New fossils in China push back timeline for complex animals

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Summary

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Scientists have reported new fossils from China’s Yunnan province that they say provide a first detailed look at how Earth moved from simple, unrecognizably basic marine life to the complex animals that later came to dominate the planet. The fossils, described in a study published in the journal Science, are dated to about 539 million years ago—during the waning end of the Ediacaran period. Researchers said the finds show that parts of the evolutionary shift toward complex animal life happened millions of years earlier than many scientists had thought.

More than 700 fossils found in the region offered a window into life from the late Ediacaran, a time when simple but strange animals are known to have lived in the oceans, often described as flat or two-dimensionally arranged. But the Science paper reported that many of the fossils in the trove are instead remnants of more complex animals that lived three-dimensionally—moving through the water and eating. Those traits, the researchers said, were previously thought to have emerged at least about 4 million years later, during the Cambrian period known for the appearance of complex and recognizable animal life.

Frankie Dunn, a co-author and paleontologist at the Museum of Natural History at Oxford University, described the fossils as a key missing link for understanding the transition. Dunn said the discovery gives researchers “This really is the first window we have into how basically the modern animal-dominated biosphere was formed and developed and came through this weird Ediacaran transitional interlude,” adding, “We go from a two-dimensional world, and within the geological blink of an eye, animals have diversified. They’re everywhere. They’re doing everything, and they’re changing biogeochemical cycles. They’ve changed the world.”

The new fossils were found in a roadside exposure near a United Nations Chengjiang world natural heritage site known for other fossils. Dunn said the area includes multiple layers and that the landscape allows researchers to “literally walk through time, geological time, in a landscape.” He said one location in particular provides a “snapshot” where evolutionary forces come together and become visible in the fossil record.

In that spot, Dunn said the fossil group includes both earlier odd life forms and early examples of organisms that would evolve into modern animals. He said what stands out in the more modern lineages is left-right symmetry in bodies. Nearly all animals alive today share similar left and right features as well as traits including a head and an anus, and before the Chinese fossils were found, scientists had seen traces of this symmetry in fossil tracks but not the organisms themselves.

Ross Anderson, a co-author also at Oxford’s Museum of Natural History, said the new fossils make it possible to connect the earlier traces to the actual animals that produced them. Anderson said, “Now we know what’s making them because we have those fossils for the first time,” describing the discovery as the first direct evidence matching the observed body-plan patterns.

The new findings also address a dispute in paleontology that Dunn described as “rocks versus clocks.” In broad terms, genetic analyses have suggested an early common ancestor for animals such as humans and starfish in the Ediacaran period, but Dunn said fossils showing the transition had not been there. Dunn said the new fossil site suggests that “perhaps the rocks and the clocks are in closer agreement than we thought,” portraying the relationship between timing in rocks and timing inferred from evolutionary change as tighter than previously supported by fossils.

Emily Mitchell, a paleontologist at the University of Cambridge who was not part of the research, said the study fits the overall expectation that the Ediacaran contained animals and that there must have been a transitional stage before Cambrian animal life. Mitchell said the new study “makes a huge amount of sense because the Ediacaran contains animals, we know there must have been a transitional stage between them and the Cambrian fauna. But until now we didn’t really have any evidence of this.” At the same time, she noted the lack of direct evidence had been the key gap.

The Associated Press reported that outside scientists questioned whether the evidence was sufficient to label the fossils as complex animals. Jonathan Antcliffe at the University of Lausanne was among those who questioned the interpretation, while most experts contacted by the news organization said the fossils were consistent with complex animal life.

Even with a revised timeline, the researchers said the fossils also open new questions about how and why the transition unfolded. Dunn said he is interested not only in when it happened but also in the mechanisms, describing questions about feedbacks between Earth and life, or between life and life. Dunn said, “I’m really interested in understanding, not just when it happened, which is interesting, but how it happened and why it happened the way that it happened,” adding, “So whether there are feedbacks that we can disentangle between Earth and life or between life and life. Once you have Ediacaran on the sea floor, is it inevitable that you’ll end up with something approaching a Cambrian explosion? They’re the kinds of questions that I find really interesting.”

Other scientists said environmental change and developmental biology likely mattered. Dunn said life on Earth started about 3 billion years ago but took another 2.4 billion years before complex animals developed, and he described subsequent rapid multiplication and diversification. Charles Marshall, a paleontologist at the University of California at Berkeley who was not part of the research, said the Cambrian explosion was sudden because of the already rich developmental system in place, saying, “The Cambrian explosion was sudden because of the already rich developmental system that was in place.”

Duncan Murdock, a curator of Oxford’s museum where many of the authors work, said the transition involved interactions among animals that changed Earth’s systems. Murdock said, “Once animals turned up and started eating each other and churning up the sediment, they changed the planet forever. And the planet that we live on is very much built on the foundations from the Ediacaran and Cambrian.”