Analysis of Ancient DNA Suggests A Previously Unknown Type of Extinct Human Ancestor

Denisova cave from the outside. / Bence Viola.

DNA isolated from a tiny bone fragment from the finger of a being that walked the earth between 30,000 and 50,000 years ago has possibly added a new branch to the human family tree.

The piece of bone was found in the Denisova Cave in the Altai Mountains, Russia.  Thanks to the preserving effects of the area’s relatively chilly temperatures, researchers from the Max Planck Institute for Evolutionary Anthropology were able to recover mitochondrial DNA (mtDNA) suitable for sequencing.

Their results, published online today in the journal Nature, show that compared to present-day humans, mtDNA from the so-called “Denisova hominin” contains twice as many differences as do mtDNAs from Neanderthals.  From this the researchers conclude that the Denisova hominin was a member of a previously unknown species of human ancestor.  Further analysis allowed them to estimate that the most recent common maternal ancestor shared by all three species lived about one million years ago.

“Their discovery is remarkable not just for the insight it gives into the human past.  For the first time a hominin has been described, not from the morphology of its fossilized bones, but from the sequence of its DNA,” wrote Terence A. Brow, in a News & Views article that accompanied the Nature study.

There is evidence that both Neanderthals and anatomically modern humans were in the area of the Denisova cave at around the same time as the individual whose mtDNA was sequenced in the current study. This suggests that each wave of hominins that moved out of Africa and into Eurasia did not replace the last.  Instead, multiple lineages coexisted for long periods of time.

More work will be needed to definitively determine the relationship of the Denisova hominin to Neanderthals and modern humans.  Ongoing research is attempting to isolate nuclear DNA for further comparisons and to look for evidence of interbreeding.

Update 3/29/10: There’s quite a bit of discussion about whether or not the Denisova hominin really is a distinct species. A good place to learn more about the debate is Carl Zimmer’s blog The LoomThe Atavism also has a post about this topic.






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