The creature wasn't much, says their paper published Thursday in the prestigious journal Science. No more than a slug-like little beastie about the size of a rice grain.
But it crawled around looking for tasty algae some 585 million years ago — 30 million years earlier than any previously known animal was able to move on its own.
"We basically have found the very first evidence of animals that can move," said geobiologist Kurt Konhauser. "Suddenly, we've gone from a sponge-type of animal that just sat there to something that is capable of moving around."
The fossils were found in silt stone excavated in the South American country of Uruguay. The stone was probably created a few hundred metres from the shore of a primordial ocean.
The only trace of the soft-bodied creature is the tracks it left. But that's enough to suggest a little of what it was like.
"It could move up and down through the sediment," said Konhauser.
"Based on the size of the track, we know we're looking at something a couple millimetres in width, maybe a few millimetres in length. We know it was pushing its way through the sediment because at the sides of the burrows are little elevated ridges.
"When we have a close look at the actual sides, there are what looks to be little tracks that are kind of like footprints — some kind of primitive foot or claw it was able to move itself forward with."
That's about all that science can tell us about the ancestor of everything that swims, runs, crawls or flies.
And even that was hard to establish. It took the team four years and repeated trips to Uruguay to nail down the date of the tracks to the satisfaction of the scientific community.
But the value of the find isn't just in the new benchmark for the emergence of mobile animals. It also appears to lend weight to two theories about the Earth's ancient past and the development of life.
It seems to fit with the "snowball Earth" theory, which holds that most of the planet was once covered with glaciers. Those ice sheets ground the early Earth's rugged rocks into fine silts, and life became possible when the glaciers melted and released vast amounts of mineral-rich silts into the sea.
It makes sense that the first animals would evolve where those resources were richest, Konhauser said.
As well, geneticists have used the pace of evolutionary change to project backwards to when they believe animals would first appear. The find made by Konhauer and his team fits right into those models.
"We're pushing real fossils into a time frame when people predicted that these things might have existed," Konhauer said.
"It's linking all these predictions together. Now we've got real proof that they might actually be right."
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