They were the width of a few hairs pressed together, but the microscopic fossils discovered in China were enormous in their implications.

The fossils turned out to be the oldest examples of a bilaterian – animals that display bilateral symmetry, meaning their right and left halves are mirror images. The remarkable 2004 discovery pushed back the genesis of complex animal life by as many as 50 million years.

USC College paleontologist David J. Bottjer was among the group that discovered the fossils – period-sized blobs believed to have skimmed the ocean floor with suction-cup mouths some 580 to 600 million years ago.

In the August edition of Scientific American magazine, Bottjer wrote about his experience and these minute, yet developed, creatures. Looking like teensy gumdrops or squashed helmets, they contain tissue layers, a gut, mouth and anus.

In Bottjer’s article, which includes color graphics, he described collecting a truckload of black rocks in Guizhou Province in 2002 with other researchers, including then-USC graduate student Stephen Q. Dornbos. The group joined forces in their quest for the earliest bilaterians at the urging of Eric Davidson, a molecular biologist at Caltech.

Bottjer, a professor of earth and biological sciences, recalled the certainty of another participant, Jun-Yuan Chen, a paleontologist at the Chinese Academy of Sciences in Nanjing. Chen, a pioneer in the study of early animal life, was certain that specimens of bilaterian animals would be found in the ancient rock heap. He was right.

But it took incredible patience and work to uncover the fossils, which measure about 200 micrometers across. The team sliced the samples into thousands of see-through-thin layers and examined them under a microscope. Finally, among the 10,000 slides, the collaborators discovered 10 examples of the fossil type they had been seeking. After more months of painstaking analysis, the group confirmed the examples were fossils of miniscule bilaterian animals.

“We were pretty excited when we saw what we had,” Bottjer recalled. “It was sort of a ‘holy cow!’-like experience.”

They named the find Vernanimalcula, meaning small, spring animal. The name refers to the time they lived after glaciers covered the planet.

The discovery is crucial. It suggests that the earliest ancestors to modern-day animals developed before the Cambrian explosion. That so-called explosion period, 488 to 542 million years ago, envelops the time on Earth when most animal groups first appeared.

In his article, Bottjer suggests that the famous Cambrian explosion was more accurately “the exploitation of newly present conditions by animals that had already evolved the genetic tools to take advantage of these novel habitats.”

Rather than solely genetics, it may have been the critters’ ability to grow large that led to the explosion. The growth spurt, Bottjer said, may have been caused by a drastic rise in dissolved oxygen in seawater. More oxygen for breathing reduces size constraints.

Despite the findings, the quest for fossils of early bilaterians has not ended.

“There’s got to be older stuff out there,’’ Bottjer said. “We have to hope that we can find even older rocks that contain these tiny things.”