Thursday, September 18, 2008
neanderthals5
A tall, cheerful Swede, Pääbo is the main engine behind a breathtaking scientific tour de force: the attempt, expected to be completed next month, to read out not just single Neanderthal genes, but the entire three-billion-letter sequence of the Neanderthal genome. Traces of DNA in fossils are vanishingly faint, and because Neanderthal DNA is ever so close to that of living people, one of the biggest hurdles in sequencing it is the ever present threat of contamination by modern human DNA—especially by the scientists handling the specimens. The precautions taken in excavating at El Sidrón are now becoming standard practice at other Neanderthal sites. Most of the DNA for Pääbo's genome project, however, has come from the Croatian specimen, a 38,000-year-old fragment of leg bone found almost 30 years ago in the Vindija cave. Originally deemed unimportant, it sat in a drawer in Zagreb, largely untouched and thus uncontaminated, for most of its museum life.
Now it is the equivalent of a gold mine for prehistoric human DNA, albeit an extremely difficult mine to work. After the DNA is extracted in a sterile laboratory in the basement of the Max Planck Institute, it is shipped overnight to Branford, Connecticut, where collaborators at 454 Life Sciences have invented machines that can rapidly decipher the sequence of DNA's chemical letters. The vast majority of those letters spell out bacterial contaminants or other non-Neanderthal genetic information. But in the fall of 2006, Pääbo and his colleagues announced they had deciphered approximately one million letters of Neanderthal DNA. (At the same time, a second group, headed by Edward Rubin at the Department of Energy Joint Genome Institute in Walnut Creek, California, used DNA provided by Pääbo to read out snippets of genetic code using a different approach.) By last year, dogged by claims that their work had serious contamination problems, the Leipzig group claimed to have improved accuracy and identified about 70 million letters of DNA—roughly 2 percent of the total.
"We know that the human and chimpanzee sequences are 98.7 percent the same, and Neanderthals are much closer to us than chimps," said Ed Green, head of biomathematics in Pääbo's group in Leipzig, "so the reality is that for most of the sequence, there's no difference between Neanderthals and [modern] humans." But the differences—less than a half percent of the sequence—are enough to confirm that the two lineages had begun to diverge around 700,000 years ago. The Leipzig group also managed to extract mitochondrial DNA from two fossils of uncertain origin that had been excavated in Uzbekistan and southern Siberia; both had a uniquely Neanderthal genetic signature. While the Uzbekistan specimen, a young boy, had long been considered a Neanderthal, the Siberian specimen was a huge surprise, extending the known Neanderthal range some 1,200 miles east of their European stronghold.
So, while the new genetic evidence appears to confirm that Neanderthals were a separate species from us, it also suggests that they may have possessed human language and were successful over a far larger sweep of Eurasia than previously thought. Which brings us back to the same hauntingly persistent question that has shadowed them from the beginning: Why did they disappear?
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Now it is the equivalent of a gold mine for prehistoric human DNA, albeit an extremely difficult mine to work. After the DNA is extracted in a sterile laboratory in the basement of the Max Planck Institute, it is shipped overnight to Branford, Connecticut, where collaborators at 454 Life Sciences have invented machines that can rapidly decipher the sequence of DNA's chemical letters. The vast majority of those letters spell out bacterial contaminants or other non-Neanderthal genetic information. But in the fall of 2006, Pääbo and his colleagues announced they had deciphered approximately one million letters of Neanderthal DNA. (At the same time, a second group, headed by Edward Rubin at the Department of Energy Joint Genome Institute in Walnut Creek, California, used DNA provided by Pääbo to read out snippets of genetic code using a different approach.) By last year, dogged by claims that their work had serious contamination problems, the Leipzig group claimed to have improved accuracy and identified about 70 million letters of DNA—roughly 2 percent of the total.
"We know that the human and chimpanzee sequences are 98.7 percent the same, and Neanderthals are much closer to us than chimps," said Ed Green, head of biomathematics in Pääbo's group in Leipzig, "so the reality is that for most of the sequence, there's no difference between Neanderthals and [modern] humans." But the differences—less than a half percent of the sequence—are enough to confirm that the two lineages had begun to diverge around 700,000 years ago. The Leipzig group also managed to extract mitochondrial DNA from two fossils of uncertain origin that had been excavated in Uzbekistan and southern Siberia; both had a uniquely Neanderthal genetic signature. While the Uzbekistan specimen, a young boy, had long been considered a Neanderthal, the Siberian specimen was a huge surprise, extending the known Neanderthal range some 1,200 miles east of their European stronghold.
So, while the new genetic evidence appears to confirm that Neanderthals were a separate species from us, it also suggests that they may have possessed human language and were successful over a far larger sweep of Eurasia than previously thought. Which brings us back to the same hauntingly persistent question that has shadowed them from the beginning: Why did they disappear?
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# posted by roodbont @ 2:49 PM 
