coming to any definitive conclusion on exactly how, when and where vertebrates came to colonise the earth's surface.