Story: When was New Zealand first settled?
Page 6 – Rat DNA
Although Pacific rats cannot speak, their DNA tells many stories. In the 1990s scientists began to take a closer look at mitochondrial DNA, which is maternally inherited. Variations in DNA from different populations reveal how they are linked, and how long they have been separated. While the DNA of the Pacific rat (kiore) in New Zealand links mainly to rats from the acknowledged Māori homeland of East Polynesia, some DNA shows links to Norfolk Island. New Caledonia is the closest large tropical island to New Zealand. Voyaging canoes might have used Norfolk Island as a stepping stone in a voyage from New Caledonia to New Zealand, or even on a return voyage.
In the period 50–150 AD there were people, and Pacific rats, living in New Caledonia, Fiji, and West Polynesia. But there are no radiocarbon dates showing settlement this early in East Polynesia. So if rats did arrive in New Zealand around 50–150, then in all probability, the people who brought them did not come from East Polynesia but rather from western island groups.
Computer simulations show that it was just as easy for voyaging canoes to sail to New Zealand from New Caledonia, Fiji, or West Polynesia, as it was from East Polynesia. An early canoe landing in New Zealand from this region remains a possibility. In such a scenario the crew either left, died, or were all male (and therefore died out) – but the rat survived. Future analysis of DNA from rat bones dated at 50–150 AD from these western island groups may prove revealing.
While this possibility is interesting, it is emphasised that the settlement of New Zealand from 1250–1300 was definitely from islands in East Polynesia, as shown by DNA of Māori descendants, and other evidence such as early artefacts.
Gnawed seeds and snails
Forest seeds are commonly eaten by Pacific rats, who leave tooth marks on woody seed cases. These gnawed seed cases, found preserved in sediment and peat deposits, can be radiocarbon-dated as direct evidence of rat presence. Seed cases have so far been dated from sites in Taranaki and the Coromandel. The earliest evidence of seed consumption by rats, dated by radiocarbon, is from the late 13th century, or 700 years ago. Seed cases older than 800 years show no rat damage. As nibbled seed cases have only been found above or within the Kaharoa ash layer, they must be more recent than 1314 (±12).
Rats also ate land snails. So researcher Fred Brook looked at gnawed fossil shells of native land snails in Northland. The snails survived the rat attacks, and their shells repaired themselves, but they still displayed the tooth damage. Because of this, he was sure that the rat damage did not occur on empty shells at some later date. Brook only found rat-damaged shells dating from 1250 onwards. Shells older than this date displayed no rat damage.
Research on gnawed forest seeds and gnawed snail shells is consistent with the view that rats (and humans) first arrived around 1250–1300. It suggests that the early radiocarbon ages for rat bones may not represent the true age of these rats.