Animal Beginnings Go Back A Billion Years New Study Finds Evolution Started Millions Of Years Earlier Than Previously Thought
More than a billion years ago, squishy little critters in a primordial soup started the evolutionary process that eventually led to people, penguins, elephants, earthworms and the thousands of other species now in the animal kingdom.
A new study pushes back by hundreds of millions of years what is thought to be the era when the variety of creatures now living may have shared a common ancestor.
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, who co-authored the study with other gene researchers at the State University of New York at Stony Brook.
That finding, to be published today in the journal Science, is far different from conclusions historically drawn from the study of fossils, said Levinton.
Fossils first appeared in the Earth’s geologic record about 500 million years ago, an era often referred to as the “Cambrian explosion” because of its apparent suddenness and variety.
But Levinton and co-authors Gregory A. Wray and Leo H. Shapiro say their study shows the animals that left those first fossils were really late arrivals in the long parade of evolution.
“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, said Levinton.
“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, said Levinton, 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 only later, during the Cambrian period, 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, said Levinton, 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 there are fossilized bacteria that date that far back. Plants probably appeared next and then later the early primitive “squishy” animals.
Levinton and his colleagues arrived at the new origin date for animals by mathematically tracing backward the changes that are known to occur in genes.
“There is a characteristic rate of change,” said Levinton. “Different proteins evolved at different rates, but at a relatively constant rate. It is almost like setting a clock. You can get a calibration from the changes.”
They used seven different sequences of genetic change and examined a variety of different phyla, or animal types.
“The genes have different rates of change, but they average to be 1.2 to 1 billion years for the emergence of the different phyla,” said Levinton.
Geerat J. Vermeij, a biologist at the University of California, Davis, said the study is an important advance in the understanding of evolution pacing.
“This separates out the events of the Cambrian explosion from the very early divergence of animals,” said Vermeij.
He said there are questions among biologists about the rate of genetic changes, but that the study’s important finding is that the changes occurred gradually and started at a point much earlier than the Cambrian.
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