The oldest-known tracks of a creature apparently using legs have been discovered in rock dated to 570 million years ago in what was once a shallow sea in Nevada.
Scientists think land beasts evolved from ancient creatures that left the sea and evolved lungs and legs. If the new finding is real — the discoverer anticipates skepticism — it pushes the advent of walking back 30 million years earlier than any previous solid finding.
The aquatic creature left its "footprints" as two parallel rows of small dots, each about 2 millimeters in diameter. Scientists said today that the animal must have stepped lightly onto the soft marine sediment, because its legs only pressed shallow pinpoints into that long-ago sea bed.
The tracks were made during what is called the Ediacaran period, which preceded the Cambrian period, the time when most major groups of animals first evolved. Scientists had once thought only microbes and simple multicellular animals existed prior to the Cambrian, but that notion is changing, said Ohio State University Professor Loren Babcock.
"We keep talking about the possibility of more complex animals in the Ediacaran — soft corals, some arthropods, and flatworms — but the evidence has not been totally convincing," Babcock said. "But if you find evidence, like we did, of an animal with legs — an animal walking around — then that makes the possibility much more likely."
Soo-Yeun Ahn, a doctoral student at Ohio State, presented the discovery today at a meeting of the Geological Society of America.
Babcock was surveying rocks in the mountains near Goldfield, Nevada, with Hollingsworth in 2000 when he found the tracks.
"This was truly an accidental discovery," he said. "We came on an outcrop that looked like it crossed the Precambrian-Cambrian boundary, so we stopped to take a look at it. We just sat down and started flipping rocks over. We were there less than an hour when I saw it."
Little can be gleaned about what sort of creature it was, but Babcock is "reasonably certain — not 100 percent" that it was an arthropod, such as one resembling a centipede or millipede, or by a leg-bearing worm. It might have been about as wide as a pencil and may have had multiple, spindly legs.
In 2002, other researchers reported a similar fossil trail from Canada that dated back to the middle of the Cambrian period, about 520 million years ago. Another set of tracks found in South China date back to 540 million years ago. At approximately 570 million years old, this new fossil not only provides the earliest suggestion of animals walking on legs, but it also shows that complex animals were alive on Earth before the Cambrian.
"I expect that there will be a lot of skepticism," Babcock said about the discovery. "There should be. But I think it will cause some excitement. And it will probably cause some people to look harder at the rocks they already have. Sometimes it's just a matter of thinking differently about the same specimen."