Tracing the origins of the Pacific Islanders has been hampered by taboos against taking DNA samples from ancient remains. So rather than sampling human DNA, scientists have been studying DNA from the rats the ancient Polynesians carried everywhere they went. Now a paper in this week's Proceedings of the National Academy of Sciences casts doubt on a popular theory about the origin of Polynesians.
The colonization of the Pacific began in earnest only 3500 years ago, explains biological anthropologist Lisa Matisoo-Smith of the University of Auckland in New Zealand, one the paper's authors. Around that time, the so-called Lapita culture appeared on islands off New Guinea and swept across the entire Pacific Ocean. Among other things, this culture is characterized by a culinary preference for the Pacific rat, Rattus exulans, which are found everywhere the Polynesian voyagers landed. In many places, the rats, originally from Southeast Asia, still thrive and occasionally still make a nice lunch. "They are actually quite tasty," Matisoo-Smith confides. Her group previously used genetic analysis of rat DNA to trace the migrations of Polynesians from island to island (ScienceNOW, 15 December 1998).
Now they've tackled a deeper puzzle: the origins of the Polynesian ancestors. Based on language similarities, some scientists claim that Polynesians swept into Oceania from Taiwan, bypassing any existing peoples, a popular theory known as the "Express Train." Others think the Lapita culture developed slowly in the area around New Guinea. Genetic analysis of Polynesians' DNA has not helped, as modern Polynesians are genetically diluted, and local tribes usually refuse scientists access to their ancestors' remains.
Again, Matisoo-Smith and her colleague J. H. Robins looked at genetic relationships among the rats. They teased DNA fragments out of ancient rat remains from Lapita deposits all over the Pacific and 20th century museum specimens from Southeast Asia. Rats living today in Southeast Asia are not closely related to the ones the colonizers took with them in their canoes to Oceania, they found. This means that a rapid Express Train is not likely. Rather, the colonizers may have stemmed from a much older population in the island region west of New Guinea, where rats still live with DNA that is similar to that of their Pacific brethren.
The rat DNA is an important piece in the puzzle, says population geneticist Martin Haase of the National Museum of Natural History in Paris, France. But he would like to see additional evidence before being convinced that the rats have derailed the Express Train.