Fossil footprints discovered in Poland have put back the date for the first 'tetrapods' or four-legged animals by 18m years. Evolutionary biologist Per Ahlberg explains the find. Video courtesy of Nature.

The oldest footprints of four-legged creatures ever discovered have forced scientists to reconsider a critical period in evolution: the point at which fish crawled out of the water onto land to evolve into reptiles, mammals and eventually humans.

The "hand" and "foot" prints are 18m years older than the earliest, previously confirmed fossil remains of "tetrapods" or four-legged vertebrates and were left by lizard-like creatures up to 2.5 metres long.

The discovery, reported in tomorrow's issue of the journal Nature, was made in a former quarry in the Holy Cross Mountains in south-eastern Poland. The fossil footprints can be reliably dated to the early Middle Devonian period, around 395 million years ago.



Philippe Janvier of the Muséum National d'Histoire Naturelle in Paris said the finding was as significant as "the first footprint of Neil Armstrong on the moon" and described its effect as akin to "lobbing a grenade" into the previous consensus of when the shift from water to land occurred.

Until now, experts had believed that the earliest tetrapod fossils, traced to about 375 million years ago, had split from their fishy ancestors a few million years earlier and then gone on to conquer the land.

"These prints push back the divergence of fish and four-legged vertebrates by almost 20 million years," said Janvier. "The evolutionary tree as we consider it now remains the same, but the timing of the tree changes."

Tetrapods are thought to have evolved from a family of fish known as elpistostegids, which had a similar body and head shape to tetrapods, but paired fins rather than four feet.

However, the footprint tracks are 10 million years older than the oldest elpistostegid body fossils. They suggest that the fossil elpistostegids were late-surviving relics rather than transitional forms.

Janvier, who said he is convinced that no animal other than an "elusive tetrapod" could have left such imprints, said: "It's really the first evidence we have of an animal with legs and digits walking on land at that time."

The paper's co-author, Professor Per Ahlberg from Uppsala University in Sweden, describes several tracks of different sizes and characteristics as well as a number of isolated prints around 15cm wide. There are distinct "hand" and "foot" prints, with no evidence of a dragging body or tail, because the animals' body weight would have been partly supported by water.

Ahlberg and his co-authors, mainly from the Polish Geological Institute in Warsaw, say their findings highlight how little we know of the earliest history of land vertebrates. They write that the prints "force a radical reassessment of the timing, ecology and environmental setting of the fish-tetrapod transition, as well as the completeness of the body fossil record".

The prints will further "shake up" scientific thinking over human origins, said Janvier, because they show tetrapods thrived in the sea, which is at odds with the long-held view that river deltas and lakes were the necessary environment for the transition from water to land during vertebrate evolution.

"The closest elpistostegids were probably contemporaneous with these tracks," he said. "We now have to invent a common ancestor to the tetrapods and elpistostegids."

Jenny Clack, a palaeontologist at Cambridge University, echoed Janvier's belief that the findings would force scientists to re-examine their beliefs about the timing of the transition to land. "It blows the whole story out of the water, so to speak," she said.

Clack added that it may also give pause for thought over what drove fish from water to land in the first place. Some theorised that tetrapods originally went ashore to lay their eggs out of reach of aquatic predators, or that their ancestors grew legs to scurry from pool to pool. She had favoured 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.

• This article was amended on 7 January 2010. The original referred to a Cambridge palaeontologist as Jenny Clark. This has been corrected.