Conventional wisdom has it that the first animals evolved in the ocean.

Now researchers studying ancient rock samples in South China have found that the first animal fossils are preserved in ancient lake deposits, not in marine sediments as commonly assumed.

These new findings not only raise questions as to where the earliest animals were living, but what factors drove animals to evolve in the first place.

For some 3 billion years, single-celled life forms such as bacteria dominated the planet. Then, roughly 600 million years ago, the first multi-cellular animals appeared on the scene, diversifying rapidly.

The oldest known animal fossils in the world are preserved in South China's Doushantuo Formation. These fossil beds have no adult specimens - instead, many of the fossils appear to be microscopic embryos.

"Our first unusual finding in this region was the abundance of a clay mineral called smectite," said researcher Tom Bristow, now at the California Institute of Technology in Pasadena. "In rocks of this age, smectite is normally transformed into other types of clay. The smectite in these South China rocks, however, underwent no such transformation and have a special chemistry that, for the smectite to form, requires specific conditions in the water - conditions commonly found in salty, alkaline lakes."

The researchers collected hundreds of rock samples from several locations in South China. All their analyses suggest these rocks were not marine sediments.

"Moreover, we found smectite in only some locations in South China, and not uniformly as one would expect for marine deposits," Bristow said. "Taken together, several lines of evidence indicated to us that these early animals lived in a lake environment."

This discovery raises questions as to how and why animals appeared when they did.

"It is most unexpected that these first fossils do not come from marine sediments," said researcher Martin Kennedy, a geologist at the University of California at Riverside.

"Lakes are typically short-lived features on the Earth's surface, and they are not nearly as consistent environments as oceans are," he explained. "So it's surprising that the first evidence of animals we find is associated with lakes, which are far more variable environments than the ocean. You'd expect the first appearance of animals to be in the most conservative, stable environments we could imagine."

It remains possible, Kennedy noted, that animal fossils of similar or older age exist that remain to be found that are marine in origin. However, at the very least, this work suggests "that animals had already taken on the ability to deal with the environmental fluctuations one sees in lake environments," he said. "That suggests that their evolutionary response is much more rapid that I would have supposed, and that the earliest animals were far more diverse than imagined."

If animals did first develop in lakes, one aspect of lake environments that could have spurred on their evolution is how much easier it is for air to percolate through them, given how much shallower they typically are than the ocean.

"The most popular explanation for the evolution of animals has to do with the increase in oxygen in Earth's atmosphere at that time," Kennedy told LiveScience. "It's possible that lakes were the first to benefit from that increase in oxygen."

The scientists detailed their findings online July 27 in the Proceedings of the National Academy of Sciences.

• Image Gallery: Dinosaur Fossils
• Image Gallery Darwin on Display

• Original Story: Oldest Animal Fossils Found in Lakes, Not Oceans