Sat. Oct 1st, 2022
The Tongariki site, built by Polynesians on Rapa Nui.  New data suggests that by the time their ancestors arrived on the island, they were already in contact with South America.
enlarge The Tongariki site, built by Polynesians on Rapa Nui. New data suggests that by the time their ancestors arrived on the island, they were already in contact with South America.

Andres Moreno-Estrada

The Polynesians were the greatest explorers of the world. Starting near Taiwan, they sailed across vast expanses of the Pacific, settling—and in some cases continuing to trade between—astonishingly remote islands from New Zealand to Hawaii. But it’s never been entirely clear whether they made the final jump, sailing from Rapa Nui to reach the nearest major landmass: South America.

There is some evidence they have, mainly the presence of South American crops in the Pacific. But there is no clear genetic signature in human populations, and the whole analysis is confused by the redistribution of people and crops after the arrival of European seamen.

Now, a new study finds clear genetic evidence that Polynesians and South Americans met — we just looked at the wrong island — and the wrong part of South America — for clear evidence. The researchers also bring up a tantalizing prospect: that South Americans were already living on a Polynesian island when the Polynesians arrived there.

Messy Genetics

The obvious place to contact Native Americans and Polynesians is Rapa Nui, also known as Easter Island. Located 3,500 kilometers from the Chilean coast, it was the closest Polynesian settlement to the Americas. The dramatic stone monuments built on Rapa Nui are also reminiscent of some of the statuary found in South America. And like other South American islands, sweet potatoes, native to America, were a staple of the diet.

But European colonizers later spread the sweet potato widely across the Pacific, making it difficult to determine when the crop arrived on distant islands. And they also dispersed people, including many of Native American ancestry, during the colonial period. This has led to some confusing and conflicting results. Two different studies of people native to Rapa Nui came to completely different conclusions about whether they shared DNA with Native Americans, possibly because not all of the island’s population mixed with postcolonial arrivals from South America.

In the case of sweet potatoes, we were able to sort things out using samples collected by early explorers and kept in museums. These predate the Spanish spreading the crop across the Pacific and indicated that the original crops used by the Polynesians originated somewhere near the intersection of Central and South America. But we have very little ancient Polynesian DNA from before the colonial period, in part because DNA is unlikely to survive for centuries on a humid and likely tropical island.

What we have instead are the current populations, which we know tend to have a complicated mix of heritages. So the large international team that decided to tackle the matter accepted the challenge of these monsters and came up with a way to untangle the heritage. To do this, the researchers obtained a large collection of Polynesian DNA samples — 166 from Rapa Nui and 188 from 16 other island populations. They also obtained DNA from 15 different groups of Native Americans along the Pacific coast.

A mix of logic and algorithms

To cope with Polynesia’s confusing genetic history, the researchers conducted a number of different analyses. One is a standard tool called ADMIXTURE that estimates whether two populations have interbred in the past. But beyond the standard tools, the researchers looked for long stretches in which DNA variations were largely identical, suggesting they were inherited from a common ancestor. The length of these pieces can also give an indication of how long ago that common ancestor existed.

As a result of this analysis, the researchers were not only able to see that the DNA from samples suggested the humans shared a common ancestor, but they were also able to identify which pieces of DNA likely came from which ancestral group, and how long ago those groups had interbred.

That turned out to be critical. The software identified pieces of DNA that appeared to come from South American groups native to Peru and Chile, the areas closest to Rapa Nui. But the researchers found that the frequency of this DNA correlated with the frequency of DNA from European sources. The amount of this DNA present also varied widely between individuals, suggesting it hasn’t been in the population long enough to reach equilibrium. All of this matches this Native American DNA that arrived when people from Spain’s South American colonies traveled to Rapa Nui, perhaps as recently as when it was annexed by Chile.

But that wasn’t the only Native American DNA found. A second set of DNA segments resemble those of Central American populations such as the Mixe and Zapotec, as well as the Zenu group from Colombia, but are closest to native Colombians. And this set had completely different properties. For starters, his presence in Rapa Nui correlated with the amount of Polynesian ancestry – not European. It also varied little between individuals, suggesting that it had been in the population for much longer and predated the arrival of Europeans in the Pacific.

Perhaps critically, this DNA is found on several islands in Eastern Polynesia. These were all settled before the European arrival and have different histories with European settlers (some Spanish, some French). This suggests that Central American ancestry is not associated with European ancestry and instead arrived when these Polynesian populations spread across these islands.

That would make the introduction of the DNA somewhere other than Rapa Nui, which was one of the last islands to settle. Instead, it was more likely to be somewhere further north, in French Polynesia. So it seems we’ve misunderstood both sides of the interactions between South America and Polynesia.

Who found who where?

Overall, the researchers conclude that South American DNA was introduced to the Marquesas, an island group northeast of Tahiti, shortly after 1100 CE. Over the next hundred years, it spread from there southwards and eventually to Rapa Nui, though it did not spread back to the core Polynesia region to the west. This is despite the fact that the sweet potato, which most likely arrived with it, was almost certainly transported westward via trade routes.

This evidence, the researchers argue, makes sense that the Polynesian word for the crop is similar to terms used for it in South America. And it gives a tantalizing hint that the San Agustin culture, which was present in coastal towns in Colombia at the time, may have influenced the later construction of large human statues on Rapa Nui.

But how did the meeting go? Given their amazing navigational skills, the obvious choice would be that the Polynesians would have reached South America as part of the expansion that took them to the Marquesas and later returned to these islands; the prevailing winds and currents make that a relatively easy trip from the Colombian coast.

But there’s another possibility, made famous by the Kon Tiki expedition in the 1940s, which showed that a raft built using technologies possibly in use on the South American coast could reach the Polynesian Islands from there. . The same trade winds and currents that would make the journey easy for Polynesians could have carried one of these rafts, whether it was deliberately sailed west or not.

Thor Heyerdahl, who led the expedition, took it as proof that the Polynesians were originally South Americans. We now know that’s not true. But that doesn’t mean they didn’t settle on Polynesian territory. “Our earliest estimated contact date is CE 1150 for Fatu Hiva, South Marquesas,” the authors write. “This is close to the date estimated by radiocarbon dating for the settlement of that island group, raising the intriguing possibility that Polynesian settlers encountered a small, already established Native American population upon their arrival.”

It’s incredibly difficult to see how we’re going to discern these possibilities unless some archaeological evidence is uncovered. But the authors suggest that obtaining DNA from more people native to different island groups could clarify the dynamics of the DNA’s distribution through the Polynesian population and advance one explanation or the other.

Nature2020. DOI: 10.1038/s41586-020-2487-2 (About DOIs).

By akfire1

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