Perhaps resource shortages due to climate and sea level change.
(At right: The Polynesian Voyaging Society's double-hulled canoe Hokule'a, its red sails wet from a passing squall, tests Polynesian sailing techniques. Jan TenBruggencate photo.)
A pair of researchers argues that past climate change and sea level have had significant impacts on human migrations that occurred thousands of years ago in the Pacific and tens of thousands of years ago in the Asian coastal regions.
They argue that a drop in sea level roughly 4,000 years ago could have dried up lagoons and estuaries in the far southwestern Pacific, suddenly creating limits in what had previously been a resource-rich area. The response, the scientists say, could have been the launch of a seafaring culture unprecedented in the history of the world--Polynesia.
In the Journal of Biogeography, Kevin Pope of Geo Eco Arc Research and John Terrell of The Field Museum, say their evidence finds that people have moved in response to periods when climate and sea level made it easier to do so—or made it hard not to. Their paper is “Environmental setting of human migrations in the circum-Pacific region.”
A big expansion into the Pacific coasts of Asia occurred during a period when conditions were favorable, 50,000 to 40,000 years ago, they write.
“It is possible that modern humans reached southern Asia by 80,000 (years before present), but the meagre archaeological evidence for these early colonizers suggests that, if they existed, their populations were kept small by the unstable environments” before 50,000 years ago.
Thailand, Australia, New Guinea, the Philippines, Indonesia and nearby areas all were inhabited in this period after 50,000 years ago, the authors say. It was an amazing era of widespread human migration, all during just a few thousand years.
“A period of relatively stable climate and sea level from(about) 45,000 (years before present) to 40,000 (years before present) supported a rapid coastal expansion of modern humans throughout much of Southeast Asia, enabling them to reach the coasts of northeast Russia and Japan by 38,000–37,000 (years before present,)” Pope and Terrell write.
Then there was a period of colder weather that inhibited further occupation of new territories. It was the last glacial period, and it lasted from roughly 33,000 to 16,000 years ago. Other sources say that glacial period peaked about 20,000 years ago. During rapid cooling or warming, sea levels can change dramatically, either flooding coastal habitations or leaving them too far from the sea to be useful.
At the end of that period, ice retreated and seas rose. The authors say that it wasn't until about 16,000 years ago that sea level rise had slowed to the point that coastal territories could be confidently occupied. That's even though they were still 300 feet below today's levels.
It seems reasonable to assume that a culture need an extended period of coastal living time to develop significant maritime skills. Yet early Asian coastal folks did have some marine voyaging capabilities, which helped them cross channels of several dozen miles to get to islands near the Asian coast.
The authors say that a marked increase in rich estuaries, reefs and lagoon ecosystems developed between 12,000 and 7,000 years ago, and that a lot of coast development occurred during this period.
Scientists use the term Holocene to refer to the period from 10,000 or 11,000 years ago to now. It is within this comparatively short period during which occurred virtually all of the history of Polynesia.
“The most notable late Holocene human expansion is the settlement of the numerous islands in Oceania, which began (about) 3500–3000 (years before present), ultimately reaching Hawaii and Easter Island in the last 1000–2000 years,” Pope and Terrell write.
The authors tackle the question of why it didn't happen earlier.
Perhaps seagoing skills hadn't yet developed to the point they reached at this period. Perhaps the culture had not yet developed a “well-adapted and 'portable' agricultural technology to support long-range colonization efforts.”
Those ideas have been suggested by others, but Pope and Terrell suggest the real reasons are different. Climate and sea level change may be the key, they argue.
Perhaps a dynamic culture with a large population capable of undertaking resource-costly voyaging missions couldn't exist until “the sea-level rise slowed down sufficiently to permit the development of coastal lagoon and estuary ecosystems capable of supporting large permanent coastal settlements.”
They figure that didn't happen until 7,000 years ago. But significant expansion by the Lapita culture—ancestors to today's Polynesians—didn't happen until 4,000 years ago. Why the lag?
The authors suggest that another period of sea level change—this time a drop in sea levels during the period when voyaging began—caused estuary, reef and lagoon ecosystems to dry up and become less productive.
So why did early Polynesians voyage? Because they could, and because they had to, Pope and Terrell write.
“We propose that a more likely environmental cause of the Lapita expansion was resource scarcity, which drove people to search for new, more productive habitats... Sea-level fluctuations in the mid-Holocene (6,000–4,000 years ago) disrupted coastal environments and settlements and may have helped to initiate the last stage of human expansion in the Pacific, namely the settling of Oceania,” they write.
Lapita is the name for an a kind of ornamented pottery found at older sites throughout many parts of the western Pacific, associated with a culture considered ancestral to the Polynesian. Lapita pottery was carried into the central Pacific but not into the youngest, generally easternmost parts of Polynesia.
© 2007 Jan W. TenBruggencate