Download Opinion Box: Thrifty Genes in Polynesia? Jim Bindon

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
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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
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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
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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
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