Washington — Newly discovered fossils have given scientists their first glimpse of what happened when Earth made the decisive transition from plants and unrecognizably simple animals to the complex life that took over the world and eventually led to us.
And it happened millions of years earlier than researchers thought.
More than 700 fossils discovered in southwestern China’s Yunnan province offer clues to life 539 million years ago during the late Ediacaran period, when simple but strange animals lived two-dimensionally in the sea without moving up and down, researchers said.
But many of the fossils in this treasure trove are the remains of more complex animals that lived three-dimensional lives, moving and eating underwater, according to a study published Thursday in the journal Science. These features were thought to arise only at least 4 million years later, during the Cambrian period, an explosion of complex and recognizable animal life.
“This is our first window into how the modern animal-dominated biosphere essentially formed, developed, and came into existence through this strange Ediacaran transition period,” said co-author Frankie Dunn, a palaeontologist at Oxford University’s Natural History Museum. “We started in a two-dimensional world, and in a geological blink of an eye, animals diversified. Animals are everywhere. Animals are doing everything, changing biogeochemical cycles. Animals have changed the world.”
Dunn said the new discovery comes after other fossils have been found in an unglamorous but multi-layered roadside outcrop a short distance from the United Nations’ Sumjiang World Natural Heritage Site, “a place where you can literally walk through time, geological time, in the landscape.” And one of those areas provides a “snapshot” where evolution brings forces together.
Dunn said the fossil assemblages at the site include both bizarre examples of life that existed in early times and disappeared, and early examples of organisms that evolved into modern animals. What is important about these more modern animals is that their bodies are almost identical on the left and right sides.
Today, almost all animals on Earth have similar features not only in the head and anus, but also in the left and right sides. Before the fossils were discovered in China, scientists had found traces of this symmetrical body shape in fossil footprints, but not the creatures themselves.
“Now that we’ve found these fossils for the first time, we now know why,” said study co-author Ross Anderson from the Oxford Museum of Natural History.
Until now, there has been conflict in the field of paleontology. Genetic analysis of trait mutations and the rate of evolution suggests that humans and starfish’s earliest common ancestor was during the Ediacaran period, but fossils and rocks do not show that this happened, Dunn said. It was called the “stone vs. clock” debate, she said.
“What our new fossil site tells us is that the rocks and clocks may actually match more closely than we thought,” Dunn said.
Emily Mitchell, a paleontologist at the University of Cambridge who was not involved in the study, said the new study is “very significant because it shows that there were animals on Ediacaran and that there must have been a transitional period between them and the Cambrian fauna. But until now, there was no evidence of this at all.”
Some outside scientists, such as Jonathan Antcliffe of the University of Lausanne, questioned whether there was enough evidence to call these fossils of complex animals, but most experts contacted by The Associated Press felt they were.
Now that scientists know when this burst of life occurred, they have more questions and a few theories.
“I’m very interested in understanding not just when it happened, but interestingly, how it happened and why it happened that way,” Dunn said. “So are there any feedbacks that can disentangle Earth and life, or life and life? Once the Ediacaran reaches the ocean floor, is it inevitable that something closer to the Cambrian explosion will happen? These 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 develop. Then they multiplied, diversified and quickly took over, Dunn said.
Charles Marshall, a paleontologist at the University of California, Berkeley, who was not involved in the study, said this is probably because Earth needed oxygen levels high enough for evolution to occur through genetic changes.
“The Cambrian explosion happened suddenly because we already had a robust development system in place,” Marshall said.
“What fundamentally changed over this period was the way animals on Earth interacted with each other,” said Duncan Murdoch, a curator at Oxford Museums, where many of the authors work. “Once animals showed up and started eating each other and stirring up sediment, they changed the Earth forever. And the Earth we live on is built on the foundations of the Ediacaran and Cambrian periods.”
___
Associated Press journalist Siobhan Starrs contributed from London.
___
Associated Press climate and environment reporting receives funding from several private foundations. AP is solely responsible for all content. Learn about AP’s standards for working with philanthropy, a list of supporters, and funded areas at AP.org.
#Scientists #evolution #early #complex #animals