Nearly 10 years after the genome of a living individual was first sequenced, and shortly after the sequencing of the genome of a 4,000-year-old male Eskimo, scientists have successfully sequenced the mitochondrial DNA (mtDNA) genome of a nearly 40,000-year-old hominin. The study, published recently in Nature , was based on mtDNA extracted from a small piece of bone of a little finger; the bone was found in the Denisova cave in the Altai Mountains in southern Siberia. Mitochondrial DNA, which is passed down the maternal line, is better conserved than nuclear DNA. The mtDNA data indicate that the hominin is probably a new human species that became extinct. The differences in nucleotide positions between the Denisova individual and the modern human are nearly double those between Neanderthals and modern humans. However, nuclear DNA genome data need to corroborate mtDNA findings for the hominin to be called a new human species, and to establish the relationship of the hominin with modern humans and Neanderthals. The same team is now working to sequence the nuclear DNA. The mtDNA data provide some fascinating information: the Denisova hominin, Neanderthals, and modern humans apparently shared a common ancestor about one million years ago; and the divergence of the Denisova hominin is about twice as old as the divergence between the Neanderthal and modern human.
The earlier notion that Neanderthals were our ancestors was overturned when it became clear they lived alongside us. The latest study shows that modern humans and Neanderthals probably lived alongside the Denisova individual, who lived about 30,000 to 50,000 years ago. A 2003 study revealed that we coexisted with another human species, Homo floresiensis, about 15,000 years ago; skeletal remains of H. floresiensis were found on an island in Flores, Indonesia. Independent of the species confirmation, the hominin discovery provides deeper insight into human evolution and suggests that the number of mass migrations of homo species out of Africa was not three, as previously thought, but more. In the case of H. floresiensis, excellent skeletal remains allowed morphological classification but comparable remains have not so far been found in the Altai. However, DNA can be teased out of the small pieces of bones found there as the cold climate favours long-term preservation.
