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

The “hand” and “foot” prints are 18m years older than the earliest confirmed fossil remains of tetrapods, or four-legged vertebrates, and were left by animals resembling lizards up to 2.5 metres long. The discovery, reported in Nature on Thursday, was made in a former quarry in the Holy Cross mountains in south-eastern Poland and can be reliably dated to the early Middle Devonian period, around 395m years ago.

Philippe Janvier, of the Museum 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 on when the shift from water to land occurred.

Until now, experts had believed the earliest tetrapod fossils, traced to about 375m 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 20m 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 10m years older than the oldest elpistostegid body fossils. They suggest that the fossil elpistostegids were late-surviving relics rather than transitional forms, and highlight how little is known of the earliest history of land vertebrates. Janvier, who said he was convinced 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 trackways of different characteristics as well as a number of isolated prints around 15cm wide. They have distinct “hand” and “foot” prints, with no evidence of a dragging body or tail, as the animals would have had their body weight supported by water.

Jenny Clark, a palaeontologist at Cambridge University, echoed Janvier's belief that the findings would force scientists to re-examine their previous beliefs over the timing of the transition to land. “It blows the whole story out of the water, so to speak,” she said.—Dawn/The Guardian News Service