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Migraciones_austronesiasGgas_human_soc One of the most incredible journeys that the human species has undergone is the Austronesian expansion of the past 4,000 years. These maritime peoples seem to have emerged from the islands of Taiwan, and pushed forward south, west, and east, so that their expansion pushed to East Africa, and the fringes of South America. There now also some circumstantial evidence that Polynesian contact with the Americas predates the Columbian Exchange. Looking at the map above in hindsight it seems natural to imagine such contacts.

Though where the Austronesians went is incredible, their origins are somewhat more opaque, but rather tantalizing. That is because their original expansion was likely just before the horizon of history. In Guns, Germs, and Steel Jared Diamond alluded to the “express train” vs. “slow boat” models of the expansion. Basically, whether the Lapita peoples rapidly pushed out from Taiwan, or whether there was a long period of coexistence with Melanesians in Near Oceania. Over the past few years genetics seems to have supported the “slow boat” model.

Here is a paper from 2012, Population Genetic Structure and Origins of Native Hawaiians in the Multiethnic Cohort Study:

The “Express Train” and the “Slow Boat” models of Polynesian migration are expected to have uniquely distinct genetic signatures on present day genomes of Native Hawaiians. Under the “Express Train” model, the proportion of admixture in Native Hawaiians of Melanesian and Asian ancestry is expected to be near zero, whereas under the “Slow Boat” model, the proportion of admixture is expected to be substantially greater than zero. To test these two models, we conducted a supervised ADMIXTURE analysis using Papuan and Melanesians as one source population of Polynesians and Han Chinese, She, Cambodian, Japanese, Yakut, and Yi as surrogates for the second source population of Taiwanese aborigines[18],[19]. Importantly, we did not fix ancestry for the Melanesians or Asians and therefore allowed for admixture within either ancestral groups–thus, mitigating bias by earlier admixture processes and allowing for accurate clusters of ancestry membership. We set K = 2 and estimated in 40 100% Native Hawaiians an average of 32% and 68% of their genomes to be derived from Melanesian and Asian origins, respectively (Figure 4). This notable proportion of Melanesian admixture (32%) among Native Hawaiians, substantially greater than zero, lends support of the “Slow Boat” model of ancestral origins.

This is not an isolated study. Y chromosomes indicate substantial Melanesian admixture, while the mtDNA does not. One inference then was a “slow boat” model predicated on matrifocality. That is, expanding Polynesian groups were centered around matrilineal lineages, and absorbed Melanesian men into their communities. The above research was from a Hawaiian data set, but the results are consistent across Polynesia in relation the proportion of Melanesian ancestry.

Case closed? No so fast! Ancient DNA has now been brought to the question, and fundamentally changed our perceptions. Genomic insights into the peopling of the Southwest Pacific:

The appearance of people associated with the Lapita culture in the South Pacific around 3,000 years ago1 marked the beginning of the last major human dispersal to unpopulated lands. However, the relationship of these pioneers to the long-established Papuan people of the New Guinea region is unclear. Here we present genome-wide ancient DNA data from three individuals from Vanuatu (about 3,100–2,700 years before present) and one from Tonga (about 2,700–2,300 years before present), and analyse them with data from 778 present-day East Asians and Oceanians. Today, indigenous people of the South Pacific harbour a mixture of ancestry from Papuans and a population of East Asian origin that no longer exists in unmixed form, but is a match to the ancient individuals. Most analyses have interpreted the minimum of twenty-five per cent Papuan ancestry in the region today as evidence that the first humans to reach Remote Oceania, including Polynesia, were derived from population mixtures near New Guinea, before their further expansion into Remote Oceanian…our finding that the ancient individuals had little to no Papuan ancestry implies that later human population movements spread Papuan ancestry through the South Pacific after the first peopling of the islands.

