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Last Updated: Tuesday, 8 June, 2004, 13:17 GMT 14:17 UK
Rat DNA clues to sea migration
Polynesia carving   PNAS
This carving shows Pacific rats on the face of a Polynesian ancestor
Scientists have used DNA from rats to trace migration patterns of the ancestors of today's Polynesians.

People are thought to have arrived in Polynesia, comprising the Pacific islands of Fiji, Tonga and Samoa, by boat some 3,000 years ago.

Rat data suggests the journey was more complex than the popular "Express Train" theory, which proposes a rapid dispersal of people from South Asia.

Details appear in Proceedings of the National Academy of Sciences.

Elizabeth Matisoo-Smith and Judith Robins from the University of Auckland, New Zealand, analysed variation in the DNA of the Pacific rat (Rattus exulans).

The genetic material was extracted from cell structures called mitochondria rather than the nucleus.

Mitochondrial DNA (mtDNA) changes at a steady rate over time, so the scientists were also able to use it to track changes in the rat population through history.

Rats on a ship

They found clear geographic patterns of rat DNA across Oceania. Rats appear in a region known as Near Oceania, which comprises the island of New Guinea, the Solomon Islands and Bougainville, after 3,500 years ago.

They apparently hitched a ride on boats used by ancient seafarers known as the Lapita, a people regarded by many researchers as ancestors of modern-day Polynesians.

The rat mtDNA types fell into three haplogroups, or types: I, II and III. Haplogroup I is found primarily in South-East Asia. Haplogroup II was found in South-East Asia and a region known as Near Oceania.

Haplogroup III is only found in an area known as Remote Oceania, comprising the islands of Vanuatu, Fiji and Samoa.

The researchers claim this result allows them to reject two well-known theories for the colonisation of Polynesia, including the Express Train To Polynesia (ETP) theory and the Bismarck Archipelago Indigenous Inhabitants (BAII) theory.

These two theories are at opposite ends of the spectrum. The ETP theory focuses on a rapid dispersal from Taiwan to Polynesia. The BAII proposes that there was no migration into Near Oceania, and that the Lapita culture arose from indigenous people in the area.

Matisoo-Smith and Robins argue that the truth was somewhere in between.

The absence of Haplogroup III rats from Near Oceania seems to preclude a progressive expansion from that area into Remote Oceania where Haplogroup III rats are common.

Instead, the researchers claim, the seafarers who brought Haplogroup III rats to Remote Oceania did not come from nearby New Guinea or the Solomon Islands but from close to the Asian mainland, completely by-passing Near Oceania.

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