By the time European explorers reached the islands scattered across the vast distances of the Pacific Ocean, most of the voyaging done by the Polynesians was among local island groups. But the locals seemed to know of places well beyond their shores, which suggested their ancestors not only colonized far-flung islands but returned to share knowledge of them. Eventually, it became clear that trade had distributed goods among Polynesian communities that later became isolated.

In recent years, modern techniques have revealed the staggering geographic extent of this trade, which included a visit to South America for some takeout sweet potatoes, which were later grown on islands throughout the Pacific. But the temporal extent of the trade remained the subject of debate, with some arguing for a brief burst of exchange and others suggesting extended trade networks persisted for a while. Now, a paper published in PNAS using another modern form of analysis argues for at least four centuries of trade.

The work relies on a site in the Cook Islands, which provided a rare archeological resource: an extended series of deposits that contained enough material that researchers could carbon date various layers. Also included were some traditional Polynesian stone adzes, tools made from a specific type of volcanic rock. The researchers behind the new paper obtained small samples from these tools and used them in another type of modern analysis called mass spectrometry.

Mass spectrometry involves breaking up a sample into its constituent chemicals, ionizing them, and then separating them based on their mass. For stone samples, this process allows researchers to identify all the elements present and even the isotopes of these elements. It's possible to use the presence of trace elements and isotope ratios to match samples to the quarry they originated in, since they'll share all the same idiosyncrasies.

Not surprisingly, the majority of the tools came from quarries in the Cook Islands, many from the island where the tools were uncovered (called Mangaia). But over a third had been carried there from other island groups. The closest of these were the Austral Islands, where the quarry is only 650km away. But there were also tools from Samoa (1,700km) and the Marquesas, where the quarry was over 2,400km from the site where the tools were discovered.

Trade started prior to 1300CE, when the earliest tools were discovered. And, while the most distant location in the Marquesas drops off within 200 years, trade with Samoa appears to continue through nearly to the time of discovery by Europeans. The authors do note, however, that the longer trade routes become relatively disused starting in the 1500s, something that's also apparent in the non-stone tools, such as fish hooks, also found at the site.

The big caveat, or course, is how long a tool was used before ending up lost or discarded and buried at the site. The one argument against tool use bias is that the steady stream of tools from almost all locations probably indicates that there's no particular bias to when tools ended up at the site.

The authors argue that the extent of trade makes it clear that colonization of new islands wasn't a process where goods and people were exchanged only for long enough to establish a human presence in the new location. Instead, trade continued for an extended period during which it would have "supported socially mediated imperatives such as the acquisition of high status goods, the fostering and maintenance of strategic alliances, and establishing individual and group prowess or 'mana.'"

PNAS, 2016. DOI: 10.1073/pnas.1608130113 (About DOIs).