WASHINGTON (AP) — Newly discovered fossils have given scientists their first real glimpse of when Earth made a crucial transition from plants and unrecognizably simple animals to the complex creatures that took over the world and would eventually lead to us.

And it happened millions of years earlier than researchers thought.

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

But a study in Thursday’s journal Science said many of the fossils in this trove are remnants of more complex animals that lived three-dimensional lives, traveling up through the water and eating. Those are traits that had been thought to only spring to life at least 4 million years later in the Cambrian period, during what was called the Cambrian explosion of complex and recognizable animal life.

“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,” said co-author and paleontologist Frankie Dunn of the Museum of Natural History at Oxford University. “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 finds were a short distance from a United Nations Chengjiang world natural heritage site for other fossils in an exposure along a roadside that’s not glamorous, but has different layers “where you can literally walk through time, geological time, in a landscape,” Dunn said. And one of those areas provides a “snapshot” where evolution brings forces together.

Complex animals with symmetry developed

In that spot, Dunn said, the group of fossils includes both bizarre examples of life that existed in earlier periods and disappeared, along with early examples of organisms that would evolve into modern animals. What's important in those more modern animals are that their bodies are mostly the same on the left and right.

Nearly all of the animal life on Earth now have similar features on left and right sides, as well as a head and an anus. Before the fossils discovered in China, scientists saw traces of this symmetric 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 Museum of Natural History.

Help in settling ‘rocks versus clocks’ debate

Until now, there was a conflict in the field of paleontology. Genetic analysis of how fast traits mutated and evolved suggested that humans and starfish had their earliest common ancestor in the Ediacaran period, but the fossils or rocks weren't there to show it happening, Dunn said. It was called a debate of “rocks versus clocks,” she said.

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

Emily Mitchell, a paleontologist at the University of Cambridge who wasn't part of the research, 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.”

Some outside scientists, such as Jonathan Antcliffe at the University of Lausanne, questioned whether there's enough evidence to call these fossils of complex animals, 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 happened, they’ve got more questions and some theories.

“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,” Dunn said. “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.”

Life on Earth started 3 billion years ago, but it took another 2.4 billion years before complex animals developed. Then they multiplied, diversified and took over rapidly, Dunn said.

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

Marshall said, "The Cambrian explosion was sudden because of the already rich developmental system that was in place.”

“What fundamentally changed across this period is the way the animals on the planet interacted with each other," said Duncan Murdock, curator of Oxford's museum, where many of the authors work. "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.”

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Associated Press journalist Siobhan Starrs contributed from London.

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