By LEE BOWMAN
Scripps Howard News Service
November 24, 2006
The largest comparison of gene codes yet done determined that the last common ancestor of humans and Neanderthals lived about 700,000 years ago. And researchers found that no matter how much interbreeding may have taken place in the nearly half-a-million years that the two were separate species, it left little or no mark on the genetic code of either one.
Although scientists have been fascinated with Neanderthals since the first bones were found in Germany 150 years ago, the relatively few specimens and artifacts found with them have produced a lot more questions than answers about how our cousins lived alongside our human ancestors in Europe and western Asia until the Neanderthals disappeared about 30,000 years ago.
"All the theories on Neanderthals have been based on a few artifacts," said Edward Rubin, senior author of a study appearing in the journal Science and a researcher at the U.S. Energy Department's Lawrence Berkeley National Laboratory.
"It's been a bit like studying ancient Egypt with all the stones, but no hieroglyphics. When we were able to crack that code, we began to learn about everyday life. By having a Neanderthal gene sequence, we'll be able to learn things about their biology that we can never learn from the bones."
Rubin's team collaborated with a team from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, led by Svante Paabo, a group that first extracted ancient mitochondrial DNA from a Neanderthal sample a decade ago.
Despite the impressions from various movie themes, getting DNA out of samples more than a few hundred years old is tricky, and especially so with rare Neanderthal bones.
For one thing, most of the bones have been handled by a lot of human researchers over the years, contaminating them with human genes. Then, too, old bones attract bacteria and plants that leave their own genetic material behind.
Luckily, after researchers sifted through 70 Neanderthal bones, one from a cave near Vindija, Croatia, proved to have minimal genetic material from modern humans. Working with a couple of grams of bone, the two teams used separate techniques to tease out more than 65,000 and 1 million base pairs of Neanderthal genes, respectively, after filtering out millions of genes from bacteria and plant material.
"We've shown that we can use this process to go after the entire Neanderthal genome with some expectation of success," said Paabo, whose team published its results in the journal Nature.
So far, by randomly comparing the Neanderthal genes with some of those from the complete human gene map, the researchers calculated that our cousins differed from us by about 3 million base pairs of genes, out of a total genetic code of more than 3 billion pairs.
By contrast, the genetic code of chimpanzees, which diverged from human ancestors more than 6.5 million years ago, differs from our species by about 50 million base pairs.
Having a nearly complete Neanderthal map, which the researchers think they'll be able to draft in a few more years, would not only explain a lot about the traits of the hardy Ice Age clan, but also contribute a great deal to understanding how modern humans made the last evolutionary steps to what we are today, and help understand many of the genetic quirks that contribute to diseases in humans today.
"Any scientists studying a particular gene that's functionally important would be able to look and compare counterparts not only for chimps but for Neanderthals, and see what's happened with humans," Paabo said.
"We're never going to bring a Neanderthal back to life, but by comparing what we know from the human genome, the more we will know about Neanderthals and humans," Rubin said. "We can't go back in time, but this data can serve as a DNA time machine that can tell us things we can never know from artifacts."
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