After studying its genetics, a scientist in Australia thinks he can now explain how people colonised the islands of the Pacific.

This has long been a hot subject for debate among researchers. Some have always thought that humans moved out rapidly from South East Asia, through Melanesia, with very little mixing between the different colonist groups.

Others argue the movement was much more disjointed, occurring over an extended period from different Melanesian populations.

However fast they moved, it is likely they carried animals on board their canoes - either intentionally or as stowaways.

To try to settle the argument, Christopher Austin, an evolutionary biologist, has studied the mitochondrial DNA of the Lipinia noctua lizard, which lives alongside humans on Pacific islands ranging from Hawaii in the northeast to Easter and Pitcairn island in the southeast.

Mitochondrial DNA is passed down virtually unchanged from mother to child. It mutates at a steady rate and therefore provides a useful evolutionary clock that allows scientists to track genetic lineage.

Genetic similarity

Austin, from the South Australian Museum in Adelaide, says his analysis supports the fast hypothesis - humans and lizards caught the "Polynesian express train".

"The extreme genetic similarity between the different colonies indicates rapid colonisation from a single source, which I take as support of the express-train hypothesis," Austin says in the science journal Nature.

"Although they are geographically part of Micronesia, the people of Kapingamarangi Atoll are Polynesian in origin.

"The L. noctua from there are also of the central/eastern clade, which strengthens the association between L. noctua and human colonisation," he adds.

Archaelogical, linguistic and genetic data show humans migrated east from Taiwan between 3500 BC and 1600 BC.

In a separate study, Dr Lisa Matisoo-Smith, at the University of Auckland, used the mitochondrial-DNA technique on rats to show that the Southern Cook and Society Islands formed the focus from which migration to other islands began.