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Oldest Footprints Show When Life On Earth Got Legs

Tiny fossil tracks found in South China firmly date appendages back to the Ediacaran period

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smithsonian.com

Around 540 million years ago, the oldest ancestors of almost every living creature on Earth burst onto the scene during an orgy of evolution known as the Cambrian Explosion. Unlike the single-celled animals and microbes that lived before the geological age, these creatures had it all: heads, tails, abdomens, shells, and lots of tiny little legs.

That’s at least the story told by the available fossil record. But because creatures from the immediate period before the Cambrian, known as the Ediacaran, were soft-bodied, making their fossils smushy and hard to interpret, researchers have long suspected that the Cambrian “explosion” wasn’t quite as explosive as it seems, and that in fact, some of these structures, especially appendages, may have evolved in the eons before the Cambrian period.

Now, reports Hannah Devlin at The Guardian, researchers claim they have discovered teeny-tiny tracks from the Ediacaran, the oldest footprints ever discovered and evidence that appendages evolved before the Cambrian shift.

These particular fossilized prints were uncovered in the upper Ediacaran Dengying geological formation, which dates back some 551 to 541 million years ago, in the Yangtze Gorges area of South China. According to a press release, the prints consist of two rows of irregular tracks arranged in a series. The researchers believe that the tracks, just a few millimeters in length, represent a bilateral animal—some ancestor of worms or insects—that was able to lift its body off the surface and walk along the bottom of the river in which it lived. There are also burrows associated with the tracks, meaning the creature may have dug into mats of microbes and sediment looking for oxygen and food. The study appears in the journal Science Advances.

In the last two decades, researchers have found multiple sets of fossil bug prints, pushing back the appearance of appendages. LiveScience reports that in 2002, for instance, a little fossil trail discovered in Canada was 520 million years old. Though a set of tracks found in Nevada in 2008 was identified in 570 million-year-old rock, researchers dispute whether those were really bug tracks or just naturally occurring indentations.

This latest find is the first to date firmly from the Ediacaran. “The rock that contains the fossil has been very well dated between 551 and 541 million years old,” lead author Zhe Chen of the Chinese Academy of Sciences tells AFP. “Previously identified footprints are between 540 and 530 million years old. The new fossils are probably up to 10 million years older.”

Shuhai Xiao, geobiologist at Virginia Tech University and the paper’s senior author, tells Devlin that discovering these tracks helps researchers get a handle on Earth’s earliest history. “Animals use their appendages to move around, to build their homes, to fight, to feed, and sometimes to help mate,” he says, pointing out that the activity of even tiny animals can change the geochemistry of Earth and impact the climate. “It is important to know when the first appendages appeared, and in what animals, because this can tell us when and how animals began to change to the Earth in a particular way.”

As to what that earliest little walker looked like, we may never know. According to the press release, while the footprints remain etched in stone, no fossils were found nearby, and it’s possible none will ever show up.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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