‘No one could have predicted.’ DNA offers surprises on how Polynesia was settled

The peopling of Polynesia was a stunning achievement: Beginning around 800 C.E., audacious Polynesian navigators in double-hulled sailing canoes used the stars and their knowledge of the waves to discover specks of land separated by thousands of kilometers of open ocean. Within just a few centuries, they had populated most of the Pacific Ocean’s far-flung islands. Now, researchers have used modern DNA samples to trace the exploration in detail, working out what order the islands were settled in and dating each new landfall to within a few decades.

“The whole question of the settlement of Polynesia has been going on for 200 years,” says University of Hawaii, Manoa, archaeologist Patrick Kirch, who was not involved in the research. “This is a really great paper, and I’m happy to see it.”

Archaeologists already had hints of how this great exploration took place. Studying the styles of stone tools and carvings, as well as languages, of the people on the various islands had suggested the original ancestors traced back to Samoa and that the expansion ended halfway across the ocean in Rapa Nui, or Easter Island. But they disagreed on whether it happened in a few centuries, beginning around 900 C.E., or started much earlier and lasted 1 millennium or more.

To learn more, Stanford University computational geneticist Alexander Ioannidis and Andrés Moreno Estrada, a population geneticist at Mexico’s National Laboratory of Genetics for Biodiversity, compared the DNA of 430 modern individuals from all across Polynesia (most of them collected for previous studies), and then eliminated later genetic input from European people. Because the researchers knew Polynesians had journeyed stepwise from island to island, their genetic analysis utilized a genetic phenomenon known as a population bottleneck. When a few dozen to a few hundred individuals from already-isolated island populations settled a new island, and then a subset of that group left to settle an additional island, and so forth, their genetic diversity would have shrunk with each voyage—like a telescope in reverse.

“It’s a mode of dispersal that’s different from any other place in the world,” Moreno Estrada says. “We can tell who comes from which island.”

In a paper published today in Nature, Ioannidis, Moreno Estrada, and colleagues identified genetic patterns specific to the founder population on each island. By analyzing the DNA, the team could trace the sequential journeys to each subsequent island, “like pearls on a string,” says University of Tübingen geneticist Cosimo Posth, who was not involved in the study.

To estimate how many generations went by between each island discovery, the scientists measured the length of shared genomic sequences between founder populations. Together, the data showed who descended from whom. That made it possible to not only show that two populations were related, but which came first.

“Getting deep-grained details on direction? That was impossible before,” says University of California (UC), Santa Cruz, population geneticist Lars Fehren-Schmitz, who was not involved with the work.

The analysis suggests canoes set sail from the shores of Samoa—more than 2000 kilometers north of New Zealand—around 800 CE. The explorers arrived first on Rarotonga, the largest island in a chain now called the Cook Islands. Successive explorers moved in all directions, island hopping over the course of centuries and eventually reaching all the way to Rapa Nui, 6500 kilometers from Samoa and 3700 kilometers off the coast of Chile, by 1210 C.E.

And because the genetic evidence allowed the researchers to reconstruct the order in which the islands were settled, they could spot connections between islands that might not seem intuitive based on the geography. For example, they argue that three island cultures known for carving massive stone statues—Rapa Nui, Raivavae, and the North and South Marquesas—shared a common founder population in the Tuamotu Islands, even though they are thousands of kilometers apart and geographically closer to other parts of the Pacific.

Geneticists showed the people who carved the enigmatic moai statues of Rapa Nui were related to far-flung islanders with similar megalithic traditions.Carlos Aranguiz/iStock

Those three islands also hold the earliest genetic traces of Native American ancestry among Polynesians. That suggests ancient Polynesians first contacted the Americas around 1100 C.E., when the seafarers were beginning their last, and longest, expeditions. “That’s something no one could have predicted through archaeology or oral history,” Moreno Estrada says.

The vast distances and difficult journeys meant less of the back-and-forth that comes with typical migrations across continents, Moreno Estrada says. Instead, islands were settled by small groups who lived more or less in isolation until the next group of explorers set sail bearing their genetic legacy. “In most situations you get gradients of genetic difference, with everyone moving around. You don’t see that here,” he says.

But Kirch says that may be too simplistic: Tools and other archaeological evidence suggest ancient voyages weren’t simple one-way trips, and that intermarriage and contact between populations continued after islands were settled. “Their analysis makes it seem like there’s a discrete set of migratory movements,” he says. “The archaeology suggests there’s a lot of back and forth, with continued interaction … between islands.”

By mapping out the genetic variations specific to each island’s isolated population, the data might help guide research into medical conditions that disproportionately affect Polynesians, or even people on specific Pacific islands—knowledge that massive genetic data sets based mainly on European and Asian populations would miss. In a recent preprint, for example, Ioannidis found that Polynesian ancestry is associated with higher risk for severe cases of COVID-19.

A bit like the Polynesian navigators who used the stars and waves to guide their voyages centuries ago, the researchers hope the new knowledge can chart a course to better health for people living in Polynesia today. “In Hawaiian we say ‘I ka wā ma mua, ka wā ma hope,” says study co-author Keolu Fox, a Kānaka Maoli, or Native Hawaiian, and a geneticist at UC San Diego. “It means we’re ‘walking backwards into the future.’”

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