WASHINGTON — It was an unfortunate accident, but a lucky break for modern science.
About 38,000 years ago, a Neanderthal man living in what's now Croatia broke his left arm, forcing him to use his other arm for most tasks. That increased the mass and density of the bone in the upper right arm, and preserved his DNA for researchers — using a dentist's drill — to recover many millennia later.
With that bit of material, along with scraps of DNA collected from half a dozen other Neanderthal fossils, scientists have now completed a rough partial draft of the genome of humans' prehistoric cousins.
The Neanderthals lived for hundreds of thousands of years in Europe and western Asia, but went extinct about 30,000 years ago. They were replaced by Cro-Magnons, the ancestors of modern humans.
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Svante Paabo, a geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his team of researchers announced their achievement Thursday, on the 200th anniversary of the birth of Charles Darwin.
"We really need to compare ourselves with our closest relatives," Paabo said. "They're not very different from us."
By contrasting the two genomes, scientists hope to discover "what makes humans human, and what makes modern humans the way they are?"
The goal is a "catalog of all the differences between the human and Neanderthal genome," Paabo said. "For the vast majority, human DNA looks like Neanderthal DNA."
Despite the similarity, Paabo ruled out any attempt to use the genome to bring a Neanderthal back to life.
"We won't be able to recreate a Neanderthal from DNA even if we wanted to," he said. "It is and will remain impossible."
The Neanderthal DNA, however, could shed light on how early humans increased their brain power and developed the ability to use language. The two species share a variant of a common gene, FOXP2, that plays a role in the ability to speak.
"We cannot say from this they could speak," Paabo said. "We can just say there is no reason to assume they couldn't speak."
According to Paabo, humans and Neanderthals shared a common ancestor about 830,000 years ago. The two lines gradually diverged until truly modern humans, Homo sapiens, arose about 200,000 years ago.
"Neanderthals were a separate branch of humanity, our closest relatives,'' said Henry Harpending, an anthropologist at the University of Utah, who was not part of Paabo's team. "I call them human."
Paabo was skeptical about speculation that humans and Neanderthals may have mated with each other, even though the two species overlapped in Europe for thousands of years.
If Neanderthals contributed to the human gene pool, "it was very small, if anything. It's tiny," he said.
However, researchers may now be able to see if they can find human DNA in a Neanderthal. "Interbreeding is a two-way street," he added.
Paabo said the decoding, or sequencing, of the Neanderthal genome required "revolutionary new technology" developed by a firm called 454 Life Sciences, a division of pharmaceutical giant Roche in Branford, Conn. The technology uses fiber optics to read stretches of DNA at a blinding speed.
"This was a humongously challenging project," said Michael Egholm, a vice president of 454 Life Sciences. "The amount of DNA is extremely limited."
The draft covers about 63 percent of the 3.2 billion base pairs — the chemical units that make up DNA — in the Neanderthal genome.
So far, the team has gone through the genome once, meaning that many gaps and errors remain.
Over the next two years, they will repeat the process 10 to 20 times to achieve much greater accuracy.
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