American Scientist

The trick about studying the past is figuring out where relevant documents might be stored. I use "documents" in the broadest sense, because documents are not only written objects but also physical ones. The documents of anthropology are usually tools, ceramics, signs of buildings like walls or postholes, human remains, traces of DNA, cut or burned animal bones—tangible debris that has survived the ages. Instead of leafing through dusty archives, anthropologists dig trenches, sifting dirt for clues that humans once were there.

Looking for very early traces of human settlement in a region is difficult, whether the region is a huge continent, a small island, or in between. In most cases, the number of initial settlers was small and their remains and artifacts scarce. If there is a distinctive and common item that can be traced unquestionably to human presence—a new kind of document—then interpreting the archaeological record becomes a lot easier. 

The Right Documents

The archaeology of Remote Oceania is a perfect example of document discovery. The timeframe of human arrival in the western area (the region of Fiji, Tonga and Samoa) is reasonably clear because of the distinctive Lapita ceramics that are found in archaeological sites dating from about 950 B.C. in Fiji, Tonga and Samoa. But over time the people who had so thoughtfully littered the western islands with bits of Lapita ceramics when they first arrived made fewer and fewer ceramics. By the time people left the western region to migrate into eastern Polynesia (including New Zealand, Hawaii, the Marquesas, Rapa Nui or Easter Island and Tahiti), they had stopped making pots entirely. Thus, dating the second, eastern part of the colonization of Remote Oceania is difficult.

When humans arrived in East Polynesia is particularly significant because so many of the islands of Oceania had experienced ecological catastrophes by the time of European arrival. For example, the deforestation of Rapa Nui has become an icon of human folly and ecocide, despite recent challenges to this idea. What pattern of settlement and catastrophe prevailed in the entire region?

The archaeological and ecological record spanning human arrival in East Polynesia is most detailed and densest in New Zealand. At A.D. 1285 to 1300, Wairau Bar on South Island is the oldest well-dated archaeological site in New Zealand. On that site Maori butchered more than 8,000 moas and consumed more than 2,000 moa eggs; in upper layers, there are huge numbers of seal bones, too. There are postholes from buildings, cutmarked moa bones and broken eggshells, stone tools, and burials of 37 individuals whose graves included such items as perforated moa eggs, real and imitation whale teeth, necklace reels, shark teeth, bird bone tubes and adzes. Moa eggshells placed as grave goods in human burials were dated by the radiocarbon method. Because of Wairau Bar, A.D. 1300 can be used as a dividing line between the time periods before and after proven human arrival.

About 1000 B.C.—before human arrival—85 to 90 percent of the New Zealand landscape was forested. After A.D. 1300, pollen from tall trees declined precipitously, and by 1840 about 70 percent of the forest had been replaced by bracken ferns and grasses. Starch from the bracken root is a major part of traditional Maori diet. Bracken was not planted, but was maintained and encouraged by frequent burning to remove competing woody plants. Not surprisingly, after A.D. 1300, the frequency of charcoal-bearing layers increased dramatically, in some places by an order of magnitude. Charcoal and bracken spores documents the chronology of human settlement in this region.

"Fern root or aruhe is not the only reason Maori favored open country," argue palynologists Matt McGlone and Janet Wilmshurst of Landcare Research in New Zealand, writing with anthropologist Helen Leach of the University of Otago in the New Zealand Journal of Archaeology in 2006. Traditionally, forest impeded the Maori ability to travel for hunting, gathering or trade. In 1844, the Reverend William Colenso wrote about an overland trip on North Island in which his Maori guides were "vociferating loudly their being privileged to see a koraha maori (indigenous fern-land, open country) again" after traveling for days in dense forest.

Human presence unmistakably altered the animal community of New Zealand too. After A.D. 1300, there was a 50 percent decline in bird species—the loss of about 40 species—that once bred on the New Zealand mainland. In addition, a bat, several species of frogs and numerous lizards became extinct. Remains of all but two of the extinct birds have been recovered from at least one archaeological site, showing they were still present when humans arrived. During the early occupation period, fur seals and sea lions also underwent a marked contraction of their range.

The extinction that causes me personally the greatest heartache was of 11 species of large flightless moas. Most of these birds were about the size of ostriches, but much heavier. The one I would really love to have seen is the wonderful Dinornis or "prodigious bird" named by English paleontologist Richard Owen in 1843. Its bones indicate that the top of its back was over 6 feet high and its weight was about 550 pounds. To have missed seeing such an extraordinary creature severely saddens me.

