Scientists get a first look at the evolution of early complex animals

WASHINGTON (AP) — Newly discovered fossils have given scientists their first real glimpse of when Earth took over the world from plants and unremarkably simple animals to complex organisms and eventually us.

And it happened millions of years earlier than researchers thought.

More than 700 fossils found in southwestern China’s Yunnan province provide a window into life 539 million years ago, the end of the Ediacaran period, a time of simple but unique animals that lived two-dimensionally in the oceans, never going up or down, researchers said.

But a study published Thursday in the journal Science says most of the fossils in the trove are the remains of complex organisms that lived three-dimensional lives, traveled through water and ate food. They are characteristic of what was thought to be life only after at least 4 million years during the Cambrian period, known as the Cambrian explosion of complex and recognizable animal life.

“This is really the first window we have into how the modern animal-dominated biosphere formed and evolved from this unique Ediacaran transitional interval,” said co-author Frankie Dunn, a paleontologist at Oxford University’s Natural History Museum. “We go through 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 bio-chemical cycles. They’ve changed the world.”

The new finds were a short distance from the United Nations’ Xinjiang World Natural Heritage Site for other fossils on roadside display that aren’t flashy, but have different layers “where you can literally walk through time, geologic time, the landscape,” Dunn said. And one of those areas provides a “snapshot” of where development brings forces together.

Complex animals evolved with symmetry

At that location, Dunn said, the group of fossils contains strange examples of life that existed and disappeared during earlier periods, early examples of organisms that would evolve into modern animals. What is significant about those more modern animals is that their bodies are often identical on the left and right.

Almost all animal life on Earth has the same features on the left and right sides, as well as the head and anus. Before the fossils were discovered in China, scientists had seen traces of this symmetrical body type in fossil tracks, but not the critters themselves.

“Now we know what’s making them because we have those fossils for the first time,” said study co-author Ross Anderson, also of Oxford’s Natural History Museum.

Help settle the ‘rock vs clock’ debate

Until now there was conflict in the field of paleontology. Genetic analysis of how quickly traits mutated and evolved suggested humans and starfish had an early common ancestor during the Ediacaran period, but there were no fossils or rocks to show it was happening, Dunn said. It was called the “stone versus clock” debate, she said.

“What our new fossil site tells us is that the rocks and clocks are actually in closer agreement than we thought,” Dunn said.

Emily Mitchell, a paleontologist at the University of Cambridge, said the new study “makes a lot of sense because there are animals in the Ediacaran, we know there must have been a transitional stage between them and Cambrian animals. But until now we had no evidence of that.”

Some outside scientists, such as Jonathan Antcliffe of the University of Lausanne, questioned whether there was enough evidence to call these complex animals fossils, but most experts contacted by The Associated Press felt they were.

Trying to figure out how and why

Now that scientists know when this life explosion occurred, they have more questions and some theories.

“When it happened is interesting, but I’m really interested in understanding how it happened and why it happened,” Dunn said. “So there are feedbacks that we can sort out between Earth and life or between life and life. Once you have the Ediacaran on the sea floor, are you inevitably going to end up with something approaching the Cambrian explosion? Those are the kinds of questions that I find really interesting.”

Life on Earth began 3 billion years ago, but it took another 2.4 billion years for complex animals to evolve. Then they multiplied, diversified and quickly took over, Dunn said.

That’s probably because Earth had to build up enough oxygen levels and evolution had to kick in with genetic changes, said University of California, Berkeley paleontologist Charles Marshall, who wasn’t part of the research.

“The Cambrian explosion happened suddenly because there was already a rich evolutionary system in place,” Marshall said.

“What changed fundamentally during this period was the way the animals on the planet interacted with each other,” said Duncan Murdoch, curator of the museum in Oxford, where many of the authors work. “Once animals appeared and started eating each other and churning up sediment, they changed the planet forever. And the planet we live on is built on a lot of Ediacaran and Cambrian foundations.”

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Associated Press reporter Siobhan Starrs contributed from London.

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