Neanderthal and Denisovan DNA Helped Melanesians Survive in Hot Tropical Island Environment

There are genetic variants in modern Melanesians that are found in very few populations. Scientists writing in the publication Science think they have worked out where they came from—Neanderthals and Denisovans.

The cross-species fornication early humans engaged in with their near-relatives is no secret. Each and every person of non-African descent carries traces of DNA derived from ancient hominin species to a varying extent; a chunk of genetic material that comes with its own set of pros and cons.

Take, for example, Neanderthal DNA, which has been linked to depression, social fear and promiscuity, as well as diseases such as type 2 diabetes, Crohn's disease and lupus. It may even increase our propensity to smoke. And yet, at the same time, it is thought that the genetic variants inherited from Neanderthals–including those that affect skin and hair color–may have helped Eurasian ancestors survive in the colder, harsher climes of the northern latitudes.

Modeling suggests the ancestors of Africans and non-Africans diverged sometime approximately 74,000 years ago, while Melanesians diverged from East Asians at a time close to 52,000 years ago. Ignoring the last 3,000 years or so when there have been major influences from people from the west, the Melanesian population has remained relatively isolated since the split, the study authors say.

Wicked Walu Island
Genes inherited from Neanderthals and Denisovans may have helped Melanesian adapt to the tropical island environment of Melanesia. Pictured: Melanesian children float on a bamboo pontoon by Wicked Walu Island on the resort-studded Coral Coast of Fiji. TORSTEN BLACKWOOD/AFP/Getty

The hot, tropical island environment of Melanesia—a group of islands to the northeast of Australia that includes Solomon Island, Fiji, Vanuatu, and Papua New Guinea—is likely to have required certain adaptations related to diet, infectious disease and body size, they add. What's more, genetic mapping shows that Melanesians carry some of the highest percentages of ancient hominin DNA. The genome of modern Melanesians is 3 to 5 percent Denisovan and 1 to 3 percent Neanderthal.

While previous studies have focused on single-nucleotide variants (SNVs)—that is, where there is an alteration involving one nucleotide (A, T, C, G) in the DNA sequence—few have looked at copy number variants (CNVs).

These are structural variants involving changes to the number of copies of sections of DNA, which contain multiple nucleotides and can be replicated or deleted. These, the study authors say, are subject to stronger selective pressure and are more likely to be related to genotype expression—this contributes to a person's observable traits. One of those highlighted in the study was a CNV associated with Denisovans that affects the number of recurrent rearrangements linked to autism. Another is a CNV associated with HIV-1 acquisition and breast and ovarian cancers in some Eurasian populations.

While the relation between CNV and observable traits is complex, the study "tentatively" suggests these genetic differences inherited from Neanderthals and Denisovans may have triggered beneficial adaptations related to metabolism, immune response and energy expenditure in Melanesian populations that would have helped their ancestors adapt to the local environment. And CNVs inherited from ancient hominins in other populations may have had a similarly beneficial effect.

"The results tentatively suggest that CNV introgression from ancestral hominins may have allowed modern humans to adapt to new environments by providing a source of beneficial genetic variation," the study authors conclude.