Collapse_ How Societies Choose to Fail or Succeed - Jared Diamond [77]
Thus, Southeast Polynesia presented colonists with only a few potentially habitable islands. Mangareva, the one capable of supporting the largest population, was largely self-sufficient in the necessities for Polynesian life, except for lacking high-quality stone. Of the other two islands, Pitcairn was so small, Henderson so ecologically marginal, that each could support only a tiny population unable to constitute a viable human society in the long run. Both were also deficient in important resources—Henderson so much so that we moderns, who wouldn’t dream of going there even for a weekend without a full tool chest, drinking water, and food other than seafood, find it mind-boggling that Polynesians managed to survive there as residents. But both Pitcairn and Henderson offered compensating attractions to Polynesians: high-quality stone on the former, abundant seafood and birds on the latter.
Weisler’s archaeological excavations uncovered extensive evidence of trade among all three islands, whereby each island’s deficiencies were filled by the other islands’ surpluses. Trade objects, even those (such as ones of stone) lacking organic carbon suitable for radiocarbon dating, can still be dated by radiocarbon measurements on charcoal excavated from the same archaeological layer. In that way, Weisler established that trade began at least by the year A.D. 1000, probably simultaneously with the first settlement by humans, and continued for many centuries. Numerous objects excavated at Weisler’s sites on Henderson could immediately be identified as imports because they were made from materials foreign to Henderson: oyster shell fishhooks and vegetable peelers, volcanic glass cutting tools, and basalt adzes and oven stones.
Where did those imports come from? A reasonable guess is that the oyster shell for fishhooks came from Mangareva, because oysters are abundant there but absent on Pitcairn as well as on Henderson, and other islands with oyster beds are much more distant than Mangareva. A few oyster shell artifacts have also been found on Pitcairn and are similarly presumed to have come from Mangareva. But it is a much more difficult problem to identify origins of the volcanic stone artifacts found on Henderson, because both Mangareva and Pitcairn, as well as many other distant Polynesian islands, have volcanic sources.
Hence Weisler developed or adapted techniques for discriminating among volcanic stones from different sources. Volcanoes spew out many different types of lava, of which basalt (the category of volcanic stone occurring on Mangareva and Pitcairn) is defined by its chemical composition and color. However, basalts from different islands, and often even from different quarries on the same island, differ from each other in finer details of chemical composition, such as their relative content of major elements (like silicon and aluminum) and minor elements (like niobium and zirconium).
An even finer discriminating detail is that the element lead occurs naturally as several isotopes (i.e., several forms differing slightly in atomic weight), whose proportions also differ from one basalt source to another. To a geologist, all these details of composition constitute a fingerprint that may allow one to identify a stone tool as coming from one particular island or quarry.
Weisler analyzed the chemical composition and, with a colleague, the lead isotope ratios in dozens of stone tools and stone fragments (possibly broken off in the course of preparing or repairing stone tools) that he had excavated from dated layers of archaeological sites on Henderson. For comparison, he analyzed volcanic rocks from quarries and rock outcroppings on Mangareva and Pitcairn, the most likely sources of rock imported to Henderson. Just to be sure, he also analyzed volcanic rocks from Polynesian islands that were much more distant and hence less likely to have served as sources of Henderson imports, including Hawaii, Easter, Marquesas, Societies, and Samoa.
The conclusions