Asia Pacific J Clin Nutr (1995) 4: 361-365
Asia Pacific J Clin Nutr (1995) 4: 361-365
Patterns of colonisation and the
"thrifty" genotype in Pacific prehistory
Alexandra A Brewis, PhD, Geoffrey Irwin,
PhD and John S Allen, PhD
Department of Anthropology, The University
of Auckland, Auckland, New Zealand.
a symposium organised by the Australasian Clinical Nutrition Society
(New Zealand Division) held at the School of Medicine, University
of Aukland on 8-9 December 1994. Presented on pages 337-370.
The seemingly distinctive markers of Polynesian
biology - serologic, molecular, anatomical, and developmental -
have been argued to have emerged from the special circumstances
of colonisation and early settlement of the region. Models point
to the microevolutionary impact of mortality selection while voyaging,
small founding group size forcing bottlenecking, and subsequent
relative isolation of groups introducing heterogeneity through genetic
drift within Polynesia. These ideas have drawn on a melange of ideas
about the colonising experience to construct evolutionary narratives,
including that of the "thrifty" genotype in Polynesia.
The substantial problem is that there previously has been no independent
theory of colonisation and inter-island contact for the region.
We critically examine which microevolutionary forces would have
impacted prehistoric Polynesians in terms of an independent navigational
theory of colonisation, and map out the expected biological/ evolutionary
sequelae. This theory suggests that Remote Oceania was settled by
directed return voyages, that the tempo of population expansion
was probably rapid, that voyaging continued after settlement, that
whereas episodes of colonisation were directed to safety, subsequent
inter-group voyages followed the pattern of mutual inter-island
accessibility. The implication is that mortality selection would
not have been the predominant force shaping the Polynesian genotype;
the development of survival sailing strategies were designed to
remove these very risks. This makes it unlikely that the specific
experience of Polynesian colonisation would promote an efficient
insulin mechanism, and so explain, for example, the high incidence
of Type II diabetes mellitus in contemporary Polynesian groups.
First proposed in 1962 by James Neel, the concept
of the "thrifty" genotype has been posed as an evolutionary
explanation for the high prevalence of diabetes mellitus in many populations,
but most particularly in native Americans and Pacific islanders in
modernising settings1,2. The proposition is that periods
of acute nutritional stress generated a selective advantage for an
"efficient" insulin 1000 metabolism. In Polynesia, these
stresses have been posed to be provided during a risky and selectively
costly maritime colonisation process3.
Polynesia, the geographic region that encompasses
one sixth of the earths surface (Fig 1), has long been recognised
as a rare and telling arena for studying human microevolution4.
It was settled rapidly and recently, and there is a startling biological
homogeneity, given the enormous space over which the component populations
are distributed. Further, the chronology of settlement is comparatively
precise with the exception that two alternative chronologies are proposed,
and debated, for East Polynesia. These are conditions under which
microevolutionary scenarios, such as that posed to explain "thrifty"
genetics in Polynesia, can be best tested. In the Polynesian case,
this assessment has been difficult because, as we will describe in
the background section, we have been stumped in linking cause-and-effect,
and used untested assumptions about the nature of voyaging to drive
interpretations of biological history. What we have needed is a distinct
body of organised ideas about the nature of Polynesian colonisation
that can allow us to better interpret biological history in the region
that is particularly explicit about the process of colonisation and
population founding. This is because assessments of genetic diversity
between human groups do not provide any obvious or automatic explanations
of population history. In this paper we present a systematic and independent
navigational theory of colonisation, map the anticipated evolutionary
sequelae of this pattern to see if the selection-bottleneck-drift
narrative is warranted against likely voyaging and settlement strategies,
and present the implications of this model for the thrifty genotype
hypothesis as it has been applied to Polynesian populations.
