PNG kept genetic diversity after agriculture, unlike Europe
While the development of farming has often been linked to greater genetic homogeneity, in Papua New Guinea it had the opposite effect, writes Andrew Masterson.
When agriculture emerged in Europe more than 10,000 years ago, communities began to intermingle more and genetic diversity across the continent decreased.
At roughly the same time, agriculture also developed in Papua New Guinea, but the effect on the genetic composition of the human population was very different.
Genetic research published in the journal Science reveals that the Papuan population, already genetically isolated from other societies, became even more distinct. Communities became less interconnected, severing along geographic and linguistic fault lines.
Geographic division occurred not only in the sense of horizontal separation, but also in vertical terms, with a strong bifurcation between highland and lowland populations. Division along linguistic lines was particularly complex. Papua New Guineans speak about 850 languages, divided into 85 broad groups – accounting for 10% of all languages in the world.
Scientists led by Anders Bergström of the Wellcome Trust Sanger Institute genotyped 381 Papuans covering all 85 language groups, looking at 1.7 million markers. They also analysed 39 already existing whole genome sequences from the island.
In their paper Bergstrom and his colleagues say their analysis supports the idea that the people who migrated to the ancient continent of Sahul – comprising modern day mainland Australia, Tasmania and Papua New Guinea – represent “an independent instance of human genetic and cultural evolution” over more than 50,000 years.
(Other recent research pushes that estimate even further back, such as the findings at Madjedbebe in Australia’s Northern Territory that date human settlement in that area to at least 65,000 years ago.)
Gene sequencing revealed that a very sharp division between highland and lowland Papuans emerged about the same time that highland communities began to cultivate plants.
The analysis showed that lowland Papua New Guineans had a small genetic inflow from southeast Asia during the Holocene period, which began around 11,000 years ago, but only to the same degree that Australian indigenous genotypes reveal. (Bergstrom’s team states, by the way, that Papuan and Australian indigenous communities had been distinct long before the seas rose between the two land masses about 8,000 years ago.)
Highland communities, however, showed no evidence “of ancestry from non-Sahul sources”.
In an accompanying editorial, writer Ann Gibbons observes that the DNA analysis suggests that the emergence of agriculture was not in itself a sufficiently powerful cultural force to encourage communities to interbreed.
This suggests that the genetic cultural homogenisation seen after the beginnings of agriculture in Europe might have occurred as a result of later migrations rather than any intrinsic pattern associated with agriculture itself.
Bergstrom’s team suggests the genotypic divisions found in Papua New Guinea may have more than one cause. The sheer size of the mountains might have been sufficient to keep lowlanders and highlanders apart. Other factors, such as war and a practice of in-group marriage, might also have contributed to community isolation and thus the development of specific genotypes.