The extraction of mitochondrial DNA from Neanderthal specimens has proven to be a key factor in determining whether our ancient human ancestors ever intermingled with Neanderthals. Thus far, the research and testing show that modern humans carry no mitochondrial DNA from any Neanderthal 'Eve'. Mitochondrial DNA (mtDNA) from four Neanderthal specimens was recovered for the project.
Mitochondrial DNA (mtDNA) is DNA locked inside the mitochondria of the cell, but outside the nucleus. Mitochondria are tiny structures that help cells in a number of ways, including producing the energy that cells need. Each mitochondrion includes an identical loop of DNA about 16,000 base pairs long containing 37 genes (Groleau, R 2002). Going back six generations in your own family tree, your nuclear DNA is inherited from 32 men and 32 women. By contrast, your mtDNA would have come from only one of those 32 women (Groleau, R., 2002).
It is inherited virtually intact from a mother to her progeny (Behar, D., 2007), and her descendants can be identified through millions of years and hundreds of generations. And yet it does mutate gradually over time. To estimate the age of the ancestor, they assume that the random mutations occurred at a steady rate. And once they obtained an idea of how much the mtDNA had changed from the ancestor's, all they needed was the mutation rate to determine the age of the ancestor (Groleau, R., 2002). [A rate that has been determined to be 2%-4% per million years (Cann, R., 1987).]
The mtDNA samples were obtained from 1) from a piece of bone cut from the upper arm of the first recognized Neanderthal fossil, the skeleton found at the Feldhofer grotto in the Neander valley in Germany 1856, 2) a 29,000 year-old fossil of a baby recently discovered in Mesmaiskaya cave in south-western Russia, 3) an mtDNA specimen from a cave at Vindija, Croatia, 4) an mtDNA sequence from the second Neanderthal fossil found at Feldhofer, the site in Germany at which the first Neanderthal fossil was found (Foley, J., 2003).
According to the Public Library of Science Journals, "in addition to DNA sequences identical to those previously amplified from present-day humans, the Neanderthal bones Vi-77 and Vi-80 from Vindija yielded four out of 89 and 73 out of 85 mtDNA sequences, respectively, that were identical to previously determined Neanderthal sequences (Serre, D., 2004)." This confirms that they had successfully isolated Neanderthal mtDNA from often degraded fossil bones samples of Neanderthals.
The studies of Neanderthal mtDNA do not show that Neanderthals did not or could not interbreed with modern humans. However, the large differences between Neanderthal and modern human mtDNA would suggest that Neanderthals and modern humans developed separately, and did not form part of a single large interbreeding population. The Neanderthal mtDNA studies strengthens the arguments of those who posit that Neanderthals are a separate species which failed to make any lasting genetic contribution to modern humans.
What other conclusions were drawn from this research? One is that Neanderthals were not more closely related to any one race or ethnic group than any other. Another conclusion is that Neanderthals died out long before the earliest traceable human female progenitor; "the researchers determined that the common ancestor to Neanderthals and modern Homo sapiens lived as long as 500,000 years ago, well before the most recent common mtDNA ancestor of modern humans. [Therefore] Neanderthals went extinct without contributing to the gene pool of any modern humans" (Groleau, R., 2002). It is still possible there was a Neanderthal contribution to the gene pool of modern humans, a contribution that might have been erased by genetic drift or by the continuous influx of modern human DNA into the Neanderthal gene pool (Serre, D., 2004).
What other possibilities could explain this? One is that there was in fact a more recent Neanderthal 'Eve' but she either had no children, had none that survived to reproduce, OR she had only sons. The Genographic Project, a major effort to track human migrations through DNA, has reached some startling conclusions about how close the human race came to extinction -- at one time humanoids numbered possibly as few as 2,000 individuals (Rincon, P., 2008). Perhaps once the easier task of tracking whole chains of mtDNA is accomplished, the Genographic Project might be able to focus on the chore of tagging scattered fragments of Neanderthal DNA.
(ADDENDUM: I erred in hoping that Neanderthal DNA might be recovered and sequenced. I recently read some news article that mentioned that mtDNA is much more durable than regular DNA -- so the chances that any paleontologist will be able to preserve any DNA from a newly unearthed Neanderthal bones are slim to none. My bad.)
Behar, D., et al, Genographic Project Public Participation Mitochondrial DNA Database, PLOS, http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.0030104, 2007.
Cann, R., et al, Mitochondrial DNA and human evolution, Nature, www.nature.com/nature/ancestor/pdf/325031.pdf, 1987.
Foley, J., Fossil Hominids: mitochondrial DNA, TalkOrigins Archive, http://www.talkorigins.org/faqs/homs/mtDNA.html, 2003.
Groleau, Rick, Tracing Ancestry with mtDNA, PBS, http://www.pbs.org/wgbh/nova/neanderthals/mtdna.html, 2002.
Rincon, Paul, Human line 'nearly split in two', BBC Science, http://www.omninerd.com/news/Divergence_and_Near_Extinction_of_Humanity, 2008.
Serre, D., et al, No Evidence of Neanderthal mtDNA Contribution to Early Modern Humans, PLOS,