Rats as Proxies for Human Expansion in the Pacific

ResearchBlogging.org The Proceedings of the National Academy of Science has an interesting article on the settlement of New Zealand. The short version of the story is that since Pacific rats are human commensals they can serve as proxies for human expansion across the Pacific. Based on this idea, the authors of the paper used radiocarbon dating to date the first appearance of Pacific rats in New Zealand. The long version of the story serves as an instructive example on the repeatability of science and a rebuke to creationists who accuse evolutionists of arbitrarily accepting or rejecting radiocarbon dates based on their conformity to evolutionary theory. This post will explore the first part and I will return to the latter two issues in a separate post.


There are several theories as to how New Zealand was peopled. The first proposed that New Zealand was colonized around 800 AD by a small population with a low growth rate which rendered them archaeologically invisible for several centuries. The second proposed that New Zealand was settled circa the 12th century AD. This latter was supported by radiocarbon dates from the earliest archaeological site and mtDNA (which indicated a larger, late arriving population. A third proposed – somewhat more extreme than the first – proposed that New Zealand was colonized around 0-500 AD by a small population with a low growth rate that remained archaeologically invisible for about 1,000 years. This population was also assumed to be somewhat transient. The support for this third theory came from AMS radiocarbon dates on rat bones that put their introduction into New Zealand at approximately 200 BC (more on these dates in the next post).
The new research dated new rat bones from two caves (Earthquakes 1 and Predator Cave). It also looked at the dating of rat gnawed vs. intact seeds from several sites in New Zealand. 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 authors of the paper conclude:

AMS radiocarbon dates on 30 Pacific rat bones and ~100 woody seed cases are consistent in showing that the Pacific rat was widespread in New Zealand by ~1280 A.D. The dates provide no evidence for the presence of rats at any time during the preceding millennium, as suggested by previous dating of rat bones … Our findings, based on several lines of evidence, indicate that the Pacific rat was introduced by the first human colonists from central East Polynesia to both main islands of New Zealand ca.1280 A.D.

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

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2 Responses

  1. I am a first year anthropology student and therefore do not process adequate information to eiher agree or disagree with the findings. However, I do have a question. The premise as oudtlined is base entirely on the belief that the rats came over with man. Other migration theories on other species proposed migration was independent of man. Is there any reason not to assess the same conclusion with the Pacific Rate (i.e. independent migration)?
    Thank you,
    Michael Peterson

  2. As I indicated in the post, Pacific rats are human commensals and live and travel in close association with humans (for the most part). They also can not swim long distances. Additionally, they do not show up on a number of islands prior to the arrival of humans.

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