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Opinion Box: Thrifty Genes in Polynesia? Jim Bindon Linguists, archaeologists, and geneticists disagree about the origin of the Polynesians. However, most scholars agree that about 3,500 years ago pre-Polynesian ancestors carrying the Lapita cultural complex arrived in the Bismarck Archipelago, interbred with existing populations, and shortly thereafter achieved the sailing technology for substantial open ocean voyaging. Within a few hundred years these peoples surmounted one of the last great impediments to human settlement: the 500 miles of ocean between Vanuatu and Fiji, sailing against prevailing winds and currents. Both cultural and biological adaptations were required to make this crossing possible. The cultural innovations likely included improvements in sailing technology and navigational expertise and enhanced food preservation techniques. The biological adaptations probably included the robust body build that facilitated paddling the vessels and metabolic adaptations to dietary and cold stress. All of these biological adapt at i onsmaybet her e s ul tofs e l e c t i onf ora‘ t hr i f t yphe not ype ’ . The Lapita sailing vessels had a large triangular sail that precluded sailing too close to the wind. The canoes sat low to the water, making paddling possible when winds were too low or from the wrong direction. Skeletal remains from Lapita sites and measurements on modern Polynesians depict a people of substantial stature with broad shoulders and hips, and robust, long limbs. The broad body, long limbs, and sizeable muscle mass are particularly well suited to the biomechanics of paddling—a voyaging strategy that only would have come into play in situations critical to survival. Thus, this body build would have had a strong selective advantage. Houghton (1996) has modeled the severe cold stress that early Pacific voyagers would have experienced. Maximum cold stress is achieved overnight when moderately low temperatures, high wind chill, and wet clothes and skin combine to produce substantial cold stress. Overnight voyages would have been very rare prior to the “br e akout ”f r om t heSol omonI s l a nds .Thes a mebodi e st hatar ewe l ls ui t e dt o paddling canoes also have a favorable low surface area to body mass ratio, excellent for conservation of body heat in cold stress. Again, a large, robust body build would provided an advantage. Traditionally the thrifty genotype argument has been used to explain adaptation to periodic famine. In the case of the Pacific it has been invoked as an adaptive response to caloric restriction associated with voyaging and settlement of the islands (Bindon and Baker, 1997). Metabolic efficiency in storage of excess calories is achieved through over secretion of insulin which increases fat tissue formation and the accumulation of an energy store. This would also increase subcutaneous fat tissue which acts as an insulation against cold stress. It has been suggested, however, that a population as well adapted to a marine environment as the Lapita people were may not have suffered from extreme caloric deprivation during voyaging and settlement. Even so, their diet would have been drastically altered: lower carbohydrate intake and an increase in protein intake. They would have eaten through their supply of the poi-like fermented crops (taro, breadfruit, banana) that they were carrying on their voyage and then had to wait for new crops to grow before regaining their normal carbohydrate intake. Several people have argued that the thrifty genotype provides a metabolic adaptation to just such a high protein low carbohydrate diet through the metabolic shifts involved in hyperinsulinemia and insulin resistance. Recent research into thrifty genes has provided some clues that the cold- and work-adapted body build and the metabolic shift to accommodate dietary stress may be related. These adaptations may be the result of mutations in the region of the insulin gene (INS), like the variable number tandem repeat (VNTR) polymorphism near INS that causes increased transcription of both the INS gene and the close Insulin-like Growth Factor 2 (IGF2) gene. Increasing transcription of INS could generate high blood insulin levels (hyperinsulinemia) and decrease sensitivity to insulin binding in peripheral cells (insulin resistance). Meanwhile, high levels of IGF2 stimulate muscular and skeletal growth predisposing to a large, robust body. I do not mean to imply that this particular VNTR polymorphism represents the thrifty gene, but it points to a possible area for exploration and integrates the biological adaptations found in modernday Polynesians that appear to result from their voyaging history. References Bindon, J. R. and P.T. Baker. 1997. Bergmann's rule and the thrifty genotype. American Journal of Physical Anthropology, 104:201-210. Houghton, P. 1996. People of the Great Ocean: Aspects of Human Biology of the Early Pacific. Cambridge: Cambridge University Press. Stress Work Response T h r i f t y I G F 2 Result Muscle Growth Strong limbs for paddling Skeletal Growth Low surface area to mass Insulin Fat deposition: Insulate body, store calories Insulin Resistance Spare glucose, prevent ketosis Cold Diet G e n e s I N S