A revised timeline for the arrival of settlers on Mangaia island in Polynesia reveals the resilience of this population, which overcame an environmental crisis through bold measures to support a sustainable society.
During the human colonization of the Pacific islands that make up Polynesia, there was much variation in how those early societies adapted to live on islands that had greatly differing environments1. That co-variation in societies and environments has made Polynesia a useful model system for understanding social development. The Polynesians on the islands of Hawaii and Tonga managed to evolve proto-states with populations in the hundreds of thousands1. Yet the early settlers on Henderson and Pitcairn islands died out or emigrated2. In a new monograph, Tangatatau Rockshelter (Mangaia, Southern Cook Islands), Kirch et al.3 have published a reanalysis of their group's earlier archaeological fieldwork data from the Polynesian island of Mangaia, using techniques not available during the initial studies. The lessons learnt from this re-evaluation of how the Mangaians settled a fragile environment and overcame severe problems of population pressure and resource shortages might be instructive for modern societies that are similarly struggling to achieve a sustainable way of life.
The earliest inhabitants of Polynesia came from Taiwan via the New Guinea region and began to reach western Polynesia around 1050 BC4,5. Eastern Polynesia's more remote islands (including Mangaia) were colonized after an interval thought to be of around 1,000 years6 on the basis of the dating of human-associated materials carried out in the 1990s. This timeline and the archaeological data suggested that the Mangaians faced a gradually unfolding resource crunch over 1,500 years that then plunged them into violence and cannibalism.
This timeline was proposed on the basis of the radiocarbon-dating methods available in the 1990s, which included the analysis of large charcoal samples in which the plant species burnt were not specifically identified. Dating methods available nowadays differ in two respects: accelerator mass spectrometry enables the dating of tiny samples, and improvements in methods of studying plant fragments have allowed identification of the plant species that provided the charcoal sources. Kirch et al. applied those methods to their Mangaian studies, particularly in the analysis of a large overhanging rock, known as the Tangatatau Rockshelter, which was a site of human activity for many centuries. This permitted much of Mangaia's archaeological history to be traced in one long continuous sequence, rather than relying on a reconstruction that combines 'snapshots' of many sites occupied at different times.
These new analytical techniques show that Mangaia was settled only around AD 1000, a thousand years later than originally believed7. The conclusion of a 2,000-year 'long pause' between the settlement of eastern and western Polynesia has also been reached for other eastern Polynesian islands8. In retrospect, it seems that the erroneous dating from the 1990s was because much of the charcoal came from long-lived tree species that had already been growing for centuries before people arrived and burnt them. Now, by dating only short-lived plants (including some introduced by the Polynesians themselves) and the bones of Polynesian-introduced rats, Kirch and colleagues have revised their proposed habitation timeline, revealing that Mangaia's resource crunch developed within just a few centuries of island habitation.
The authors found that the crunch arose from initially exponential human population growth together with resource depletion in a vulnerable environment. Polynesians began by felling forests to subsist, using slash-and-burn agriculture on volcanic slopes. But Mangaia's thin, old soils were leached of most nutrients, and, once eroded, could support only fern regrowth. Most native landbird species and some seabirds were exterminated. After depleting reef populations of large molluscs and fish, Polynesians had to switch to smaller species offering less meat for more work. They raised pigs prized for rituals, but this was an inefficient use of resources because 10 kilograms of the crop plant taro are needed to produce 1 kg of pork.
By AD 1400, food shortages and population pressures resulted in violence and cannibalism that were still remembered in oral traditions of the 1800s when Mangaians encountered Europeans9. The bones of 41 people excavated from an archaeological site show the marks of having been butchered, cooked and discarded in the same way as pig bones were.
Kirch and colleagues studied how the Mangaians tackled their resource problems after this nadir. The islanders switched from slash-and-burn agriculture to high-yield irrigated agriculture in flat swamplands, fertilized by soil eroded from the slopes (Fig. 1). They also had the good luck to obtain the South American sweet potato, which is a much more productive crop than taro. Pigs disappeared from the archaeological record on Mangaia, which was probably the consequence of a courageous decision to kill all the pigs, as occurred on the Pacific island of Tikopia10. By eating taro themselves instead of feeding it to the pigs, the Mangaians gained ten times more calories. For protein, they developed sustainable methods of harvesting freshwater fish, eels and ducks from ponds.
As a source of mammalian protein, Mangaians replaced pigs and people with rats. When talking to Europeans in the 1800s11, Mangaians spoke of delicious food using the expression, “It is as sweet as a rat!”. Although violence persisted on Mangaia, it became moderated by political unification under a paramount chief, and cannibalism ended.
These discoveries raise other questions. How rapid were the landbird extinctions following the arrival of Polynesians? The authors' data show that the birds were gone within 500 years, but the time-resolution of these data is coarse. Only additional dating of bird bones, together with species identification, will provide the answer. I'll bet that the extinctions took only a few decades. The same question remains to be answered for other faunas that evolved in the absence of modern humans. Were such creatures not afraid of humans and therefore particularly easy hunting targets?
What ended Polynesia's long pause? Were the wide seaways between eastern and western Polynesia crossed because of changes in wind patterns and canoe technology12? Or did one lucky voyager discover a large, uninhabited archipelago and bring back the news, leading to a subsequent stream of voyages13? Another mystery is how South American sweet potatoes reached Polynesia across 4,000 kilometres of ocean14. Did some Native American predecessors of the explorer Thor Heyerdahl make an amazing one-way voyage from South America to Polynesia? Or perhaps Polynesians made an even more amazing journey from Polynesia to South America and back again.
This study of the Mangaians illustrates how ancient people developed a sustainable society, changed their expectations about what type of food to eat, and thereby escaped from their population and resource trap. Will our society follow their example of adaptive flexibility, or will we instead vanish like the populations of Pitcairn and Henderson islands?