Fossils Push Back Earliest Complex Animals 40 Million Years

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 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 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.

Original article here.