Although the exact source(s) of the Polynesian ancestors
remain(s) unidentified, molecular studies- including of mtDNA lineages5
and globin gene variability6 - confirm a genetic history
of Asian immigrants having admixed with non-Austronesians in Near
Oceania, around the northern coast of Papua New Guinea, resulting
in a complex colonising genotype(s). A roughly similar pattern is
evinced in linguistic and archaeological data. Current debates focus
on the speed with which the later Asian migrants moved through the
area, and (consequently) the degree of admixture, both prior to and
following initial colonisation of Polynesia7,8.
Figure 1. The Pacific region.
There are several aspects of Polynesian population
biology that have been posed as defining. For skeletal biologists,
these have been relatively short lower limbs, tall and musculated
skeleton9,10, characteristically pentagonal and brachycephalic
head form with wide cranial base angle (over 140 degrees), high frequencies
of "rocker" jaw11, femoral and humeral bowing,
(possibly) oval shaped fovea capita, tibial and talar squatting facets12,
and reduced frequencies of shovelling on incisors13. Molecular
and serological markers of Polynesian populations have been: near
fixity in the nine base-pair deletion in region V of mtDNA14;
lack of diversity in HLA; low frequency of a a a globin gene variant; an
absence of A1 blood type with relative absence of type B15;
and an almost startling lack of overall genetic variation, such as
within the (non-coding) D-loop region of mtD 1000 NA16.
Polynesian groups in non-industrial settings may also display precocious
development and, maybe, relatively low age at menarche.10
None of these markers are specific to Polynesians, although they are
generally regarded as defining because they are displayed at considerably
different frequencies to other Oceanic groups, particularly those
in West and East Micronesia and Near Oceania.
These characteristically Polynesian population traits
(serological, molecular, developmental or anatomical) have often been
argued, like "thrifty" genotypes, to have emerged from the
particular circumstances of colonisation and early settlement of the
region, in that they would create and enforce opportunities for microevolutionary
change. There has emerged in the literature a seeming trilogy of microevolutionary
explanations for the emergence of the Polynesian pheno/genotype, all
related to notions of the relationship between the colonising and
settling experience and the evolutionary sequelae that would emerge
under such conditions. The basic scenario invoked is: selection through
mortality and bottlenecking during colonisation, followed by genetic
drift through relative isolation. (Mutation is rarely invoked as an
explanation because the time-scale since colonisation, less than two
millennia, provides insufficient time for the accumulation of new
For example, in developing an explanation for the
anatomical markers of the Polynesian skeletal phenotype, Houghton
has posed that the crucial evolutionary force was selection through
mortality, where only the better adapted would survive to settle new
islands, so to be ancestral to contemporary Polynesians:
The large and muscular body form that is still evident
in Polynesia, and which on a growing archaeological record was widespread
through the western Pacific in prehistory, is an appropriate adaptation
to the island environment. The body form arose within the Pacific
as a consequence of the selective pressures of voyaging and small-island
This is based on the notion that the sea is a dangerous
environment, and that while sailing technology of the proto-Polynesians
is acknowledged to have been sophisticated, the process of colonisation
across ocean is seen as inherently risky, by sheer force of distance
and the changing nature of the nautical climate. Under such a scenario,
Houghton argues that Polynesian body form evolved a cold climate form-
a stocky, large body that would be better suited to the conservation
of heat during voyages in which people were exposed to the elements.
Houghton, however, also acknowledges genetic drift
as an integral force in the evolution of the Polynesian form, noting
that selection combined with the "inevitable processes
of genetic drift in this world of islands,... provides sufficient
explanation for the large range of human morphological variation"18
(emphasis ours). The assumption of the inevitability of bottlenecking
during colonisation and subsequent drift through isolation in producing
the Polynesian form is implicit in virtually all the regional scholarship.
The seeming homogeneity of Polynesian biology is often posed as a
priori evidence of founder effect due to small founding group
size. For example in the globin gene system, low frequency of a a a -chromosomes in Eastern
Polynesians are argued to result from founder effect, with overall
globin gene data suggesting "...considerable genetic drift and
founder effects during the colonization of Polynesia"19.
