Washington, August 8 : The complete Neanderthal mitochondrial genome sequenced from a 38,000-year-old bone has shown that the last common ancestor of Neanderthals and humans lived about 660,000 years ago, give or take 140,000 years.
Analysis of the new sequence confirms that the mitochondria of Neanderthals falls outside the variation found in humans today, offering no evidence of admixture between the two lineages, although it remains a possibility.
The findings open a window into the Neanderthals' past and help answer lingering questions about our relationship to them.
"For the first time, we've built a sequence from ancient DNA that is essentially without error," said Richard Green of Max-Planck Institute for Evolutionary Anthropology in Germany.
The key is that they sequenced the Neanderthal mitochondria-powerhouses of the cell with their own DNA including 13 protein-coding genes-nearly 35 times over.
That impressive coverage allowed them to sort out those differences between the Neanderthal and human genomes resulting from damage to the degraded DNA extracted from ancient bone versus true evolutionary changes.
Of the 13 proteins encoded in the mitochondrial DNA, the researchers found that one, known as subunit 2 of cytochrome c oxidase of the mitochondrial electron transport chain or COX2, had experienced a surprising number of amino acid substitutions in humans since the separation from Neanderthals.
"We also wanted to know about the history of the Neanderthals themselves," said Jeffrey Good, also of the Max-Planck Institute.
For instance, the new sequence information revealed that the Neanderthals have fewer evolutionary changes overall, but a greater number that alter the amino acid building blocks of proteins.
One straightforward interpretation of that finding is that the Neanderthals had a smaller population size than humans do, which makes natural selection less effective in removing mutations.
That notion is consistent with arguments made by other scientists based upon the geological record, according to co-author Johannes Krause.
"Most argue there were a few thousand Neanderthals that roamed over Europe 40,000 years ago," he said.
That smaller population might have been the result of the smaller size of Europe compared to Africa.
The researchers now hope to get DNA sequence information for Neanderthals that predated the Ice Age, to look for a signature that their populations had been larger in the past.
Technically, the Neandertal mitochondrial genome presented in the new study is a useful forerunner for the sequencing of the complete Neandertal nuclear genome.