A series of fossils unearthed in southwestern China has revealed the origins of complex life in unprecedented detail, and pushed its beginning back by at least 40 million years.

The specimens come from the Doushantuo formation, a layer of sediments deposited about 590 million years ago, just before the Ediacaran period's primordial fauna gave way to the kaleidoscopically complex creatures of the Cambrian explosion.

During the Ediacaran, even the most structurally complicated animals had flat bodies with simple symmetry, like living quilts or mattresses. It was only during the Cambrian that animals developed what's known as bilateral symmetry — a distinct front and back, top and bottom.

The Doushantuo fossils date to the cusp of this transition, and are so finely preserved that scientists can distinguish the structures of individual cells. The latest fossils, described Monday in the Proceedings of the National Academy of Sciences, aren't even fully formed animals, but embryos.

Using synchrotron radiation microtomography — a microscopy technique that combines thousands of of X-rays taken from different angles — researchers reconstructed the embryos in three-dimensional detail. They found that the embryos were bilaterally symmetrical, and were organized so differently that they belonged to two distinct taxonomic groups. For those groups to be so different, bilateral symmetry must have been around for a while. Some scientists have suspected as much, but without such solid evidence.

"These bilaterians had already diverged into distantly related groups at least 40 million years before the Cambrian radiation," wrote the researchers. "The last common ancestor of the bilaterians lived much earlier than is usually thought.

Images: PNAS

See Also:

*Citation: "Complex embryos displaying bilaterian characters from Precambrian Duoshantou phosphate deposits, Weng’an, Guizhou, China." By Jun-Yuan Chena, David J. Bottjer, Gang Li, Michael G. Hadﬁeld, Feng Gao, Andrew R. Cameron, Chen-Yu Zhang, Ding-Chang Xian, Paul Tafforeau, Xin Liao, and Zong-Jun Yin. Proceedings of the National Academy of Sciences, Vol. 106 No. 42, Oct. 20, 2009. *

*Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecosystem and planetary tipping points. *