Chew clues: A fossilised nut of a native New Zealand tree, bearing the distinctive nibble marks of a Pacific rat.

Credit: Landcare Research

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SYDNEY: A new analysis of rat bones has challenged a controversial claim that Polynesians first arrived in New Zealand 2,000 years ago. The first colonists really did arrive 700 years ago, says the report.

"There are multiple lines of evidence to suggest this, and rats are now paralleling that story," said study coauthor Trevor Worthy at the University of Adelaide in South Australia.

In a paper published today in the U.S. journal The Proceedings of the National Academy of Sciences, researchers led by Janet Wilmshurst, from environmental research company Landcare Research in Lincoln, New Zealand, report new evidence that the earliest known forerunners of the Maoris arrived between 1280 and 1300.

Pacific stowaways

In an earlier 1996 study, in the U.K. journal Nature, a different team of researchers revealed that rat bones found at several sites - including predator cave in the north of the South Island - were up to 2,000 years old. As no land mammals (apart from bats) were present in the country prior to human habitation, these rats were assumed to have arrived from elsewhere.

"As the Pacific rat or kiore cannot swim very far, it can only have arrived in New Zealand with people on board their canoes, either as cargo or stowaways. Therefore, the earliest evidence of the Pacific rat in New Zealand must indicate the arrival of people," said Wilmshurst who is a palaeoecologist. Rat remains have been used to track and date human colonisation of islands throughout the pacific.

However, the finding of rat bones dating to 2,000 years ago was inconsistent with the oldest human bones and archaeological remains known from New Zealand and also significantly pre-dated the earliest evidence of deforestation, fires, and other human damage to the environment.

"There is no supporting ecological or archaeological evidence for the presence of the Pacific rat or humans until 1280 to 1300 and the reliability of the bone dating from that first study has been questioned," said Worthy.

At the time of the first analysis, the carbon dating used was stretched to the limits of what was technologically feasible, he said, and the rat bones had to be destroyed to perform the analysis, meaning the tests could not be repeated.

In order to settle the question, Wilmshurst, Worthy, and their team returned to two of the same sites that yielded the bones for the 1996 study and dug more rat remains from the same depth or deeper in the sediments. They also excavated swamps on both the major islands of New Zealand, looking for fossils of large seeds produced by a local species of woody tree, which rats feed on today.

Despite the discovery and dating of more than 30 rat bones from the sediments, none were proved to be more than 680 years old. The researchers also dated 50 seeds that had been chewed by rats, with the oldest dating to 1280. More than 100 unchewed seeds were found and these yielded dates up to 3,000 years old.

"Selective reading of the evidence"

Furthermore, the scientists said that the widespread extinction of New Zealand's native birds – such as the moa – didn't begin until after 1300 which is consistent with the latest finding.

David Lowe, a geoscientist at Waikato University in New Zealand, who was not involved in the study, said he is confident the new results are correct. He argued that the various lines of evidence for first settlement around 1300 are now numerous and substantial.

“The new rat bone ages clearly show that the ‘old’ dates published back in 1996 are untenable because they cannot be reproduced," said Lowe. "The dates on the rat-gnawed woody seed cases from sites in both North and South islands have provided a very strong line of support for the new rat bone ages.”

Richard Holdaway, coauthor of the original 1996 study, and now at Palaecol Research, in Christchurch on New Zealand's South Island, stood by his date of 2,000 years old for the rat bones.

He said that although New Zealand wasn't widely colonised until at least 1290, it had been visited by humans, carrying rats with them, over 1,000 years prior to that. He also disputed the claim that trends in the extinction of New Zealand fauna agreed with an arrival date for rats and people of around 1300.

"There are many data - published and unpublished - which speak to the extinction of small organisms on the main islands of New Zealand before [widespread] human colonisation," said Holdaway. "Only a selective reading and citation of the evidence could unequivocally take the opposing view."

Holdaway further noted that the oldest rat bones would have been the rarest in the sediments, making them the least likely to be found.