Evolution may have started earlier than previously thought
<Picture: graphic>October 25, 1996
Web posted at: 5:20 p.m. EDT=20
WASHINGTON (AP) -- 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.=20
In a study published Friday 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.=20
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.=20
This finding is far different from conclusions that generations of =
scientists have drawn from the study of ancient fossils, said Levinton.=20
"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."=20
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, Levinton said.=20
"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."=20
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Full Summary from Science Now
http://www.sciencenow.org/html/961024a.htm
Thursday, 24=A0 October 1996 6:00 PM
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Animal Life's Ancient Origins
Animals appeared on Earth more than a billion years ago--twice as early =
as previously estimated, according to a provocative study published in =
the 25 October issue of Science.
Paleontologists had traced the origins of animal life to roughly 565 =
million years ago, during an explosion of diverse animal life-forms in =
fossils dated to the Cambrian Period. However, microbiologist Gregory =
Wray of the State University of New York, Stony Brook, and others =
question whether the fossil record tells the whole story. If more =
ancient animals were too small or too "squishy," Wray says, they would =
have left no trace in the fossil record.
Wray and his colleagues therefore attempted to trace animal =
life--defined as multicellular creatures whose cells are held together =
by collagen--back to its origins without heading to a paleontology dig. =
Instead, they used a "molecular clock." They assumed that nucleotide =
bases--the building blocks of DNA--change into other nucleotides at a =
constant rate. Wray's team counted nucleotide differences in DNA of =
living species from 16 phyla, which are groups of related species. They =
examined species sharing a known common ancestor, such as humans and =
carp. By plotting the number of nucleotide changes against the estimated =
time since the pair diverged, the researchers calculated "how fast the =
clock was ticking," Wray says. His team applied this rate to species =
pairs with no known common ancestor--such as humans and clams--to =
determine how much time had elapsed since the species had diverged. =
Winding their molecular clock backward through time, Wray's team =
estimated that the mother of all animals must have been born 1 billion =
to 1.2 billion years ago.
Not everyone is convinced that Wray's clock ticks at a constant rate, =
however. Although the study is interesting, says Harvard University =
paleontologist Andrew H. Knoll, "molecular clocks are not Timexes." =
University of California, Davis, geologist Geerat J. Vermeij agrees that =
it's better not to assume a constant rate. "What I am advocating is that =
people sit down and test this assumption," he says.
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