Siberian rocks clock biological big bang.

The first U.S. geologists granted access to work in a remote corner of northeast Siberia have succeeded in dating the evolutionary explosion at the beginning of Earth's Cambrian period-a biological burst that produced almost all major groups of modern animals in an astonishingly short span of time.

Prior to the so-called Cambrian explosion, animals had simple body plans lacking hard parts, and worm-like organisms ranked as the most complex creatures. The Cambrian period brought a leap in innovation with the appearance of animals sporting novel features such as shells, skeletons, legs, and antennae. That event transformed life. The Cambrian marked the birth of most complex animal phyla on Earth today, including the arthropods, mollusks, echinoderms, and our own group, the chordates. Since then, advanced animals have stuck with those same basic body plans; no new ones have evolved.

While charts of geologic time generally show the Cambrian beginning around 570 million years ago, research in the last decade has revealed that date as too early, making it difficult to pin down the length of the explosion. The new dating work suggests the peak evolutionary frenzy actually began 530 million years ago and lasted only 5 to 10 million years.

"People have thought that it was fast, but the timescale was so poorly calibrated that nobody could begin to think about [evolutionary] rates," says geochronologist Samuel A. Bowring of the Massachusetts Institute of Technology. Bowring and his colleagues from MIT, Harvard University, and the Yakutian Geoscience Institute in Yakutsk, Russia, report their findings in the Sept. 3 SCIENCE.

The researchers collected rocks from the early Cambrian period at several sites near the mouth of Siberia's Lena River. They dated zircon crystals within the rocks using a method that relies on the radioactive decay of uranium to lead. When zircons form in underground magma chambers, they incorporate uranium atoms and exclude lead atoms. As the uranium decays, though, lead accumulates in the crystals. By measuring the ratio of remaining uranium to lead, researchers can judge the zircons' ages.

Bowring's group determined the Cambrian started 544 million years ago, a date much later than the traditional one but within the range hinted at by previous measurements. While some creatures appeared for the first time in the fossil record during this primary stage of the Cambrian, the diversity of known fossil groups jumped most dramatically during the period's next two stages, the Tommotian and the Atdabanian.

From their work on the Siberian rocks and those found recently in New Brunswick, Canada, Bowring and his colleagues dated the beginning of the Tommotian at approximately 530 million years ago. Work by other researchers has suggested the Atdabanian ended 525 million years ago. Because of uncertainties in the dates and the starting point of the stages, Bowring judges that the Cambrian explosion lasted at most 10 million years and as little as 5 million.

Interest in the Cambrian has surged in recent years, in part because of remarkable fossil discoveries made in southwest China and northern Greenland, which coincided with a reevaluation of fossils found early this century at Canada's Burgess Shale (SN: 7/11/92, p.22). The new dating work will help researchers trying to understand what caused the flowering of new phyla and why it never recurred. Some scientists think the explosion followed an environmental change, such as a rise in the oceanic oxygen concentration. Others suggest that genetic or developmental innovations within organisms allowed them to form new types of body architecture.

"If we try to find out what mechanisms -- ecological or genetic -- caused this rapid diversification, then surely the absolute rates are very important because they put some limits on what you can suggest," says Stefan Bengston, an expert in Cambrian fossils at Sweden's Uppsala University.