The assumption of relative population isolation providing
the mechanism for genetic drift is also widely acknowledged, even
accepted. In the classic model of Kirch and Green20, the
implicitness of this process in Polynesian population history is used
to develop complex models of linguistic/ biological/ socio-cultural
divergence within Polynesia. That is, following an homogenising process
at colonisation of the region, that the contemporary and more recent
prehistoric Polynesian form has subsequently diverged within
Polynesia from a more standardised ancestral form. Kirch draws on
this thread with reference to social behaviour:
Since colonising parties of Polynesians were
small, and therefore representative of only a portion of the
cultural-behavioral variability inherent in the mother
population, something akin to the founder effect known
to geneticists...may have operated to produce rapid change in island
populations (emphasis added)21.
More specifically with reference to biology, Green
poses that the current phenotypic forms in Near Oceania have been
diverging from the forms that were ancestral to Polynesians, experiencing
more genetic divergence than the Polynesians and "..that a lot
of this change is due to gene flow from near Oceania as far as Fiji
in the last 2500 years, which did not affect the more isolated populations
of...Polynesia"8. Especially in Eastern
Polynesia, the initial burst of colonisation was followed by relative
isolation (with selection/ adaptation in specific island environments),
so that genetic divergence increased through time away from a more
homogenous founding gene pool. He says:
There is no evidence for cultural, biological or
linguistic replacement, but only of gradual evolution of speakers
of a Polynesian language with a Polynesian culture and physical
phenotype out of their immediate predecessors. It was from ancestral
Polynesian populations of West Polynesia that those of East Polynesia
(and later the Outliers) derive. In East Polynesia, because of
genetic isolation, drift through small founding population size,
and natural and cultural selection an unusual physical form
in the Mongoloid population radiation occurred (emphasis added)5.
It is, arguably, in interpreting genetic data that
this vague model of selection-bottleneck-drift has been most explicitly
invoked. One of the most explicitly developed examples of this is
Flint et al, who argue on the basis of data from six DNA minisatellites,
that the lack of gene diversity in Polynesians compared to Melanesians
is a product of this specific pattern of settlement history22:
... our knowledge of the colonization of Polynesia
suggests it probably involved bottlenecks. In addition to...geographic
bottlenecks, colonization is estimated to have cost approximately
half a million lives... Furthermore, once settled, island populations
are known to have suffered catastrophic diminution in size... The
minisatellite data, revealing a low gene diversity in Polynesia,
would appear to confirm...the process of settling so many small
and well-separated islands in so short a time, the Polynesians have
become relatively homogenous. ...The most likely explanation
for low genetic diversity in Polynesia is that colonization involved
small numbers of settlers passing through numerous bottlenecks
Interestingly, they maintain this conclusion despite
their observation that genetic diversity in thes 1000 e Polynesian
DNA minisatellites was less than they expected, and less than that
seen, for example, within highland Papua New Guineans and Australian
Such models, placing primacy on selection during colonisation,
and the role of founder effect and subsequent drift on the creation
of the Polynesian form, are based (variously) on a trilogy of assumptions
about the nature of the colonisation and settlement of the region:
that voyaging was risky, that founding populations were small, and
that after colonisation Polynesian populations were relatively isolated.
These ideas about the origins of Polynesian biological distinctiveness
have drawn on a melange of ideas about the nature of the colonising
experience to construct evolutionary narratives. The substantial problem
is that, while a reliable chronological framework has been developed,
there has been no independent navigational theory of Pacific colonisation
and inter-group contact. This is perhaps most obviously the case with
respect to how founder effect has been built into narratives about
population history in Polynesia. For example, Green recognises:
...there currently seems to be a consensus that
initial colonization was always by small propagules....
This agreement stems not from estimates derived from the archaeological
evidence of founder and settlement period sites in tropical Polynesia...,
as from the various voyaging and colonisation models employed by
pre-historians, which often tend to incorporate findings from the
simulation studies of demographers23.
The Independent Colonisation Model24
Through Island Southeast Asia and western Melanesia
as far as the end of the Solomons was a sheltered voyaging corridor
of large, often intervisible islands separated by short water gaps.