These results strong indicate that the original Lapita migration did not mix with Melanesians. And, the ancient samples share common ancestry with modern Polynesians, so that their heritage persists down to the present. Looking at the distribution of Melanesian ancestry they concluded this admixture occurred on the order of ~1,500 years before the present (their intervals were wide, but the ancient samples serve as a boundary). Additionally, in line with the Y and mtDNA the X chromosome indicated more of the ancient ancestry than the autosome. The authors conclude that “it is also possible that some of these patterns reflect a scenario in which the later movement of Papuan ancestry into Remote Oceania was largely mediated by males
who then mixed with resident females.”

The take home message than is that we need to be more modest with our models. Without ancient DNA it seems likely that we would not have stumbled onto this result; the ancestry deconvolution methods which date admixture have wide confidence intervals when you go back far in time.

 
• Category: Science • Tags: Genetics, Genomics 
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  1. Possibly of interest also: Third, the cluster of ancient individuals does not overlap with present-day populations, indicating that the data are from a population that is not present in unmixed form today (Fig. 1).

    The distinctiveness of the ancient individuals is also highlighted by their high differentiation from all present-day groups (0.05 < FST < 0.26; between all modern individuals and the ancient Vanuatu individuals, using the statistic FST, which compares withinand between-group squared allele frequency differences) (Extended Data Table 3).

    So that 0.05 is around the kind of differentiation between the early Neolithic Anatolia group and the early Steppe group (per the recent Lazaridis 2016 paper), where Sardinians approx 0.016 from Neolithic Anatolia and WHG at 0.097. Seems like quite a bit of structuring.

    In a paper in June (http://www.nature.com/ejhg/journal/vaop/ncurrent/full/ejhg201660a.html?WT.feed_name=subjects_genomics), the Kankanaey Igorots from the Philippines were thought to be the closest representatives of early Austronesian expansion, however they are included in this, so it seems like they can’t have Fst less than 0.05 from the ancient Lapita individuals.

    I wonder where this differentiation comes from. Whether there is more admixture between even the Austronesians who were left behind (even with the isolated Kankanaey), or whether it’s more like the Austronesians on the expansion picked up some other ancestry that just wasn’t Papuan-like. Or it could also be due to strong drift in the different groups, which Fst is suspectible to, however the outgroup f3 statistics (Extended Figure 2a) do also show that the Lapita individuals are far from the most recent proxies.

  2. “our finding that the ancient individuals had little to no Papuan ancestry implies that later human population movements spread Papuan ancestry through the South Pacific after the first peopling of the islands”.

    Oh dear. Maju will not be pleased. I argued with him for years that this was the most probable scenario in the region.

    “Whether there is more admixture between even the Austronesians who were left behind (even with the isolated Kankanaey), or whether it’s more like the Austronesians on the expansion picked up some other ancestry that just wasn’t Papuan-like”.

    A bit of both, but mainly the former. There has been a huge movement into SE Asia of people with a more Mongoloid phenotype than the people of the remote Pacific have. Another point Maju became very heated over. In fact we basically have a cline between East Asia and Papuans, but the overlap survives only in the remote Pacific. The cline has become very steep just to the east of Wallace’s Line. As for your second suggestion, the most common Y-DNA haplotype in the remote Pacific is C1b2a1a-P33. This is not a New Guinea haplotype. There we have C1b2a1c-B46. Both derive from C1b2a1-M208, the Nusa Tenggara, or southern, branch of Indonesian C1b2a. So the Austronesian expansion did not pick up any Papuan ancestry but some related genes from nearby Indonesia.

  3. One thing that always surprises me when I see maps of the Polynesian expansion, is why they never made it to Australia (or did they, but left it alone because it was already populated?)

    • Replies: @John Massey
    Possibly deterred by climate. They did engage in trading with the Australian mainland. The dingo had to come from somewhere.
    , @terryt
    It is almost certain that the Austronesian expansion moved along the northern coast of New Guinea. New Guinea is much closer to the Philippines than is Australia. Besides which as a proportion of the whole island just a small proportion of both Australia and New Guinea are suited to a fishing economy. Which makes the Timorese situation interesting. The modern population of Timor is much more similar to that from New Guinea than it is to the remainder of SE Asia, However there are some haplotypes present in northwest Australia that almost certainly originated in New Guinea and whose arrival may be related somehow to the Austronesian expansion.
  4. “results are consistent across Polynesia in relation the proportion of Melanesian ancestry”

