Story: When was New Zealand first settled?
Page 5 – Rat bones
The Pacific rat – an earlier arrival?
The Pacific rat, known to Māori as kiore, is a poor swimmer and could only have arrived in New Zealand with humans. In the mid-1990s a scientist radiocarbon-dated Pacific rat bones excavated from caves in both the North and South islands, and came up with dates as early as 50–150 AD. This meant that humans must also have arrived by this time, some of them with rats on board. The discovery was a scientific bombshell and the possibility of such an early arrival caused hot debate.
In a Hawke’s Bay cave known as the Hukanui rock overhang, a single rat bone, reportedly found beneath the ash layer from the Taupō eruption of 232 AD, returned a radiocarbon date of 134–320. This was consistent with its position below the ash layer. The bone, which was excavated in 1959 by an amateur archaeologist, lay in a matchbox in the Dominion Museum for 40 years before it was dated. How the bone got beneath the 232 Taupō ash layer is still in dispute.
However, a second rat bone from the Taupō ash layer at the Hukanui rock overhang was dated at around 1300, and a pigeon bone from the same layer was dated at 3,000 years old. As bones in undisturbed sediment layers should be about the same age, this raised some difficult questions. The veracity of the ‘matchbox bone’ is still being debated.
Can rat bones be trusted?
Other researchers, such as Atholl Anderson, raised questions about the reliability of rat bones for radiocarbon dating, including:
- Could burrowing animals have moved the rat bones beneath certain sediment layers?
- Bones from some cave sites gave dates that differed by hundreds of years from the known dates of the layers from which they were unearthed. This suggested that there was something wrong with the rat dates – that for some reason they were not giving reliable ages.
- Laboratories might not have prepared the rat bones correctly before dating them or, as happened at an overseas laboratory, the bones were somehow being contaminated during the dating procedure.
- The bones might contain old carbon that the rat had intercepted through the food chain, and so the dates may not accurately represent the true age of the rat.
Before more scientists are convinced of the early arrival scenario, there would need to be a new find of rat bones from beneath the 232 Taupō ash layer, or a corroboration of the 50–150 dating of rat bones, using other dating methods. The most recent radiocarbon dates of rat bones, published in 2004, do not support the early arrival scenario. Six dates on rat bones from one of the South Island cave sites, carried out by two different laboratories, all came out at 1250 or younger.
If people did arrive as early as 50–150, and introduced the rat, they either died out or did not stay, as there are no other traces of human settlement until 1250–1300.