The dating project, in one of the largest studies of its kind, has shown that the country was not visited by humans over 2000 years ago, as some previous research suggests.

An international team of researchers, led by Dr Janet Wilmshurst from Landcare Research, spent 4 years on the project which shows conclusively that the earliest evidence for human colonisation is about 1280-1300 AD, and no earlier. They based their results on new radiocarbon dating of Pacific rat bones and rat-gnawed seeds. Their results do not support previous radiocarbon dating of Pacific rat bones which implied a much earlier human contact about 200 BC.

The original old rat bones dates have been hotly debated ever since they were published in Nature in 1996. The ages are controversial because there is no supporting ecological or archaeological evidence for the presence of kiore or humans until 1280-1300 AD and the reliability of the bone dating has been questioned. This is the first time that the actual sites involved in the original study have been re-excavated and analyzed.

Dr Wilmshurst and her team researchers re-excavated and re-dated bones from nearly all of the previously investigated sites. All of their new radiocarbon dates on kiore bones are no older than 1280 AD. This is consistent with other evidence from the oldest dated archaeological sites, Maori whakapapa, widespread forest clearance by fire and a decline in the population of marine and land-based fauna.

“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” Dr Wilmshurst said.

The dating of the rat bones was also supported by the dating of over a hundred woody seeds, many of which had distinctive tell-tale rat bite marks, preserved in peat and swamp sites from the North and South Islands.

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“These rat-gnawed seeds provide strong additional evidence for the arrival of rats, and therefore humans, and are an indirect way of testing the veracity of the dates we have done on rat bones,” said Dr Tom Higham, Deputy Director of the Oxford Radiocarbon Accelerator Unit at Oxford University.

Rats leave rows of narrow grooves or bite marks on woody seed cases when they gnaw open the seed, and these distinctive teeth marks can be seen with the naked eye. “The width of the teeth marks left on the woody seeds exactly match those of a rat's two front teeth, and cannot be mistaken for any other seed predator. We have dated over 100 individual seeds, some rat-gnawed, others intact or bird-cracked, which show that rat gnawed seeds only occur in both the North and South Islands of New Zealand after about 1280 AD”, Dr Wilmshurst said.

With over 165 dates on seeds and bones from a large number of sites, the overwhelming evidence suggests that rats and their human carriers did not reach New Zealand until about 1280 AD.

“The earliest dates for rat and human arrival are strikingly consistent with the oldest dates from archaeological sites, the first large clearances of forest by fire, and declines or extinctions of marine and land-based fauna. It now seems certain that the first M ori settlers arrived in New Zealand sometime about 1280 AD. The consistent picture emerging now is that settlement was much later than the old rat bone dates led many people to believe” said Professor Atholl Anderson, from the Australian National University.

This age has several important implications; firstly rat predation only began after 1280 which is much shorter period than previously implied and makes the risk to currently declining populations of rat-sensitive species more pressing as they could be diminishing faster than previously assumed. Secondly, colonisation did not involve a protracted delay between initial discovery and subsequent colonisation, an idea implicit in earlier theories. The first people arriving in New Zealand from tropical east Polynesia initiated an immediate and rapid transformation.

“A precise date for the arrival of the rat helps us to fully understand both the history of human settlement and the past and present ecological impacts of kiore on native fauna and flora. It also allows the human settlement of New Zealand to be placed more accurately in the context of the broader settlement pattern of East Polynesia” Dr Wilmshurst said.

Dr Wilmshurst and her colleagues are now turning their attention to other islands in East Polynesia where similar controversies exist over the timing of initial human settlement. Dates on rat-gnawed seeds and rat bones are likely to be just as helpful in resolving controversies on other Pacific islands where rats were also introduced prehistorically.