ROANOKE TIMES
Copyright (c) 1996, Roanoke Times
DATE: Friday, October 25, 1996 TAG: 9610250092
SECTION: NATIONAL/INTERNATIONAL PAGE: A-1 EDITION: METRO
DATELINE: WASHINGTON
SOURCE: Knight-Ridder/Tribune
You may have to add 500 million years of ancestors to your family tree.
A study being released today argues that the major branches of life around us probably sprang up twice as early as was previously believed. The prevailing theory is that the ancestors of birds, fish, reptiles, mammals and most plants appeared in one spectacular eruption in the Cambrian era, about 540 million years ago.
The new theory uses mutations in genes as a sort of genetic clock, and suggests a more gradual evolution, starting more than a billion years ago. While the idea of a genetic clock is not new, the study has found a way to predict how fast the clock runs.
``Many paleontologists have read the fossil data literally and said there was an explosion of life in the Cambrian era,'' said Gregory Wray, the chief proponent of the new theory. ``We've brought together a new kind of evidence - the major groups of animals evolved at least twice as long ago.'' The distinction may not affect everyday life today, but scientists find the search for origins fascinating. Since we live in a period when many species are becoming extinct, knowing when our ancient ancestors were born could give insights into what happens during evolutionary changes.
Starting with Charles Darwin, scientists have puzzled over the fact that the fossil record shows a burst of crustaceans, vertebrates, invertebrates, sponges - virtually all the prototypes of life today except single-celled creatures and possibly some plants - in a 20-million-year blip around the Cambrian period. Those dates have been estimated through carbon dating.
Evolution tends to proceed in fits and starts, spurred by sudden changes in the environment, but a spurt of this magnitude would have been nothing short of evolution on jet engines.
``We would have to believe evolution had been occurring at a rate that has never been equaled since or even close to being equaled,'' said Wray, a scientist at the State University of New York at Stony Brook. ``This would be the granddaddy of all those spurts.''
Wray's conclusion, likely to be questioned in the field, is that scientists have focused on the Cambrian period because that's where the fossil trail began. But that doesn't mean there were no animals before, Wray said.
It could be, he said, that pre-Cambrian animals were very small and didn't have skeletons, which are the only body parts that typically get preserved. Also, the rocks that preserve skeletons might not have been formed yet.
``The question'' about the Cambrian period, he said, ``is did they come into being at that time or did they acquire their skeletons at that time?''
To find out, Wray and his fellow molecular evolution researchers constructed a ``genetic clock.''
It works like this:
All living creatures have genes - chemical codes that build bones and muscles and gills and wings - which are passed on from species to species along the tree of evolution.
The extent of differences in genes between species indicates how far apart the species are on the tree, and when they shared a common ancestor.
A gene that helps build hemoglobin, for instance, a component of blood, will be more similar in monkeys and human beings than, say, in human beings and dolphins.
If you knew how fast such genetic changes happened, you could calculate when the ``dolphin gene'' and the ``human gene'' must have been identical - when dolphins and human beings shared a common ancestor.
Wray and his colleagues calculated how fast the gene changes were happening by studying vertebrates - animals with backbones.
Then they compared seven genes across numerous species and, using the clock, calculated when they all shared a common ancestor, about 1 billion to 1.2 billion years ago.
LENGTH: Medium: 72 linesby CNB