It's hard to get a bearing on the kind of organisms that existed before life absolutely exploded onto the scene during the Cambrian Period. Fossils from the Ediacaran Period before that are relatively scant, since lifeforms hadn't yet evolved the kinds of shells and body types that fossilize easily, but they did leave other traces of themselves behind. Now, paleontologists have discovered a set of tiny fossilized burrows, created by some of the earliest complex organisms more than half a billion years ago.

The fossils in question date back to the transition between the Ediacaran and Cambrian Periods, about 541 million years ago. That places them just before the Cambrian explosion, an event where life hit the ground running and diverged into almost all of the animal kingdom's broadest groups, called phyla, over a relatively short period of about 25 million years.

"This is an especially exciting find due to the age of the rocks – these fossils are found in rock layers which actually pre-date the oldest fossils of complex animals – at least that is what all current fossil records would suggest," says Russell Garwood, an author of the new study. "The evolutionary events during the Ediacaran–Cambrian transition are unparalleled in Earth history. That's because current fossil records suggests that many animal groups alive today appeared in a really short time interval."

Rather than physical remains like bones, the new finds are trace fossils: tracks and burrows made by ancient organisms. Discovered in sediment in western Brazil, these fossilized tunnels are tiny, measuring between 50 and 600 microns wide. That means the creatures responsible would have been about the size of a human hair.

In order to find burrows this small, the team used a process called X-ray microtomography, which allows researchers to build a 3D computer model of an object without damaging the original.

The researchers compare the culprits to modern-day roundworms, and they probably used a similar undulating method of getting around. Using muscles in this way implies a level of complexity to the organisms that didn't seem to have been around that early – at least, according to the fossil record. That's because back then, animal remains weren't hardy enough to fossilize well, making these trace fossils the earliest record of their existence.

"Our new fossils show that complex animals with muscle control were around approximately 550 million years ago, and they may have been overlooked previously because they are so tiny," says Luke Parry, lead author of the study. "The fossils that we describe were made by quite complex animals that we call bilaterians. These are all animals that are more closely related to humans, rather than to simple creatures like jellyfish. Most fossils of bilaterian animals are younger, first appearing in the Cambrian Period."

The age of these complex creatures is backed up by DNA studies called "molecular clocks." According to this idea, DNA evolves at a steady rate, so tracing back the path can help identify when two species split from a common ancestor, and ultimately peg when different types of life first arose. Molecular clock studies have previously suggested that some complex organisms should have arisen during the Ediacaran, which is in agreement with the Brazilian trace fossils.

"Our discovery highlights an unexplored window for tracking animal evolution in deep time," says Parry.

The research was published in the journal Nature Ecology & Evolution.

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