An evolutionary road less traveled
Until expanding agricultural development and modernization encroached on their forest homelands, the Mlabri lived mostly as nomadic hunter–gatherers in the forests of northeastern Thailand and western Laos. This lifestyle is unique among the other so-called hill tribes of Thailand - who all farm - raising the possibility that the Mlabri descended from the ancient Hoabinhian hunting–gathering culture of Southeast Asia. But, in a new study published in the freely-available online journal PLoS Biology, Mark Stoneking and colleagues reveal that the Mlabri are actually descended from farmers, who subsequently reverted to a hunting–gathering lifestyle. This overturns our assumptions that contemporary hunter–gatherer groups represent the pre-agricultural lifestyle of human populations.
Stoneking and colleagues compared the genetic diversity of the Mlabri with that of six other agriculture-based hill tribes. Not only did all of the other hill tribes show "significantly higher" variation, but this lack of variation hasn't been found in any other human population, indicating a "severe reduction in population size" for the Mlabri. This reduction likely happened 500 to 800 years ago, Stoneking and colleagues conclude, and at most 1,000 years ago. But how? Since genetic analyses can't distinguish between a population bottleneck and a founding event, the authors used simulations to calculate the amount of population reduction required to completely eliminate mtDNA diversity, arriving at "not more than two unrelated females" and "perhaps even only one." Linguistic studies suggest that the Mlabri language arose after speakers of a related language, probably Tin, split off and came into contact with another, as yet unknown language, an event that likely happened less than 1,000 years ago.
The genetic and linguistic evidence indicates that the Mlabri were "founded" between 500 to 1,000 years ago by one female and one to four males from an agricultural culture. With too few hands to farm, this tiny group likely turned to hunting and gathering. Altogether, Stoneking and colleagues conclude, these findings caution against automatically assuming that contemporary hunter–gatherer groups "represent the pre-agricultural lifestyle of human populations, descended unchanged from the Stone Age." Interestingly, the authors' scenario of Mlabri origins is not so different from the traditional story told by the Tin.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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