The wild and domestic pigs now found on Pacific islands are descended from pigs brought in sea canoes by migrants from Taiwan and neighboring China four thousand years ago. This is evident from a complex genetic analysis of more than a hundred pig genomes up to 3,000 years old. She was published this week in Scienceby a team led by Greger Larson and Laurent Frantz (both Queen Mary University of London).
Until now, it was believed that the Austronesian-speaking migrants had only brought their pigs from the islands they visited along the way, in their millennia-long migration from island to island. The first phase went from Taiwan via the Philippines south to the Indonesian archipelago – all areas with their own indigenous pig species and breeds – and from there it went east to Melanesia (Papua New Guinea, Vanuatu).
The next phase from about 2,800 years ago then enters Polynesia: Samoa and in more historical times (1,100 years ago) it reaches distant Hawaii, as well as New Zealand and Easter Island (both about 800 years ago). From Indonesia, Austronesian-speaking groups also sailed west across the Indian Ocean. They reached Madagascar around 1,500 years ago.
Hardly any mixing with local pigs
Surprisingly, in the first waves of pig dispersal across the Pacific Islands, the pigs taken remained relatively purebred, with virtually no admixture of local pigs on the islands where they were already found, such as the Philippines and Sulawesi. Only the pigs that remained there mixed with other species and breeds, whether in the wild or not.
This pattern fits wonderfully with the behavior of migrants themselves, who did not mix with local populations on their way to further destinations, insofar as they were present on the islands visited (especially in the Indonesian archipelago). This was already apparent from DNA analyzes almost ten years ago. Apparently the Austronesian migrants, pigs and all, kept to themselves for a long time.
As for the pigs, the researchers suspect that the breeds brought from Taiwan apparently had specific characteristics that their herders wanted to preserve. Suitability for sea transport would undoubtedly have been one of them, but these properties are not known in detail. A specific anatomical molar shape is the clearest external feature of this Pacific variant of the East Asian pig (Sus scrofa).
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Mixing with European pigs
The DNA analysis was complex because it was mainly possible to examine the genomes of recent pigs from the area: 54 current ones, 60 from the last century (1871-1993) and only three ancient ones. One from the Philippines, 500 years old, one from Palau (Micronesia), 1,400 years ago, and one from the early phase of the expansion, from Vanuatu, 2,900 years old. The current genomes were heavily mixed with other East Asian races in all the hundreds to thousands of years after the first wave of expansion across the Pacific Ocean. And more recently, there were also mixtures with European pigs that came with the colonizing powers from the nineteenth century onwards.
The family trees have also been clouded by the many genetic bottlenecks to which the already small pig populations were subjected by their many sea voyages. With such a sudden reduction in the population (because, for example, half are lost in a storm or die on arrival), all kinds of coincidental shifts take place in the genetic signatures (genetic drift).
All these mixtures and random genetic shifts led to the fact that the previous (much more limited) DNA analyzes seemed to indicate that the Austronesian-speaking migrants only acquired their pigs on the affected islands after leaving Taiwan. Only by complex ‘scrubbing’ of the later influences and especially by the almost 3,000 year old pig genome from Vanuatu were the researchers able to obtain a new picture.
They also do not rule out that new research will yield further refinements, for example that other pigs were also brought from Taiwan at the time (which look much more like ‘normal’ East Asian pigs), but that only those with the current Pacific signature survived the first genetic bottlenecks.
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