Deflating the biological Big Bang.

Charles Darwin and generations of biologists since him have struggled to explain the sudden appearance of animal fossils just after the start of Earth's Cambrian period, 543 million years ago. If evolution were a slow process, as Darwin believed, how could so many groups of animals emerge so abruptly in a biological version of the Big Bang?

A trio of molecular biologists has finally come to Darwin's rescue. Using genes as a clock to time the speed of evolution, they find that various animal phyla emerged about a billion years ago, leaving time for a protracted period of evolution. Gregory A. Wray, Jeffrey S. Levinton, and Leo H. Shapiro of the State University of New York at Stony Brook report their results in the Oct. 25 Science.

To construct their molecular clock, the Stony Brook scientists analyzed the DNA sequences of seven genes in living animals. Gene sequences change naturally with time. By assuming that the genes mutated at a relatively constant rate, the researchers could use the genetic differences among animals to test how long ago their ancestors split apart.

According to this analysis, the branching of several invertebrate phyla took place about 1.2 billion years ago. Invertebrates diverged from chordates-the phylum to which vertebrates belong-about a billion years ago.

These conclusions have upset some paleontologists because the fossil record shows no sign of such early animals. The oldest fossilized creatures hail from the Vendian period at the end of Precambrian time, roughly 565 million years ago. "Paleontologists like to have hard evidence in the form of fossils," says Wray. "But there is the distinct possibility that we're not going to find fossils [from these earliest animals] because they might have been tiny and squishy and not able to leave a fossil record."

If so, the creatures would have evolved for hundreds of millions of years before they eventually grew large enough to produce recognizable fossils at the end of the Precambrian.

Douglas H. Erwin, a paleontologist at the Smithsonian Institution's National Museum of Natural History in Washington, D.C., says the new study improves on past molecular clock efforts. But he takes issue with the technique because Wray and his colleagues calibrated the clock for only the last 500 million years. To pinpoint the emergence of animals, they had to extrapolate rates of genetic change much further back than their calibration point. If genes mutated more quickly early in animal history-as some scientists suspect-then this technique would overestimate the age of animal phyla.

Wray and his colleagues contend that they tested the reliability of the molecular clocks by examining rates of genetic change for different animal groups and for much simpler organisms, such as yeast and fungi. In all cases, the tests indicated steady rates of mutation.

Bruce Runnegar of the University of California, Los Angeles hails the new study because it supports estimates he obtained in 1982 by using blood protein as a molecular clock. "The molecular sequence information is showing us very strongly that the radiation of the animal groups can't have happened at the Cambrian-Precambrian boundary. I don't think we know how early it happened, but it's some long time before the Cambrian," says Runnegar.