The extinct Denisovans live on, if somewhat tenuously, in our genes. Analysis of the DNA of Melanesians has revealed that their genome retains traces of the Denisovans to an extent similar to the presence of Neanderthal genes in people of European or Asian descent.
Few of us are fully human, if by that you mean having a genome entirely descended from the Homo sapiens who left Africa 60,000 years ago. Interbreeding took place with Neanderthals, accounting for an estimated 1.5 to 2 percent of the genes of Europeans and Asians. Even many Africans have some of these Neanderthal genes, probably from journeys back to Africa. A smaller portion of the modern human genome comes from the Denisovans, a population we know only from a tiny fossil record.
There is evidence that Denisovan alleles (gene variations) are more common among people of East Asian and Australian Aboriginal descent, with a particularly high concentration in those genes that provide our immunity. An analysis of the genomes of 1,523 people from around the world has confirmed this.
The new study, which has been published in Science, included 35 people from the Bismark Archipelago, New Guinea. Comparing the genes of living people with the Denisovans gave very few matches for most of the global population, other than the New Guinean Melanesians, where it was between 1.9 and 3.4 percent.
The cave in Siberia in which the first Denisovan fossils were found. ЧуваевНиколай / Wikimedia Commons CC-By-SA-3.0
The other significant observation to emerge from the study is that all the Denisovan DNA appears to have entered the human gene pool at the same time. This does not mean that there was only one assignation between a modern human and a Denisovan, but whatever cross-species fraternization occurred, it took place over a relatively limited period. In contrast, it was already known there were at least two periods where Neanderthal genes contributed to human development, and the paper reports that three actually occurred; one of these occurred after Melanesians became largely genetically isolated from the southeast Asians.
Looking at where the Denisovan genes appear in the genome, the authors report, “We find significantly more overlap in regions depleted of Neanderthal and Denisovan lineages than expected by chance.” They see this as evidence that differences in the frequency of Neanderthal and Devisonan genes between modern populations is influenced by the usefulness of the inherited genes to particular conditions, as much as the proportion of ancestry the extinct species represent.
Denisovan alleles are credited with providing the altitude tolerance that allowed humans to settle Tibet. Those same alleles are rare among inhabitants of low-lying areas who don't need them.
Most of what we know of the Denisovans comes from a portion of a finger bone – from which we were fortunately able to extract DNA – and two teeth. Their evolution, geographic coverage and extinction are almost entirely mysterious.
The oldest known DNA from members of the Homo genus was at one time thought to have come from Denisovans. However, a paper published this week concludes that these ancient people were more closely related to Neanderthals.