LONDON (AP) — The water-dwelling ancestors of modern-day mammals, reptiles and birds emerged onto land millions of years earlier than previously believed, researchers report.

A set of fossilized footprints show that the first tetrapods — a term applied to any four-footed animal with a spine — were treading open ground 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.

The work appears in Thursday’s issue of the journal Nature.

Until now, scientists thought 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, 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.

Although at least some of the footprints may have been made in shallow water, paleontologist Per Ahlberg, one of the article’s co-authors, said it was nevertheless clear from the shape of the toe prints and the nature of the sediment that the animals spent time walking around on land.

“We know from site that you have rain drop prints and mud cracks in the sediment,” he said, noting also that the prints appeared far too crisp to have been made underwater.

While the find is important, it also challenges the commonly accepted notion that tetrapods colonized the surface from lakes or river beds. Ahlberg and his colleagues argued that the footprints were first created in what was probably a lagoon-like environment at the time, adding that a coastal location made sense because shifting tides could strand small marine animals, giving our fishy forebears an incentive to explore open land.

Although she acknowledged their importance, Clack warned against drawing conclusions exclusively on small marks left by animals 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 colonize the earth’s surface.

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

She said some theorized 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.

All those theories were called into question 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.

Ahlberg said paleontologists were already scouring the area for more evidence of footprints — and fossils of the animals themselves.

“Obviously the hunt is on,” he said.

(This version CORRECTS that animals’ marks were small, instead of the animals themselves.)