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.
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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