An ancient sponge, no bigger than a grain of sand, is forcing scientists to rethink their explanation of early animal evolution.

The tiny sponge, which measures just 1.2 millimeters wide, appears 60 million years earlier than expected, boasting evolutionary features that scientists didn't think appeared until the Cambrian explosion. It was discovered by researchers in a ancient rock formation in southwestern China's Guizhou Province.

The Cambrian explosion is a period of time -- beginning 542 million years ago and lasting 20 million years -- when most animal phyla first appeared in the fossil record. The proliferation of fossils have convinced many scientists that the phenomenon marked a period of rapid diversification and adaptation.

But the newly studied sponge -- detailed in the journal PNAS -- puts such an explanation into question.

"This specimen is of an animal that had already evolved a number of fundamental sponge traits," researcher David Bottjer, a professor at the University of Southern California, explained in a press release. "It implies that by the time this animal was living, most of the developmental genes for sponges had evolved."

The sponge lends credence to the argument that early animal adaptations didn't happen all of sudden, but began earlier and happened more gradually. Proponents of this explanation contend that the Cambrian explosion was not the result of rapid speciation, but the simply the result of environmental and biological factors which made fossils likely to be preserved -- and found some 540 million years later.

"Fundamental traits in sponges were not suddenly appearing in the Cambrian Period, which is when many think these traits were evolving, but many million years earlier," Bottjer said. "To reveal these types of findings, you have to use pretty high-tech approaches and work with the best people around the world."

Researchers say a key to better understanding the timeline of early animal evolution will be locating and studying more smaller, older, hard-to-find fossils. New imaging technologies, like electron microscopy, have made the detailed observation of older, smaller fossils possible.

"These organisms don't have all the bells and whistles that modern creatures do," Bottjer said. "But this particular fossil has enough complexity that we can say we hadn't been dating the early evolution of animal traits properly."