Before the Cambrian 550 million years ago, most life on Earth was composed of bacteria and single-celled animals. But then something happened to cause an "explosion" of complex multi-cellular body forms. Scientists have long been puzzled about why this burst of diversity occurred.

Some have suggested that a sudden rise in oxygen allowed larger and more complex life forms to appear and develop. Others have suggested that animal complexity started long before the Cambrian, and that we had only failed to find fossil evidence of it.

Reporting in the journal Science , Jun-Yuan Chen of Nanjing Institute of Geology and Paleontology in China, David Bottjer of the University of Southern California, and colleagues described 10 bilateral fossils they discovered in the Doushantuo Formation in China.

The fossils are the first evidence of bilateral animals -- body plans that have a left and right side, a top and bottom, and a mouth and anus. Early animals like sponges and cnidarians have radial symmetry - rounded body plans that, when cut in half from any direction, are always two mirror images.

"This discovery helps us to learn more about the murky origins of bilaterian animals, which are most of what we see on Earth. You and I would have to go scuba diving to see cnidarians and sponges," Bottjer said in the Science Magazine release.

Before this discovery, the earliest bilateral fossils were mollusk-like Kimberella, Ediacaran fossils that appeared about 20 to 30 million years before the Cambrian. "These fauna were soft a lot of them were just big flat sheets with compartments," according to Bottjer.

"There were probably some bilaterians around. We just don't have much of a record. Then you go back farther in time and we don't have any record of any of these Ediacaran animals. That's where our fossils come in," he told Science.

The Doushantuo Formation in China's Guizhou Province is a phosphorite rock formation that developed in a shallow sea 580 to 600 million years ago. These rocks are already famous for having yielded the oldest known multi-celled embryos. The scientists cut thousands of thin slices from the rock and examined them under a microscope.

In addition to the bilateral animal fossils, the scientists found sponges, cnidarians, eggs and embryos. These soft bodies were preserved due to the phosphate, which, thanks to special conditions, quickly worked its way into the cellular structures.

The bilateral animal fossils measure only a few hundred microns across - they would look like a mere speck to the naked eye, measuring about the same width of two to four human hairs. Pits located near the front end of the body may have been sensory organs. The miniscule animal probably moved through the water by flexing its body. The specimens probably looked like little helmets.

The researchers named the animal Vernanimalcula guizhouena, meaning "small spring animal from Guizhou." The rocks formed after the "winter" of the Earth's final Snowball Earth episode, when glaciers are believed to have covered the planet.

Some paleontologists are doubtful about whether the fossils really are bilateral life forms, or even if they are fossils at all.

But if the features are bilateral fossils, they may point to a trend in pre-Cambrian animal development. If Vernanimalcula were swimming around 600 million years ago, then the Cambrian Explosion may be a reflection of increases in body size rather than in animal complexity.

The Vernanimalcula fossils suggest that "maybe complex animals were around beforehand, and it was just the ability to grow large that caused the Cambrian Explosion," Bottjer said in the Science release.

"The Cambrian Explosion may have had a really long fuse," he added.