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A new study pushes back by hundreds of millions of years the start of the evolutionary process that led from tiny squishy creatures in the ocean to the wide diversity of species, including humans, now in the animal kingdom.

In a study published today in the journal Science, researchers at the State University of New York at Stony Brook trace the genetic heritage of animals back in time to a point when the variety of animals now living may have shared a common ancestry.The grand divergence - the theoretical start of genetic changes that led to many species - began slowly about 1.2 billion years ago and is still reshaping the animal world, says Jeffrey S. Levinton, a co-author of the study.

This finding is far different from conclusions that generations of scientists have drawn from the study of ancient fossils, said Levinton.

"Up to now, it has been believed that the higher animals emerged about 545 million years ago, at the beginning of what is known as the Cambrian period," said Levinton. "Our data suggests that it happened farther back, about 1.2 billion years ago, and that it occurred over a span of about 200 million years."

The animals that first began to divide into the different phyla, or species types, were fragile and not likely to leave a fossilized imprint in rock that was then forming, Lev-in-ton said.

"The early representatives of the animal groups were probably very small and soft-bodied and not very preservable," he said. "It is probably that what existed were little squishy things that didn't have many of the characteristics of the modern animal groups."

What happened, Levinton said, is that the "squishy things" developed slightly different genetic patterns that continued to change in separate directions, eventually evolving into the thousands of animal forms that now exist.

It was not until the Cambrian period, he said, that the various animal groups began developing the hard-bodied characteristics that could be preserved in rock. Those changes may have occurred relatively rapidly, perhaps over eight million years, he said, but they were based on a genetic foundation that evolved earlier.

All of the animals a billion years ago lived in the ocean, probably migrating up and down the water column, grazing on plankton that already existed, he said.

Life is thought to have started about 3.5 billion years ago and some fossilized bacteria date that far back. Plants probably appeared next and the early primitive "squishy" animals later.