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Rat DNA Offers Clues to Pacific Colonization, Study Says

John Pickrell
for National Geographic News
June 9, 2004
 
Wherever people go, rats go too. Now researchers are making use of this fact to help understand patterns of human migration. A new genetic comparison of rats from various Pacific islands presents clues about the origins of Polynesian people.

The study, published this week in the science journal Proceedings of the National Academy of Sciences, strikes a blow against one popular theory of how remote Pacific islands were colonized, and argues that the migration was a slow and complex process.



"People in the Near Oceania region of the Pacific have a long history of interaction with other groups," said study author Elizabeth Matisoo-Smith, a biological anthropologist at the University of Auckland, New Zealand. "[This] make[s] understanding the prehistory of the region difficult using human genes alone."

Noting that Polynesians and their ancestors carried plants and animals as they migrated, Matisoo-Smith said, "We can use the genetic variation in those species to track the movement of the people who carried them."

The technique is similar to one used by archaeologists to uncover clues about early peoples by pinpointing the origins of stone or clay used in ancient tools and pots, Matisoo-Smith said.

Epic Journey

Pacific settlement is thought to have begun around 40,000 years ago with the migration of people from Asia into New Guinea and Australia. By 30,000 years ago, these early people had sailed onto the Solomon Islands and the Bismarck Archipelago, a region known as Near Oceania.

Colonization of so-called Remote Oceania—the region east of the Solomon Islands that encompasses Vanuatu, Fiji, Tonga, Samoa, and far beyond—did not begin until much later. Precisely how this transpired has been the subject of vigorous debate for centuries.

One popular theory is known as the "express train" model. It argues that the Lapita people, who first populated Remote Oceania, represent a second wave of colonists who rapidly swept their way from Taiwan through Southeast Asia and Near Oceania and out into Remote Oceania. According to the theory, the process, which started 3,500 years ago, took just a few centuries to complete.

Advocates believe that the Lapita are the ancestors of the Polynesians and that they had little or no mixing with existing populations in Near Oceania. Evidence for the theory comes from striking similarities between languages spoken by indigenous inhabitants of Taiwan and Pacific Islanders.

But other theories suggest different scenarios. One holds that the Lapita people were made up of existing Near Oceania inhabitants alone, with little input from Taiwan. Another argues that the Lapita descended from people who did come from Asia, but underwent plenty of mixing with people in New Guinea, the Solomon Islands, and other parts of Near Oceania as they migrated.

The debate has been difficult to settle for a number of reasons. First, there is low genetic variation in ancestral Polynesian populations. (This was further exacerbated by population crashes in the wake of introduced European diseases.) Second, interbreeding makes it virtually impossible to trace prehistoric migration patterns using human DNA alone.

Rat Snack

But Matisoo-Smith and University of Auckland colleague Judith Robins have a valuable tool to arbitrate the debate: the DNA of Pacific rats (Rattus exulans), which were carried in the canoes of the Pacific's early colonizers as food items.

The rats offer several advantages to researchers: The rodents are a distinct species from later European stowaways, and therefore did not interbreed. The Lapita and later Polynesian people carried the rats everywhere they traveled, according to Matisoo-Smith. And the rodents are unable to swim, which means humans are responsible for the rats' dispersal.

The researchers trapped rats and collected DNA samples on numerous Pacific islands. They also took ancient DNA samples from rat bones unearthed by Pacific archaeologists. By comparing the same types of genes among different populations, Matisoo-Smith created an evolutionary tree illustrating how island rat populations were related to one another.

The results revealed that rats form three isolated populations, perhaps linked to three distinct populations of Pacific colonists. The data suggests a slow and complex scenario of migration into Remote Oceania involving lots of interaction along the way.

The finding may refute the express-train theory, and it is backed up by archaeological evidence of trade in obsidian stone between islands, according to Matisoo-Smith.

No Quick Fix

"The paper is an important contribution to the debate about Pacific settlement," said Russell Gray, an evolutionary biologist at the University of Auckland in New Zealand. (Gray was not involved in the study.)

"The use of a commensal species that were deliberately taken on colonizing voyages is a really nifty idea," said Gray, who has himself traced the origins of Pacific islanders using both human genetics and language. (Commensal species are those like rats and cockroaches that live alongside human settlements.)

"This is a prime example of the potential for cross-illumination between archaeology and genetics," said Melinda Zeder, a zooarcheologist at the Smithsonian Institution in Washington, D.C.

"Stowaway rats are able to provide a much more robust test of competing models of migration in the Pacific than traditional archaeological data," Zeder said.

According to Matisoo-Smith, integrating data from the genetic analysis of rats and other species (such as pigs, dogs, and chickens) with human genetics, the analysis of local languages, and archaeology may yet provide further insights into the peopling of the Pacific region.

Still, there may be no easy solution to the debate. "The settlement of [the Pacific] was the last major human migration, and it seems to grab the public's imagination," Matisoo-Smith said. "But there are not simple answers. Like most human endeavors, the settlement of the Pacific was complex. And that complexity should be recognized and celebrated."

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