Neanderthal Bone Yields Complete Mitochondrial Genome

Discovery Channel

Neanderthal Bone Yields Complete Mitochondrial Genome
Jennifer Viegas, Discovery News


Aug. 7, 2008 -- DNA extracted from a 38,000-year-old Neanderthal bone has just enabled scientists to sequence the complete mitochondrial genome for the human-like species, according to a paper that will be published tomorrow in the journal Cell.

The remarkable feat, which has led to at least three major discoveries about the extinct stocky European individuals, represents a breakthrough for studies on the human family.

"This is the first complete mitochondrial genome sequence from an extinct hominid," lead author Richard Green explained to Discovery News.

Mitochondria, which an individual inherits from his or her mother, are cellular powerhouses that possess their own DNA and include 13 protein-coding genes. The researchers sequenced the Neanderthal mitochondria 35 times to ensure their findings were as accurate as possible.

After studying the newly completed genome, Green, a researcher at the Max-Planck Institute for Evolutionary Anthropology in Germany, and his team first concluded that the Neanderthal mitochondria falls outside the range of variation found in humans today, offering no evidence that interbreeding occurred between them and us.

The researchers are quick to add that such interbreeding could still have happened and that the Neanderthals' "exact relationship with modern humans remains a topic of debate."

Clearer is the fact that Neanderthals and humans split from a common ancestor around 660,000 years ago. The researchers based this initially upon prior research that determined humans and chimpanzees diverged from each other six to eight million years ago.

They calculated mtDNA sequence changes for both humans and Neanderthals since that time. These accumulated changes then "let us calculate how long ago was the most recent common ancestor of humans and Neanderthals," Green said.

He added, "This common ancestor likely looked something like Homo erectus." This extinct hominid is believed to have been super strong with a relatively large head and brain.

What most surprised the scientists was how little purification acted upon the Neanderthal's DNA, meaning that the elimination of slightly deleterious alleles, or variant gene forms, didn't occur very often within the population.

"One sensible explanation for this could be a very small effective population size," Green said, explaining that only a few thousand Neanderthals may have roamed Europe around 40,000 years ago, close to when they went extinct.

It's unclear if this was a general feature of the Neanderthal population, perhaps explained by the fact that they had to deal with repeated glaciations, or if some population bottleneck "happened late in the game," he said.

Perhaps the biggest modern human revelation to come out of the project is that there was an explosion of certain amino acid substitutions within the human genome after the Neanderthal/human split.

"What we can say is that there was a lot of change in a very short time within modern humans," Green said. "Further work will be necessary to say what the consequences of these changes were."

John Hawks, assistant professor of anthropology at the University of Wisconsin, Madison, told Discovery News that he has "been waiting a long time for this sequence to come out," adding that "all previously reported sequences of Neanderthal mtDNA" were fragmentary when compared to this one.

Geneticist David Reich at the Harvard Medical School also agrees that the newly sequenced genome "is exciting and important."

"The most striking thing about the paper is that it shows that the authors are able to get an extremely reliable DNA sequence out of a (38,000-year-old) Neanderthal fossil especially when they do a large amount of DNA sequencing," Reich told Discovery News, mentioning that it then "becomes obvious that the sequence the authors are obtaining is correct."

Green and his team are already at work on yet another Neanderthal genome project -- sequencing the complete Neanderthal nuclear genome -- that should be finished by the end of the year. It should answer, once and for all, whether or not modern humans and Neanderthals interbred to such a degree that the mixing would have resulted in a Neanderthal genetic contribution to the modern human gene pool.


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