This was a zone of Pleistocene colonisation which occurred in the
period approximately 50,000-30,000 years ago. Beyond this corridor,
in a region known as Remote Oceania, islands generally become further
apart and smaller and their natural resources more attenuated. The
difference was sufficient to arrest further colonisation for some
25,000 years, until deep-sea explorers had developed maritime technology
and exploration strategies which allowed them to search offshore and
Further east, between West and East Polynesia and,
in Micronesia, the Marianas and the islands lying to the east, there
is a biogeographic distinction between the islands of the continental
Western Pacific and the smaller volcanically-based ones on the Pacific
Plate, to the east of the Andesite line. Continental islands
are often more diverse in geology, flora and fauna than oceanic islands,
which may be high volcanic islands or low-lying and often short of
good soils and fresh water. The most extreme examples being atolls
which are fringes of coral above an underwater volcano.
Some 3500 years ago, two distinct episodes of maritime
colonisation, characterised by two distinct archaeological signatures,
spread into western Micronesia and Island Melanesia. The latter was
associated with an entity known as Lapita which, while materially
distinctive, is nevertheless ambiguous in cultural, linguistic and
biological terms. However, when this entity spread beyond the Solomons
into the Remote Pacific, it represented an integrated navigational
system. Lapita in Fiji/West Polynesia is identified as ancestral to
subsequent Polynesian peoples and cultures. However, whether it carried
the full inventory of information which was to develop into Polynesian
is a question which must be raised.
Colonisation was obviously deliberate because canoes
carried the domesticated plants and animals and the people needed
1000 to establish breeding populations. Moreover computer simulations
which consider real winds and currents proved that the major voyages
did not happen at random but were the result of directed navigation.
But while this much was clear, much remained that was unknown and
in particular the strategies of exploration which were used as the
first Pacific navigators penetrated an unknown ocean.
Sailing between known and unknown islands has its
risks, but sailing into an empty ocean can be fatal. Some earlier
theories of Pacific colonisation preferred many explorers to die at
sea, but there is nothing to show that Pacific explorers were careless
with their lives. Their options were to search and find, to search
and die, or to search and return. From the practical point of view,
a non-suicidal sailor the best strategy goes a long way to explaining
why the thrust of Pacific exploration was against the easterly trade
winds. Without a doubt, it is safest to sail first in the direction
which is normally upwind because one can expect the fastest trip back.
The hard way is really the easy or safe way and this simple paradox
is one of the keys to explaining the trajectory of human settlement.
Practically every radiocarbon date in the remote Pacific supports
the view that colonisation went first against the prevailing winds
and only then across and down them.
Sailing upwind also provides the means to find the
way home by latitude sailing. This was evidently developed during
the settlement of Polynesia and simply involves using the altitude
of ones origin island, while still upwind of it, and then running
with the wind along the latitude. Experimental evidence shows that
the error in estimating latitude without instruments is matched by
the ability to detect the presence of land from offshore by observing
bird behaviour and other signs.
A second point to be made about latitude sailing is
that it is possible to make mistakes in dead-reckoning, to lose track
of ones position and still get back. This was more than just
a safeguard in prehistory. One could safely continue to search upwind
with the knowledge of what lay behind. It meant that upwind exploration
could virtually escape from navigational limits. Given the immensity
of the Pacific Ocean we now have an insight into how it might have
been explored. The ultimate limit of early voyages into the east was
set by the human body and spirit almost without navigational constraint.
To sail with some safety across the prevailing winds
requires a knowledge of islands to leeward of the starting island,
in case that cannot be reached on the return journey. Sailing downwind,
on the other hand, usually requires returning by a different route.
The circumstances of exploration changed in the remote Pacific as
geographical knowledge was added to navigational knowledge, and the
range of feasible options increased. Increasing experience and skill
were needed to manage the long exploratory probes eventually made
into different and more difficult weather systems on the high-latitude
extremities of Polynesia, and to South America.
