Rat points to Polynesian origins

By SIMON COLLINS science reporter

Research on domestic rats has pinpointed Halmahera, a four-pronged island between Borneo and New Guinea, as the most likely pre-Pacific homeland of the Polynesians.

A 12-year study by Auckland University biological anthropologist Lisa Matisoo-Smith has traced the genetic origins of the Pacific rat, Rattus exulans, which migrated with humans in canoes throughout the Pacific Islands and New Zealand during the past 3000 years.

The results, published today in the US Proceedings of the National Academy of Sciences, show that the Pacific rat can be divided into three genetic groups, which coincide in Halmahera.

The Indonesian island is just outside one of the genetic groups in the Southeast Asian region of Borneo, Sulawesi and the Philippines, and is the only place that has rats of both the second group, extending from the Philippines southeastwards to the Solomon Islands, and the third group, which scattered through the rest of the Pacific.

The evidence, which earned Dr Matisoo-Smith a reputation as "the rat lady" as she travelled around collecting rats, confirms human evidence from mitochondrial DNA, which is passed on from mothers to their children.

"The Polynesian mutation in mitochondrial DNA - the Polynesian motif - is traced back to Halmahera," she said.

But she stressed that the settlement of Polynesia, the last part of the Earth to be settled by humans apart from Antarctica, was a complex process which involved continuing interaction among widespread groups of islands, possibly including several migrations separated by hundreds of years.

Both human genetic and language evidence still point to an even earlier homeland, in Taiwan and ultimately in mainland China.

Anthropologists believe the development of agriculture about 10,000 to 15,000 years ago allowed an explosion in the human population of East Asia which drove people to migrate to new islands, leaving Taiwan about 5000 years ago.

But Dr Matisoo-Smith found no evidence that Pacific rats ever lived in Taiwan, at least until recently, blowing apart the so-called "express train to Polynesia" theory that people moved quickly southwards from Taiwan and into the Pacific Islands without significant interaction with the people already living en route in eastern Indonesia and New Guinea.

The evidence suggested that people moved along a "voyaging corridor" through eastern Southeast Asia and into Melanesia with a mixture of intrusion of some new cultural practices, integration of some practices from the local inhabitants, and innovation.

By 3500 to 3300 years ago, the combination of all these elements had created a new culture, labelled "Lapita" after its distinctive pottery, which stretched from New Britain to Samoa. Lapita people developed permanent villages, a range of horticultural crops, fish hooks, seagoing canoes and domesticated animals such as pigs, dogs, chickens and rats.

The rats were carried deliberately because they were an important part of the people's diet.

But the fact that the third genetic group of Pacific rats is found only in Halmahera and the remote Pacific, and not in other parts of "near Oceania" from the Philippines to the Solomons, raises a question over how people moved from Halmahera into what is now Polynesia.

Dr Matisoo-Smith is investigating two possible routes.

First, and most likely, people and rats may have moved southeast down the north coast of New Guinea and through the Solomons to Vanuatu, Fiji, Samoa and eventually to the rest of Polynesia.

In that case, there should be archaeological evidence of the third genetic group of rats in the area down to the Solomons and further east.

Alternatively, or perhaps through a secondary migration, they may have moved northeast into Micronesia, and into the Southeast Pacific from the north.

If that happened, Dr Matisoo-Smith believes it was much later, because many Micronesian atolls became habitable only as the sea level dropped in the past 2000 years.

Halmahera

Halmahera is the biggest island in the Moluccas (Maluku) between Borneo and New Guinea.

* It is about one-sixth the size of the North Island with a population in 1990 of 138,000.

* Its location at the boundary between Southeast Asia and the Pacific has made it a key link in the migration of people and their animals from Asia out to Melanesia and Polynesia.

The Proceedings Of The National Academy Of Sciences USA