ANIMAL LIFE ON EARTH MAY BE TWICE AS OLD.

Byline: John Noble Wilford The New York Times

New molecular research shows that the first simple forms of animal life, progenitors of creatures small and great, from beetles to humans, may have appeared on Earth more than 1 billion years ago, about twice as early as once widely thought.

The findings, based on chemical clues in living creatures, would mean that long before animal organisms grew bones, shells and spines that left a clear mark in the fossil record, minute creatures crawled or slithered between grains of mud or sand in primordial waters. They had no skeletons to be fossilized and uncovered by paleontologists, not even enough substance to leave detectable burrows and tracks.

But for half a billion years, these cryptic organisms presumably were slowly evolving, diverging into different lineages and finally becoming big and complex enough to leave a profusion of fossils, beginning about 565 million years ago in the late Vendian geological period and extending into the Cambrian period.

The emergence then of early animal fossils was so sudden and pronounced, over a relatively short time of 20 million years, that the phenomenon is known as the ``big bang of animal evolution.''

A team of researchers at the State University of New York at Stony Brook made the new discovery using patterns of genetic changes as molecular clocks, a technique that is becoming increasingly fruitful in determining evolutionary history so deep in time that fossils are either absent or of uncertain value.

The scientists, led by Dr. Gregory A. Wray, a molecular biologist at Stony Brook, examined detailed records of seven different genes in different species, revealing their rates of divergence over time from common ancestors. They were thus able to estimate how long the structures of these genes had been changing. They found this to be 1 billion to 1.2 billion years back to the origin of animals. The earliest evidence for life, single-cell organisms resembling bacteria, is 3.6 billion years old.

The results of the analysis are being reported in today's issue of the journal Science. They supported the similar findings of an earlier study in the 1980s that were viewed as intriguing but not definitive. The new work involved substantially more data and extended the putative origin of animals back another 100 million to 300 million years.

In their report, Wray and his colleagues, Dr. Jeffrey S. Levinton and Dr. Leo H. Shapiro, said the analysis of genetic divergences ``cast doubt on the prevailing notion'' that all the major animal groups, or phyla, ``diverged explosively during the Cambrian or late Vendian, and instead suggest that there was an extended period of divergence,'' beginning about a billion years ago.

Other scientists described the findings as interesting and provocative, certain to stimulate debate and more research.

``It gives us a welcome new perspective on the Cambrian explosion,'' said Dr. Andrew H. Knoll, a paleontologist at Harvard University. ``But how much it will cause us to completely rethink the world, I don't know.''

Many paleontologists, who look at the past through the lens of fossils, are expected to question the reliability of the molecular-clock methods for probing deep time.

In a commentary accompanying the report, Dr. Geerat J. Vermeij, a paleontologist at the University of California at Davis, cautioned that acceptance of the proposed ancient origin of animals hinged on the accuracy of the calibration of genetic changes in living organisms and the assumption that the rate of divergence was constant through time. Scientists are not sure such an assumption is valid.

Dr. Bruce Runneger, chairman of the Earth and Space Sciences Department at the University of California at Los Angeles, welcomed the new findings because they seemed to confirm the results of his more limited study based on blood proteins, published in 1982.