http://evolution.berkeley.edu/evolibrary/news/100501_xwomanNews update, August 2015Since we first reported on this story in 2010, X-woman's lineage has been the subject of a flurry of scientific investigations that have reshaped our view of these early human relatives. First, though, a bit of terminology: the lineage has received a new moniker —
X-woman is a Denisovan, a name that comes from the cave in which the fossils were first discovered. Further research in that location uncovered two additional Denisovan fossils: teeth. In addition, scientists have discovered a femur in a cave in Spain that has genetic ties to Denisovans, but there is still uncertainty about to which lineage the original owner of this bone belonged.
Scientists have also delved into the evolutionary history of Denisovans by decoding their entire genome — not just the mitochondrial genes. The bulk of the Denisovan genome (i.e., their nuclear DNA) tells a very different story than their mitochondria do. Though Denisovan mitochondrial DNA is equally closely related to Neanderthal and human mitochondria, the rest of their genome is not.
It now seems that Denisovans and Neanderthals are more closely related to one another than either is to Homo sapiens.The other update on Denisovan evolutionary history is that Denisovans interbred with both Neanderthals (
about 17% of Denisovan DNA seems to come from Neanderthals) and ancient humans. Denisovans contributed in their own small way to the human lineage through this interbreeding. The highest percentage of Denisovan DNA in modern humans is found in Melanesian populations, which can trace nearly 5% of their genomes back to Denisovans. As results from newly discovered fossils and ancient DNA accumulate, the human branch of the tree of life is getting bushier, and interbreeding seems to be more common than we had previously thought. This might explain why the Denisovan mitochondrial DNA at first led us astray; perhaps X-woman's mitochondrial genome was introduced to Denisovans through interbreeding with another, as yet unknown hominin lineage.