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Neanderthal Mitochondrial DNA
Ancient DNA has been retrieved and analyzed from Egyptian mummies and from animals such as quaggas, mammoths, moas and marsupial wolves. The first analysis of mitochondrial DNA (mtDNA) from Neanderthals was published in 1997.
The specimen was taken from the first Neanderthal fossil discovered, from Feldhofer Cave, in the Neander Valley, Germany. A small sample of bone was ground up to extract mtDNA.
The Neanderthal mtDNA sequences were substantially different from modern human mtDNA (Krings et al. 1997, 1999). Researchers compared the Neanderthal to modern human and chimpanzee sequences. Most human sequences differ from each other by on average 8.0 substitutions, while the human and chimpanzee sequences differ by about 55.0 substitutions. The Neanderthal and modern human sequences differed by approximately 27.2 substitutions. Using this mtDNA information, the last common ancestor of Neanderthals and modern humans dates to approximately 550,000 to 690,000 years ago, which is about four times older than the modern human mtDNA pool. This is consistent with the idea that Neanderthals did not contribute substantially to modern human genome.
A second mtDNA sequence, announced in 2000, was derived from a 29,000 year old Neanderthal found in Mezmaiskaya Cave, Russia (Ovchinnikov et al. 2000). Although the Mezmaiskaya Cave sequence was slightly different than the Feldhofer Neanderthal, the two Neanderthal mtDNA sequences were distinct from those of modern humans. These results confirmed the earlier findings that showed that Neanderthals were unlikely to have contributed to the modern human genome. As with the previous study of Neanderthal mtDNA, results were consistent with separation between the Neanderthal and modern human gene pools or with very low amounts of gene flow between the two groups.
Further mtDNA sequences confirmed sequence differences between Neanderthals and modern humans. Researchers compared Neanderthal mtDNA to that of modern humans from different geographic regions. If Neanderthals had interbred with modern humans in Europe, then researchers would have expected to find more similarities between Neanderthals and Europeans than between Neanderthals and other modern humans. However, Neanderthals were equidistant from modern human groups, which is consistent with genetic separation between modern humans and Neanderthals. However, this does not explicitly disprove admixture because interregional gene flow between modern humans could have swamped the Neanderthal contribution to Europeans (Relethford 2001).
Researchers have also studied ancient DNA from anatomically modern Homo sapiens from Europe dating to the same time period as the Neanderthals. Material from two Paglicci Cave, Italy individuals, dated to 23,000 and 25,000 years old, was sequenced. The Paglicci Homo sapiens mtDNA sequences were different from all Neanderthal mtDNA sequences but were within the range of variation for modern human mtDNA sequences (Caramelli et al. 2003). Mitochondrial DNA from the Paglicci specimens as well as other ancient humans fit within the range of modern humans, but the Neanderthals remain consistently genetically distinct. This shows that early anatomically modern Homo sapiens were not very different genetically from current modern humans, but were still different from Neanderthals. Though this evidence does not disprove the idea of Neanderthal and modern human admixture, it shows that moderns and Neanderthals did not have more genetic similarities during the Pleistocene that were subsequently lost. If interbreeding did occur, Neanderthal mtDNA sequences could have been lost due to genetic drift.