Probably the Maori felt, or feel, the same way. In 1870 Sir George Grey recorded a Maori saying, "Ka ngaro, I te ngaro, a te moa." This phrase translates as "to be lost as the moa is lost" and refers to a tragic and irreversible loss.

Enter the Rat

But with this remarkably well-documented record, why is it so difficult to date human arrival in New Zealand? Thereby hangs the tail (forgive the pun) of the kiore or Polynesian rat, Rattus exulans. Prior to human arrival, bats were the only indigenous mammals on New Zealand or the other islands of Remote Oceania. The kiore arrived in Polynesia with humans as a notorious and highly successful stowaway, like the Norwegian rat more familiar to Europeans and Americans.

In 1994, accelerator mass spectrometry (AMS) radiocarbon dating began at the Rafter Laboratory in New Zealand, making accurate dating of small bones possible. Richard Holdaway of Canterbury University and Atholl Anderson of Australian National University got the brilliant idea of using specimens of Polynesian rats as documents of human presence. Even if the founding colonies of humans were very small and slow to reproduce, the rats that came with humans were likely to have reproduced rapidly and spread quickly. Finding and dating rat bones should be much easier than finding human bones or other human traces. Their idea was to test the reliability of rat bone dates using archaeological specimens from sites of known radiocarbon age and then to extend the method to specimens from natural sites of unknown age.

Holdaway used rat bones, excavated from New Zealand sites that had been formed by laughing owls. These birds swallow their prey whole and, after digestion, sit on favorite roosts and cough up neat little packets of bones known as owl pellets. Based on gelatin extracted from these owl pellet bones, the rats were almost 2,200 years old, as Holdaway reported in 1996 in Nature. However, this meant that rats were present on New Zealand for at least 1,000 years before the earliest archaeological record of humans. Holdaway explained the discrepancy by hypothesizing that Maori had paid an early, transient visit to New Zealand—leaving rats—but did not settle there in numbers until about A.D. 1300.

The early rat dates implied that faunal and floral extinctions in New Zealand occurred in two stages. The first stage saw perhaps a drop in species abundance, but no extinctions: that was the "rats only" phase during which the rodents bred freely and ate plants, fruits, seeds, insects, lizards, snails, eggs and nestlings of ground-breeding birds. The second stage, about 1,000 years later, saw the founding of sizeable settlements of humans who hunted, fished and trapped with enthusiasm, triggering numerous extinctions. This interpretation is known as the traditional or "long chronology" of New Zealand.

Anderson addressed the dating issue by using material from an excavation he had led between 1988 and 1992 of the huge, stratified, moa-hunting middens at a site called Shag River Mouth. The remains of at least 6,000 moas show how skillful and systematic the moa hunters were. Anderson found a pattern of deliberate butchery, with moa heads and necks discarded in one pile, pelvises in another, complete ribcages in a third. The major bone of the thigh, the femur, carried big muscles and was usually intact but the big bone of the lower leg, the tibiotarsus, was smashed, for marrow or to obtain thick bone pieces for manufacturing tools. Sometimes a heap of bones was formed of only one species of moa, suggesting that Maori may have taken down an entire flock. Moas were roasted across small depressions or ovens, many of which were preserved intact.

In addition to moa remains and ovens, there were at least 43 hearths, 36 huts, many adzes and hafted blades, fish hooks, manufacturing tools, various ornaments and two intact moa eggs, one near a human burial. At the time of excavation, Anderson took nearly 50 samples for radiocarbon dating—samples of charcoal from hearths and moa ovens, bird bone, marine shell and moa eggshell—and the dates grouped tightly around A.D. 1350 to 1400.

In the face of this hard evidence that Maori had a very well-developed and effective moa-hunting adaptation between A.D. 1300 and 1400, Anderson was troubled by the early dates Holdaway had obtained for Polynesian rats.

"It just did not make sense to me," Anderson explains, "that people could have been in New Zealand for 1,000 years before they started killing moas and seals, making shell middens, flaking stone and so on. What were they doing instead? Holdaway's default position, that people had arrived and then left almost immediately, also looked improbable to me."

"To resolve the problem, I took six samples of rat bone from the same provenances as the other 50 dating samples from Shag River Mouth. The Rafter laboratory in New Zealand (which had dated Holdaway's rat bones) dated four of them between 180 B.C. and A.D. 700, but two samples that we sent to the lab in Oxford were dated to A.D. 1000–1200. Clearly something was wrong," he says. Anderson's paper suggesting that there were technical problems with dating rat bones was published in Archaeology in Oceania in 1996, at the same time as Holdaway's Nature paper reporting the early dates on rat bones.

