Ancient DNA of the Pacific rat (Rattus exulans) from Rapa Nui (Easter Island). SS Barnes, E. Matisoo-Smith, and T. Hunt.
We report analysis of ancient mitochondrial DNA sequences from nine archaeological specimens (8 femura and 1 incissor) of Rattus exulans excavated from Anakena Beach Dune on Rapa Nui. Sequence of a 239-base-pair fragment of the hypervariable mitochondrial control region reveals a single mitochondrial DNA (mtDNA) sequence of all samples corresponding to the R9 haplotype prevalent in East Polynesia. This suggests a single or very limited introduction of Rattus exulans to the island. Rapa Nui, like other remote islands of Polynesia, remained effectively isolated following colonization.
It's not published yet, but it's on the web site (link) as In Press. Because the same haplotype was found in the lowest/earliest levels of the site and also that there was little/no variation over time suggests a single introduction of the species, which points to a single set of human immigrants as well. As they not, this isn't conclusive since it's possible that people could have arrived without any more rats. This seems unlikely given other rat populations elsewhere have much more diversity.