The flux of genes on the South Seas

Huli Wigman from the Southern Highlands, Painting of Tahitian Women on the Beach by Paul Gauguin

Many demographic models utilized in genetics are rather simple. Yet the expansion and retreat of various demes in post-Ice Age Europe seems to be far more complex than had previously been assumed, though I suspect part of the rationale for the original simplicity was a preference for theoretical parsimony in the face of a paucity of data. The landscapes traversed by our species are rich and topographically convoluted. Not only does the land vary, from plains, to deserts, to mountains, but the climate shifts radically over time and space. In the pre-modern age when humans were more dependent on environmental exigencies these fluxes in ecological and climatic parameters were essential in sharping the arc of human demographic expansion and contraction.

This is why a closer examination of the prehistory of Oceania is so appealing: here you have a physical geography which is radically constrained and so reduces the degrees of freedom of human movement and habitation. Unlike Europe, South Asia, or much of Africa, the time depth of the residence of the current indigenous inhabitants of Australia is on the order of 40 – 50,000 years. It seems likely that the indigenous people of the island of New Guinea to the north are from the same original settlement of Sahul, the ancient super-continent which consisted of New Guinea, Australia, and Tasmania. After the initial sweep out to the farthest reaches of what became Tasmania, there was a later push to the east of New Guinea, to the Solomon Islands,~30,000 years before the present. Then nothing for tens of thousands of years. The march of humanity seemed to stand still on the shores of the Solomons, just as the hominin lineage had once been cordoned off from Sahul by the forbidding seas between it and Sundaland, the Ice Age peninsula of Southeast Asia which was later submerged and became the western portion of Indonesia and Malaysia. The stasis was shocked by the Austronesians, a seafaring peoples who seem to have exploded out from somewhere between Borneo and Taiwan within the last 10,000 years, likely just on the margins of written history. The most famous of th Austronesian peoples are the Polynesians, who pushed across the Pacific, and likely even had some tentative contact with the New World. A less well known case is Madagascar, whose inhabitants speak an Austronesian language with clear affinities to a dialect of Borneo. The map below shows rough distribution of Austronesian peoples:

Austronesian expansion

Of particular interest for the purposes of this post is the expanse to the east: Melanesia and Polynesia, Near Oceania and Far Oceania. A new paper in Current Biology , Demographic History of Oceania Inferred from Genome-wide Data, examines the genetics of this region of the world in light of history utilizing a ~1 million marker SNP-chip:

We developed a new approach to account for SNP ascertainment bias, used approximate Bayesian computation simulations to choose the best-fitting model of population history, and estimated demographic parameters. We find that the ancestors of Near Oceanians diverged from ancestral Eurasians 27 thousand years ago (kya), suggesting separate initial occupations of both territories. The genetic admixture in Polynesian history between East Asians (87%) and Near Oceanians (13%) occurred 3 kya, prior to the colonization of Polynesia. Fijians are of Polynesian (65%) and additional Near Oceanian (35%) ancestry not found in Polynesians, with this admixture occurring considerably after the initial settlement of Remote Oceania. Our data support a greater contribution of East Asian women than men in the admixture history of Remote Oceania and highlight population substructure in Polynesia and New Guinea.

Like Dienekes I think there’s something off with the dates they’re generating here. The archeology tells us New Guinea was settled by humans 15-20,000 years before this paper finds that they diverged from other Eurasians! We know that Aborigines are the closest to Papuans genetically, so if they separated from Eurasians less than 30,000 years ago, that would mean that the original inhabitants of Sahul were replaced after that period by the current groups. Far simpler I think to assume that something is off with their timing. Below are the primary figures, a frappe bar plot, PCA, and tree 7b which illustrates the most supported pattern of population branching and admixture.

There’s nothing too revolutionary in this paper. Rather, it seems to be an exploratory analysis of Oceanian genetics, a precursor to what may come soon. They can not, it seems, differentiate between the slow-boat and express-train models of the settlement of Polynesia, though the consistent pattern of Melanesian admixture seems to lean toward some form of slow-boat, because that is the theory which emphasizes a longer interaction with Melanesian populations.

I know I emphasized the relative simplicity of Oceania in relation to other parts of the world in terms of interpretation because of the geographical constraints, but even here there are layers and twists in the genetic and cultural bedrock. To the left is a map of the Papuan languages. From what I can tell Papuan languages are actually a negation of Austronesian and other well supported language families. The key is to notice that some parts of Near Oceania, Melanesia, have been shifted toward Austronesian languages, though New Guinea is a general exception to this pattern. Remember that humans did not move past the Solomons for ~30,000 years. Large scale settlement of Madagascar seems to have occurred only with the arrival of the Austronesians within the last 2,000 years (after a likely sojourn in East Africa!). This was a genuine cultural revolution which radically shifted the terrain of the possible. And yet by and large the Papuans resisted assimilation to the Austronesian cultural toolkit, which seems to have been otherwise so successful. Why? The Papuans were well equilibrated to their own local ecology, and the Austronesians had no comparative advantage. Rather, the Austronesians, in the form of the Polynesians, struck out into unknown waters and innovated. They found low hanging fruit by discovering trees which had been neglected.

The second interesting point is the bias toward Austronesian mtDNA, and substantial admixture on the Y lineages from Papuans even among Polynesians. The standard explanation of this is that the Austronesians had some aspect of matrilineal descent and matrilocality in terms of communal fission. I think that the Austronesians are arguably a perfect example of the leap-frog pattern of migration, and yet unlike most continental leap-frogs the genetic signal seems to be stronger on the female than male side. There is some evidence of the same in Madagascar. This indicates to me that the Austronesian maritime expansions were qualitatively different from continental leap-frogs, which often were based on the mobility of men on horses.

As I said, the picture remains broadly the same. But there are some touch ups and clarifications on the margins, and that is worthwhile. And definitely an appetizer for what is to come.

Citation: Wollstein A, Lao O, Becker C, Brauer S, Trent RJ, Nürnberg P, Stoneking M, & Kayser M (2010). Demographic History of Oceania Inferred from Genome-wide Data. Current biology : CB PMID: 21074440

Image Credit: Nomadtales, Wikimedia