A cache of microscopic fossils in rocks about 570 million years old contains the earliest record of animal life yet discovered. Scientists say that exquisitely preserved sponges and other primitive animals, described in this week's issues of Science and Nature, promise to shed light on how and when animals first arose--a question paleontologists had feared they could not address for lack of fossils from those times.
The two reports of early animals spring from the same source, a mine bored into the Doushantuo formation in south-central China for its phosphate-rich rock used as fertilizer. Two teams, working independently, scoured samples from the mine for microscopic animals after reports that the phosphate in such rocks had preserved microscopic embryos. A group led by cell biologist Chia-Wei Li of the National Tsing Hua University in Taiwan reports in Science that some of the submillimeter spheroids--at first mistaken for ancient algae--are actually tiny sponges, some of the first multicellular animals thought to exist on Earth.
The second group, led by paleontologist Shuhai Xiao of Harvard University, reports in Nature that other fossils in the mine are early-stage embryos of more sophisticated animals that have bilateral symmetry, the next evolutionary step beyond sponges. Some scientists had thought that symmetrical animals didn't evolve until 10 million to 20 million years later, just before the Cambrian explosion of life-forms.
Embryologists had suggested that it might be impossible to find traces of the earliest animals, says Andrew Knoll of Harvard, one of the discoverers, because for tens or even hundreds of millions of years these animals were probably "squishy little larvalike things" that wouldn't become fossilized. But the Chinese fossils "show the potential of phosphatization as a means of preserving incredible detail" in early animals, says paleontologist Simon Conway Morris of Cambridge University. "The big question is how much farther back this [record] will go."