    “admixture occurred on the order of ~1,500 years before the present”

    Not sure if it was taken into account or would affect these results but there has recenltly been a dramatic reassessment in dating the expansion into East Polynesia (beyond Samoa). The graphic does not reflect this, but likely it happened in two bursts much more recently than previously thought. First in the 11th century to the Society Islands and in the 13th to the more far flung islands including Hawaii and New Zealand. Perhaps the Melanesian admixture, occurring before this expansion but after Lapita, would leave a signature. East polynesians would be a recent expansion of a west polynesian population, likely Samoan, and would uniformly have the Melanesian fraction of that population. West polynesian populations like Tongan and Tuvaluan would show varying admixture into the original unadmixed Lapita.

    http://www.pnas.org/content/108/5/1815.full.pdf

    Interestingly Fiji was pioneered by Polynesians but became Melanesian. The original Lapita style pottery was replaced by the Navatu style of unknown origin in the the first millenium A.D.

    • Replies: @terryt
    The secondary expansion into furthest east Polynesia was from the Society islands, not a separate burst from the Tonga/Samoa/Fiji region. As you say, the Melanesian movement definitely reached Fiji, and may have brought to there the technology necessary to move into the more isolated islands of Eastern Polynesia. East Polynesia is therefore not the product of a recent, more Melanesian population from Fiji/Tonga/ Samoa although may have been genetically altered by the Melanesian arrival in Fiji.

    As for the Dingo. It seems to have arrived in Australia quite some time before the Austronesian expansion.
  5. @Tobus
    One thing that always surprises me when I see maps of the Polynesian expansion, is why they never made it to Australia (or did they, but left it alone because it was already populated?)

    Possibly deterred by climate. They did engage in trading with the Australian mainland. The dingo had to come from somewhere.

  6. @Tobus
    One thing that always surprises me when I see maps of the Polynesian expansion, is why they never made it to Australia (or did they, but left it alone because it was already populated?)

    It is almost certain that the Austronesian expansion moved along the northern coast of New Guinea. New Guinea is much closer to the Philippines than is Australia. Besides which as a proportion of the whole island just a small proportion of both Australia and New Guinea are suited to a fishing economy. Which makes the Timorese situation interesting. The modern population of Timor is much more similar to that from New Guinea than it is to the remainder of SE Asia, However there are some haplotypes present in northwest Australia that almost certainly originated in New Guinea and whose arrival may be related somehow to the Austronesian expansion.

  7. @swampr
    "results are consistent across Polynesia in relation the proportion of Melanesian ancestry"

    "admixture occurred on the order of ~1,500 years before the present"

    Not sure if it was taken into account or would affect these results but there has recenltly been a dramatic reassessment in dating the expansion into East Polynesia (beyond Samoa). The graphic does not reflect this, but likely it happened in two bursts much more recently than previously thought. First in the 11th century to the Society Islands and in the 13th to the more far flung islands including Hawaii and New Zealand. Perhaps the Melanesian admixture, occurring before this expansion but after Lapita, would leave a signature. East polynesians would be a recent expansion of a west polynesian population, likely Samoan, and would uniformly have the Melanesian fraction of that population. West polynesian populations like Tongan and Tuvaluan would show varying admixture into the original unadmixed Lapita.

    http://www.pnas.org/content/108/5/1815.full.pdf

    Interestingly Fiji was pioneered by Polynesians but became Melanesian. The original Lapita style pottery was replaced by the Navatu style of unknown origin in the the first millenium A.D.

    The secondary expansion into furthest east Polynesia was from the Society islands, not a separate burst from the Tonga/Samoa/Fiji region. As you say, the Melanesian movement definitely reached Fiji, and may have brought to there the technology necessary to move into the more isolated islands of Eastern Polynesia. East Polynesia is therefore not the product of a recent, more Melanesian population from Fiji/Tonga/ Samoa although may have been genetically altered by the Melanesian arrival in Fiji.

    As for the Dingo. It seems to have arrived in Australia quite some time before the Austronesian expansion.

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