Strange fossils reveal that complex life evolved on Earth much earlier than we thought

‘Amazing’ fossils reset the clock as complex animals evolved

Jackie Flynn Mogensen Edited by Claire Cameron

Approximately 540 million years ago, complex life arose in the ocean. Life rapidly changed from simple, soft-bodied animals living on the ocean floor to animals we recognize today, with bodies such as shells, cartilage, mouths and anuses, and the ability to swim, burrow, and hunt.

Scientists call this short, sharp burst of evolutionary activity the Cambrian explosion, and it has influenced the way we think about how life as we know it evolved on Earth. But the discovery of a bizarre fossil trove in China casts doubt on the consensus that the Cambrian explosion may not have been as explosive as we thought.

Hundreds of fossils discovered in southern China’s Yunnan province reveal that at least some life forms that scientists thought originated in the Cambrian period lived and thrived millions of years ago, during an era known as the Ediacaran period. Many of the fossils look alien, from earthworm-like creatures tethered to the ground to “sausage-shaped” animals to finger-like creatures with tentacles. The research results were published in a magazine science on Thursday.

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Francis Dunn, a natural history fellow at Oxford University’s Natural History Museum and co-author of the paper, said the discovery was something of a coincidence. Her colleagues at China’s Yunnan University discovered this treasure trove while searching for algae fossils on cliff faces in the area. The rock is famous for its ability to preserve ancient life.

Dunn said the serendipitous discovery yielded “some of the most important early animal fossils” discovered in recent decades. More than 700 specimens from the Ediacaran period were located there. Some were simply algae, but hundreds more were animals in “various forms,” ​​she says.

The most common animal the researchers found was a creature about the size of an adult’s index finger with a worm-like body and a disc that roots itself to the ocean floor. Their frequency (more than 100 of the new specimens were examples of this unnamed creature) suggests that they once lived in densities on the ocean floor, Dunn says.

“It was unlike anything I’d ever seen,” Dan says.

But what was most surprising, she says, was that many of the fossils discovered looked eerily like they belonged to the Cambrian period, rather than the Ediacaran period. Some, including large numbers of insects, were “bilaterally symmetrical.” This term refers to animals that are bilaterally symmetrical, or have a body arrangement in which one side mirrors the other. This important evolutionary adaptation helped early life move through sediments and water columns, develop nervous systems, and ultimately “dominate” the animal kingdom, Dunn says. Today, most animals, including humans, are bilaterally symmetrical.

Previously, scientists thought that bilaterians appeared primarily in the Cambrian, were rare in the Ediacaran, and were certainly not as diverse or flourishing.

The new fossils offer a glimpse into a “transitional world,” when simple, flexible life forms coexisted with complex bilateral creatures. Some of the specimens closely resemble Cambroernitids (animals that look vaguely similar to modern sea cucumbers), which until now have been dated only to the Cambrian period.

Dunn says this is a once-in-a-lifetime discovery. “People have been looking for fossil sites like this, but they have never been found until now,” she added. This suggests that the Cambrian “explosion” may have been more gradual. Or, as Dunn puts it, this discovery “quells the Cambrian explosion.”

Now, Dunn and her colleagues are working to formally describe all the fossils and name the new creatures. Once everything is cataloged, scientists can study where these animals fit on the tree of life.

“The fossils from this site will easily keep us busy for about 10 years,” she says.

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