The Austronesian explosion was crazier than we thought

The Austronesian languages, credit
The Austronesian languages, credit

The Austronesians were crazy and extraordinary. Starting about ~5,000 years ago they set off from the environs of Taiwan, and began to push outward. For ~30,000 years the people of Melanesia had defined the eastern edge of human habitation, but the Polynesian branch of the Austronesians blasted past that, going alway the way to Hawaii and Easter Island. At the other extreme the ancestors of the Malagasy settled Madagascar, and island which the peoples of Africa had not reached as of yet despite ~200,000 years of human habitation. We don’t know what was happening here, but it is hard to pinpoint particular cultural, environmental, or genetic forces which might result in these sorts of radical change in mores. Humans are conservative and cautious by nature. But our particular lineage of modern humans far less so than our forebears or cousins. After all we did make it Oceania and the Americas, while the others did not.

But a great unresolved question is contact with the Americas. There’s a lot of suggestive evidence, but no clincher. But two recent papers increase the probability considerably. Both are inĀ Current Biology, Two ancient human genomes reveal Polynesian ancestry among the indigenous Botocudos of Brazil and Genome-wide Ancestry Patterns in Rapanui Suggest Pre-European Admixture with Native Americans. Alexander Kim has already reviewed the nuts & bolts of the first paper. Here’s the major finding: heretofore the reasonable assumption about these Polynesian remains in interior Brazil were the product of escaped slaves, but there is an 80-90% probability that they died before any such enslavement of Polynesians could have occurred. In fact both remains may be pre-Columbian!

Cite: Genome-wide Ancestry Patterns in Rapanui Suggest Pre-European Admixture with Native Americans
Cite: Genome-wide Ancestry Patterns in Rapanui Suggest Pre-European Admixture with Native Americans

The second paper has a somewhat more subtle result. The inhabitants of modern day Easter Island are descended in the main from the Polynesians who arrived from the west. This has long been known from classical genetics and non-genetic fields. There has also been suggestion of European and Amerindian admixture. Entirely reasonable in light of Easter Island being a possession of Chile, and 19th century migratory events. What these authors did is that by looking at the distribution of ancestry outcomes in the genomes of Easter Islanders, they inferred that the admixture with Amerindians far predated that with Europeans. The rationale here is simple: recent ancestry from divergent groups tends to exhibit patterns of long alternating blocks, due to a relatively small number of recombination events. In contrast older ancestry tends to be broken up by many recombination events over the generations, until deconvolution can’t separate the two elements and they fuse as one. As an example of the latter case modern day Europeans and South Asians are compound populations whose admixture dates of ~4,000 years or more makes it difficult to trivially deconvolute their ancestral components on a genome-wide scale (though ancient DNA from Mal’ta likely can help in the case of Europeans).

Figure 4 above shows the match of two demographic models with the empirical results. M2 is one where Mestizos from Chile bring European and Amerindian ancestry into the genomes of Easter Islanders. M1 is where there is an ancient Amerindian admixture, followed by a later European one. The solid lines show the predictions, while the points show the empirical results from the samples. It is clear visually that M1 fits the data. There are many short Amerindian blocks, evident of an old admixture, as opposed to more varied and longer European blocks. The rough dates for Amerindian ancestry admixture are in the range of 1300 to 1400 A.D., which match reasonably well with when Easter Island was settled.

These results are strong. Not definitive and probably not the last word, though more Easter Islander samples can end the debate of admixture at least. But they make us wonder how incredible human migrations have been over the past ~50,000 years! Ancient people were far more daring than we had imagined, and I think we need to reconsider what “crazy” exactly is in many ways.

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