The last great migration of humans to lands unknown occurred with the colonization of East Polynesia about a millennium ago. It’s not an easy feat finding tiny islands scattered in a body of water that can engulf all seven continents with room to spare.
“In terms of the scale, risk, and magnitude of the exploration, it’s one of humanity’s momentous achievements,” said Barry Rolett, an anthropologist at the University of Hawaiʻi at Mānoa.
But the details of this accomplishment—and what drove it—have been shrouded in mystery.
Now, a new study published in the journal Proceedings of the National Academy of Sciences of the United States of America reports that humans arrived in East Polynesia 200 to 300 years earlier than previously thought.
Their arrival in East Polynesia—a culturally and linguistically distinct region spanning from the Cook Islands to Rapa Nui and Hawaii—coincides with a time of prolonged drought in their West Polynesia islands of origin in Tonga and Samoa, which may have helped spur the dangerous excursions eastward.
“It’s an impressive study and an important one,” said Rolett, who was not involved in the research. “It’s unusual for Polynesia because there hasn’t been a lot of paleoenvironmental reconstruction work done in this area.”
Tracking Human Settlement Through Mud, Charcoal, and Feces
Lake sediments and mud can be used as archives of both human environmental impact and the climate across the centuries, said David Sear, a professor of physical geography at the University of Southampton in the United Kingdom and the lead author on the new study. “We wanted to go and collect data along the route of the human colonization story of the Pacific and follow that story in the mud from the lakes and bogs.”
Because of how remote the islands are, the researchers had to bring their own inflatable boats, build their own rafts, and transport all their equipment by hand via muddy jungle paths to drill and collect cores of mud from each island’s lake. The team initially collected mud cores from Lake Te Roto on Atiu, a part of the Southern Cook Islands.
After collecting mud cores, Sear and his colleagues stored them in aluminum tubes. “You pack them into a cardboard box very carefully, put ‘fragile’ on the outside, go to the post office, pay 200 quid, and get it flown back to the U.K. under special import-export licenses, of course,” he said.
Back in the lab, researchers scanned the mud for multiple proxies of human activity, including charcoal, which is a sign of fire, and titanium, which indicates soil erosion; together, they indicate deforestation of the trees and underbrush native to the island. But the most telltale sign of human presence they looked for was something even more fundamental: feces. Specifically, fecal sterols, a fatty substance found in mammalian feces. On these remote Pacific islands, there were no mammals besides fruit bats prior to the arrival of humans and pigs.
“The idea of using fecal markers is really innovative, and it works extremely well,” said Rolett.
Together, the evidence points to an incremental migration process with humans first arriving in East Polynesia around 900 CE, followed by increased settlement activity over the next 200 years. This study “fills in a really important part of the puzzle of human settlement,” said Melinda Allen, an archaeologist at the University of Auckland in New Zealand and coauthor on the study. “And a lot of unconnected strands of evidence can now be pulled together as a result of these findings.”
Climate Change and Migration
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An extended regional drought in West Polynesia may have driven the human forays east. The researchers reconstructed regional paleoclimatic conditions of the past 2,000 years using additional lake core samples taken from islands in Samoa and Vanuatu, as well as previously published records of the Society Islands of French Polynesia. They found that the timing of human arrival in East Polynesia coincides with an intense, prolonged drought—the driest period in 2 millennia—which the researchers suggest helped drive people to migrate.
However, there are likely other factors that might have led to settlement in addition to or in conjunction with drought, said Seth Quintus, an anthropologist at the University of Hawaiʻi at Mānoa who was not involved in the current study. “It’s really hard to say that drought is what’s causing the movement of people.”
As a whole, the study “teaches us a lot about how people in the past manage and respond to different risks in their environment,” he added.
Pacific Climate Change Past and Future
Sear said that there are still more climate data to analyze from the mud cores once the labs are back open: The records his team collected go back 10,000 years, and this study looked at only the most recent 2,000. Understanding how climate has changed in the Pacific Ocean is crucial because it is “one of the big engines of the global climate system,” Sear said, and there are not many climate data from before the 1950s.
Better understanding of the region’s climate system would not only shed light on the area’s past but benefit the almost 12 million people living in the region today.
“These people are being squeezed by rising sea levels, changes in precipitation, increasing temperatures,” Sear said. “When you put that together, they’re amongst the most vulnerable people on the planet.”
“If we can get a better understanding of both how their ancestors changed the landscape and the climate story that goes along with that, it will help them manage their future,” Sear said. “Because, of course, one of their responses to climate change in the past is to get into a canoe and move somewhere else.”
“You can’t do that anymore,” he said. “That major adaptation strategy is no longer is available to them.”
—Richard J. Sima (@richardsima), Science Writer