The broad pattern of settlement is that deep-sea colonisation
began after 3500 years ago with the spread of Lapita through the islands
of Melanesia to reach Fiji, Tonga and Samoa by 3000 BP (before present),
or before. The evidence is that the ancestral people concerned were
few in number and travelling fast. They carried a portable economy
of plants and animals, and engaged in long-distance exchange. Available
radiocarbon dates now indicate that the Marquesas Islands in far East
Polynesia may have been settled soon after 0 AD, although not in association
with Lapita. It is likely earlier sites will be found among closer
groups such as the Cooks and Societies. By about 500 AD settlement
reached Hawaii, distant Easter Island and probably South America.
1000 Cool and more difficult sailing conditions delayed settlement
south of the tropics, but by about A.D.1000, colonists reached New
Voyaging did not end with settlement. The evidence
is that inter-island voyaging continued and, once the location of
islands was known, navigators were freed from the constraints of survival
sailing strategies. Thereafter, the frequency of communication was
affected by accessibility more than safety. Variables such as distance
and size of island target were invoked.
Implications for the thrifty genotype hypothesis
To summarise the aspects of this model that have particular
relevance for interpretations of biological history, this theory suggests
that Remote Oceania was settled by directed return voyages, that the
tempo of population expansion was probably rapid, that voyaging continued
after settlement, that whereas episodes of colonisation were directed
to safety, subsequent inter-group voyages followed the pattern of
mutual inter-island accessibility. Given this explicit model of prehistory
in the region, we can better anticipate the possible role of microevolutionary
forces for a thrifty genotype sequelae in Polynesia.
Under this model of population movement and contact,
opportunities for mortality selection were subdued; mortality selection
during voyaging was minimised by the very nature of navigational strategies,
designed as they were to minimise these specific risks. In essence,
nothing about Polynesian biology particularly suggests the heavy hand
of directional selection during the colonisation process, or even
subsequently while settled on Polynesian islands. This is not only
because of the lack of opportunities, but because many features of
Polynesian biology do not appear as forms of traits that would be
selected for/against in such a scenario. Further, implicit in this
model of exploration, settlement and post-settlement contact is that
founding populations were likely to be multiple, and relatively diverse
in source and timing.
This model provides a very different explanation of
why Polynesian biology appears relatively genetically homogenous.
Since genetic divergence, including through drift, requires isolation,
conceptualising prehistoric Polynesian groups as coming into regular
contact with each other provides a reasonable explanation for the
lack of genetic diversity found in Polynesians. While opportunities
for drift are implicit in island sequences, given the often devastating
impact of natural disasters on small island groups, they would tend
to occur at different locations at different times. But the force
of gene flow would counteract the changes in gene frequencies promoted
by drift in specific island groups. The homogeneity of the Polynesian
biology could result from population contact and the associated reduction
in opportunity for differentiation and inter-group heterogeneity.
That is, the evolutionary history of Polynesians may actually be characterised
by genetic convergence rather than divergence.
In summary, navigational expectations make it difficult
to suggest that the specific experience of Polynesian colonisation
would promote an efficient insulin mechanism, and so explain, for
example, the high incidence of Type II diabetes mellitus in contemporary
Polynesian groups. Given this voyaging theory, we propose that the
(proto) Polynesian phenotype was not a product of the colonisation
of the region because opportunities for selection were subdued and
the founding populations may have been initially externally reinforced
and subsequently admixed. Second, the independent model of Pacific
colonisation conservatively favours the idea that "thrifty"
genotypes, including that shared by Polynesians, had an evolutionary
origin/basis considerably more ancient than the period of initial
and subsequ 1000 ent voyages of settlement in the Pacific region.
This fits with the suggestion that "thrifty" genotypes may
be virtually pan-species, with a small number of (predominantly Northern
European-derived) populations having more recently experienced selection
for a "non-thrifty" genotype.
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Copyright © 1995 [Asia Pacific Journal of Clinical
Nutrition]. All rights reserved.
January 19, 1999