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Afarensis has just blogged on this new PNAS paper, “Dating the late prehistoric dispersal of Polynesians to New Zealand using the commensal Pacific rat.” The goal of this paper is to clarify when New Zealand was peopled. There are several hypotheses floating out there, two of which indicate New Zealand was either peopled 1,200 years ago or 800 years ago.

One hypothesis, suggested in this 1996 Nature article, “Arrival of rats in New Zealand,” indicates people arrived with rats roughly 2,800 years ago. This was established using carbon dating of rat bones. Rats, as explained by Afarensis, are often used as a proxy to understand human migratory patterns, because rats and humans have a longstanding commensal relationship. In February, I shared some news on how rats have been used to understand migrations of humans in the Neolithic.

I’m not a rat biologist, and I don’t fully know their ecological independence. Through my personal experiences, I understand rats have been pretty dependent on humans but because these rat bones were dated to be 2,000 years older than the first human remains in New Zealand and were excavated with no supporting ecological or archaeological context, the dates have been hotly contested.

The authors of this new PNAS revisit this study and included carbon dating of rat gnawed seeds from two caves in New Zealand. The authors hope to re-clarify the presence of human occupation of New Zealand because it just doesn’t seem feasible that rats could live for so many thousands of years without humans.

Here’s a summary of the results, from Afarensis’ blog post,

“The research specifically focussed on plant seeds from plants that had been driven to extinction by the Pacific rat (or the rats have been implicated as a possible cause). All the rat bones date to 1280 AD or younger. The un-gnawed seeds are the oldest, whereas, none of the gnawed seeds date to before approximately 700 years BP. This current research dovetails with dates on rat gnawed snails.”

The new dates all confer with the time humans are understood to be present in New Zealand, but one thing that wasn’t discussed in the PNAS paper is other sources of the ratting of New Zealands. Like I said, I’m not well versed in rat ecology. I know they live in close association with humans but that doesn’t mean rats are dependent solely on humans… they just benefit from living next to humans.

See the genus Rattus is thought to have emerged from the Murid family about 3.5 million years ago in Asia. This is well before human ancestors ventured into Asia. They did fine for a couple millions of years there, living in colonies independent from humans. It is possible that rats can live in areas without a human presence.

So if we agree that rats can live by themselves, how did they get to New Zealand? 3,000 years ago, sea levels were about the same level as they are now. Janet Wilmshurst, pictured above, and one of the authors of this paper, told the press that this particular species of rat, the kiore, “cannot swim very far, it can only have arrived in New Zealand with people on board their canoes, either as cargo or stowaway. ”

I feel that Wilmshurt is treading on very fine line, speaking with such vindication that these rats had to have come to New Zealand along with humans. Many of the non-marsupial mammals of Australia came by island hopping and rafted from the north, it is possible these rats did the same.

    Wilmshurst, J.M., Anderson, A.J., Higham, T.F., Worthy, T.H. (2008). Dating the late prehistoric dispersal of Polynesians to New Zealand using the commensal Pacific rat. Proceedings of the National Academy of Sciences, 105(22), 7676-7680. DOI: 10.1073/pnas.0801507105