It turns out this happened millions of years earlier than researchers previously thought. More than 700 fossils discovered in southwestern China’s Yunnan province open a window into life 539 million years ago, at the very end of the Ediacaran period—a time of simple but strange animals that lived almost two-dimensionally on the ocean floor, without moving up or down, researchers noted. However, a study published Thursday in the journal *Science* shows that many fossils from this collection are remnants of more complex animals that led three-dimensional lives: they moved through the water column and actively fed. Such traits were thought to have emerged at least four million years later, in the Cambrian period, during the so-called Cambrian explosion of complex and recognizable animal life. “This is really the first window we have into how the essentially modern animal-dominated biosphere originated, developed, and passed through this strange Ediacaran transition,” said study co-author and paleontologist Frankie Dunn from the Oxford University Museum of Natural History. The new fossil sites were found a short distance from the Chengjiang fossil site, a UNESCO World Heritage Site, in exposed roadside layers that may not look spectacular but reveal different strata “through which you can literally walk through time, geological time, in one landscape,” Dunn said. One of these areas provides a “snapshot” of a moment when evolution brings different forces together in the same place. At this site, Dunn noted, the fossil assemblage includes bizarre life forms that existed in earlier periods and later vanished, as well as early examples of organisms that would evolve into modern animals. A key feature of these more modern animals is that their bodies are largely symmetrical from left to right. Nearly all of today’s animal life on Earth has symmetrical left-right body features, along with clearly defined heads and anal openings. Before the fossils discovered in China, scientists had only traces of this symmetrical body type in fossilized movement tracks, but not the organisms themselves. “Now we know what made them, because for the first time we have those fossils,” said study co-author Ross Anderson, also from the Oxford Museum of Natural History. Until now, there has been a conflict in paleontology. Genetic analyses of mutation rates and trait evolution suggested that humans and starfish shared their earliest common ancestor back in the Ediacaran period, but there was no evidence in rocks and fossils that this was actually happening, Dunn explained. This was called the “rocks versus clocks” debate, she added. “Our new fossil site tells us that maybe the rocks and clocks are actually closer to agreeing than we thought,” Dunn said. Emily Mitchell, a paleontologist at the University of Cambridge who was not involved in the research, said the new study “makes a lot of sense, because the Ediacaran contains animals, so we know there must have been a transitional phase between them and the Cambrian fauna. But until now, we didn’t have actual evidence for it.” Some outside scientists, such as Jonathan Antcliffe from the University of Lausanne, questioned whether there is enough evidence to declare these fossils remains of complex animals, but most experts interviewed by The Associated Press believe there is. Now that scientists know when this explosion of life occurred, they have even more questions and several theories. “I’m really interested in understanding not just when it happened, which is interesting, but how it happened and why it happened in that way,” Dunn said, adding: “So, are there feedback loops we can disentangle between Earth and life or between living things themselves. Once you have the Ediacaran on the seafloor, is it inevitable that you end up with something resembling the Cambrian explosion? Those are the questions I find really interesting.” Life on Earth began about three billion years ago, but it took another 2.4 billion years for complex animals to evolve. After that, they rapidly multiplied, diversified, and took over dominance, Dunn said. This is likely because Earth first had to build up a high enough oxygen level, and evolution had to trigger genetic changes, explained paleontologist Charles Marshall from the University of California, Berkeley, who was not part of the research team. Marshall said: “The Cambrian explosion was sudden because of the already rich developmental system that was in place.” “What fundamentally changed during this period is how animals on the planet interacted with each other,” said Duncan Murdock, curator at the Oxford Museum where most of the study’s authors work, adding: “When animals appeared, they began eating each other and churning up sediment, forever changing the planet. And the planet we live on today is largely built on the foundations of the Ediacaran and Cambrian.”