This led Anderson and Wilmshurst to argue for the "short" chronology, which favors a more-recent peopling of eastern Polynesia that began about A.D. 900 and reached New Zealand around A.D. 1300.

As the debate proceeded, Anderson noticed that Holdaway's dates clustered according to the time at which the Rafter Laboratory had processed them. Thirty of the first 32 dates calculated by that laboratory fell between 200 B.C. and A.D. 800. Of the next 28 dates run by the same lab, 25 were from A.D. 900 or more recent. Anderson suggested that perhaps the Rafter laboratory had failed to remove contaminating, older carbon from the rat samples in pre-treatment or the rats had incorporated older carbon from their diet into their bones.

Like Anderson, Wilmshurst had found discrepancies between Holdaway's rat dates and the evidence from the sedimentary profiles she examined in the 1990s and early 2000s. In core after core—she has studied more than 150 now—she found a major change in the flora and in the frequency of charcoal-bearing levels dating to about A.D. 1300. The signature of human arrival seemed clear and matched the archaeological evidence.

In the face of skepticism, Holdaway continued to test rat bones, seeking further refinement of the long chronology hypothesis. None of the bones in his second round of testing were as old as those in the first.

In recent years, the short chronology has been strengthened by additional new dates indicating that all of East Polynesia except New Zealand—Hawaii, the Marquesas, Easter Island, the Society Islands—was colonized about A.D. 900, much later than the rat dates in New Zealand. These findings raised a new paradox. If New Zealand was colonized at 200 B.C. and the rest of East Polynesia was not settled until A.D. 900, then the Maori colonizers of New Zealand could not have brought East Polynesian culture to New Zealand when they settled there. However, Maori culture is indisputably part of East Polynesian culture.

Coming from separate perspectives, Wilmshurst and Anderson found themselves agreeing that if the date of Wairau Bar was an accurate document of a very early colonization of New Zealand by humans, then the early rat dates could not be correct. A new approach to testing the dating was needed.

Seeds of Change

In 2001 Wilmshurst recognized a new sort of document of human presence: woody seeds that had been gnawed by rats. She had found many such seeds in her botanical samples and could easily tell rat-gnawed seeds from those broken by birds. She and dating specialist Thomas Higham of the Oxford Radiocarbon Accelerator Unit tried to resolve the debate by using rat-gnawed seeds as proxies for rats. If rats had been present for more than 1,000 years earlier than the Wairau Bar site, there ought to be plenty of older rat-gnawed seeds. But they found none older than Wairau Bar in the two North Island sites.

Urged to look for evidence on South Island, Wilmshurst added Anderson and avian paleontologist Trevor Worthy from the University of Adelaide to her team for a three-year project that used a two-prong approach. First, they re-excavated Holdaway's oldest two sites—called Predator Cave and Earthquakes #1—to collect new rat bones for dating. They also re-examined bones, now in museums, that had been originally collected by Holdaway. Second, Wilmshurst and Higham collected and dated woody seeds from a further eight sites, some on South Island.

As the team reported in a 2008 paper in the Proceedings of the National Academy of Sciences, all of the rat bones they tested were more recent than A.D. 1280. The 48 gnawed seeds they dated were all more recent than A.D. 1250. Another 48 intact or bird-broken seeds ranged in age from about 2000 B.C. to about A.D. 1800. A volcanic ash from North Island, in which they found rat-gnawed seeds, provided an independent check as it was dated to A.D. 1314. Combining all of their data, Wilmshurst and her colleagues concluded that the earliest presence of rats in New Zealand was securely dated to about A.D. 1290 to 1380, a range closely matching the dates of the older archaeological sites and confirming the short chronology. Apparently, Polynesian rats took less than 80 years to spread through out North and South Islands—about the same time span recorded for the spread of the European rat, Rattus norvegicus, after its arrival.

With the short chronology established, the massive ecological impact that has been called the "dreadful syncopation" of human arrival, faunal extinction and deforestation is also validated. Within a few centuries of their arrival, the people of eastern Polynesia devastated the animals they found in their new habitat and cut down and burned much of the forested land.

And yet, as Anderson points out, from the point of view of the invading species, this was not a disaster but a triumph.

"The first duty of colonizers is to survive. That requires rapid population growth sustained by consuming the richest resources. Relentless slaughter of large flightless birds and breeding seals, and sustained burning of the forest to open up land for edible ferns and gardening was often the result, but without such environmental change people could hardly have inhabited Remote Oceania at all."

The same might be said of rats.