Addressing Ancient DNA
The difficulties in addressing ancient DNA (aDNA) are quite complex. The elegant double helix DNA chain breaks apart within a few months of an organism's death, and damage to DNA over time is not unexpected. Dr. Pããbo, speaking in a teleconference to reporters on November 13, described the search process. "We looked through around 70 different specimens of Neanderthals from around Europe and western Asia. Because these specimens are so valuable, we removed very tiny samples of just around 10 mg of bone and look for amino acid composition. ... If that looks well preserved, if it looks like there's collagen in there and that the amino acids are not too much damaged, then we go on and remove a bigger sample of about 100 mg of bone and extract DNA from that."Then we do the crucial test of looking for how much Neandertal DNA is in there relative to contaminating modern human DNA. And for that we use our knowledge about the mitochondrial genome where we can tell the difference between Neandertal and modern human. We have found that in most of these bones we have a large proportion of contamination.
"But in this one bone from Vindija, we were very lucky to find that about 1-2 percent of the human DNA comes from contaminating modern DNA, and around 98% or 99% of the human DNA is of Neanderthal origin."
A Tool for the Future
The focus of this project has been on the sequencing--that is to say, on describing the patterns of the way the DNA fits together and (in Dr. Rubin's case) in cataloging and placing the data on the Internet for use by other scholars. Not much research on the DNA has been completed, although initial findings suggest that modern humans and Neanderthals last had much 'social interaction'--at least what would generate gene flow--about 400,000 years ago or some 300,000 years before the appearance of AMH. More recent mixing--that between AMH and Neanderthals--has not been ruled out, although no evidence has been found to date.The researchers believe the real worth of the project will begin when the genome is completed, and then can be compared to chimpanzee and modern human genome sequences. Then, Pããbo and Rubin believe, researchers will have the ability to find out what traits we have that Neanderthals didn't, that enabled our success on this planet.
Said Dr. Smith, "This is simply incredible work. Who would have thought 15 years ago we'd be sitting and discussing extracting nuclear DNA from an earlier form of life?"
For More Information
Svante Pããbo was recently profiled in an extensive article in Smithsonian magazine, called Neanderthal ManSome of the following references are not as yet available online but I will add the links as soon as they become available.
James C. M. Ahern, et al. 2004 New discoveries and interpretations of hominid fossils and artifacts from Vindija Cave, Croatia. Journal of Human Evolution 46:27-67.
Tom Higham, et al. 2006 Revised direct radiocarbon dating of the Vindija G1 Upper Paleolithic Neandertals. Proceedings of the National Academy of Science 10(1073):1-5.
James P. Noonan, et al. 2006 Sequencing and Analysis of Neanderthal Genomic DNA. Science 314:1113-1118.
Richard E. Green, et al. 2006 Analysis of one million base pairs Neanderthal DNA. Nature 444:330-336.
David Serre et al. 2004. No Evidence of Neandertal mtDNA Contribution to Early Modern Humans. PLoS Biology 2(3).
