Travels with DNA in the Pacific

Travels with DNA in the Pacific

Little

than 200 years ago, James Cook’s fateful third voyage into the Pacific Ocean effectively brought to a close the era of exploration that had begun two and a half centuries earlier with Ferdinand Magellan’s discovery of Guam during his global circumnavigation. Cook’s violent death in Hawaii and the later tribulations of his then ship’s master, William Bligh, when in command of the Bounty, were vivid public reminders of the hazards and dangers faced by seamen charting the vast ocean that lay beyond the East Indies. So imagine the surprise of these intrepid Europeans when, time after time as they filled in the map of the Pacific, they were confronted (sometimes literally) by faits accomplis. Almost without exception, habitable islands, no matter how remote, were already occupied by people whose means of transport consisted of little more than outrigger canoes, and who had been in residence for so long that they had forgotten where they came from or how they got there. more

Origins of Pacific islanders surrounding the origins of Pacific islanders were not lost on the early European voyagers. The botanist Sir Joseph Banks, who accompanied Cook on his first expedition, noted "the similarity of customs ...

Stripped of some of the more contentious language, that is still a view held today by the supporters of the rather fancifully named "express train to Polynesia" hypothesis.’-3At the other extreme is a modern theory that regards all Pacific peoples as local variations from a stock-that Polynesians are, in effect, common differentiated Melanesians.4 Inherent in these two extremes are questions of dispersal that also hark back to the 18th century. Cook had wondered how "to account for this Nation spreading itself so far over this vast Ocean". The "express trainers", using archaeological and linguistic evidence to support their case, see population pressures as the catalyst for the rapid migration of protoPolynesian peoples from their southeast Asian homeland into the central Pacific (Fiji, Samoa, Tonga). Contacts with Melanesia were fleeting, recorded only in pottery and language. The entire episode was over in just a few hundred years. By contrast, the "differentiators", relying on the same archaeological and linguistic evidence, movement into the central Pacific as the natural envisage outcome of a trading and cultural continuum between Melanesia and island southeast Asia stretching back many if not tens of thousands of years. Both factions agree that the Fiji-Samoa-Tonga triangle was first settled about 3500 years ago and that this was the starting point 1500 years later for the final expansion eastwards into all remaining parts of Polynesia, ending with the occupation of New Zealand in AD 900.

The issues

traditions ... and the almost identical sameness of Language ..." of the islanders of what we now call Polynesia. He went on "there remains little doubt they came ... from the same source ... I firmly believe that it is to the Westward and by no means to the East". By East he meant America, presaging a controversy that rumbled on for almost 200 years before culminating in Thor Heyerdahl’s famous voyage from Peru to Polynesia on the balsawood raft Kon Tiki in 1947. But despite the efforts of Heyerdahl and earlier sceptics, opinion continues to favour Banks’ view, now fleshed out over the intervening 200 years by a small industry of Pacific scholars-

anthropologists, linguists, oceanographers, archaeologists, and, latterly, geneticists.

Traditionally, the Pacific (see figure) has been divided three great regions: Melanesia (black islands), Polynesia (many islands), and Micronesia (little islands). But this seemingly simple classification, originally proposed by Dumont d’Urville in 1831, has always provided fertile grounds for dissent and argument. d’Urville himself divided the islanders, in terms that we would now regard as overtly racist, into two groups-one, dark-skinned and frizzy-haired and confined to the great into

island of New Guinea and the arc of islands from the Bismarck archipelago to New Caledonia and east to Fiji; the other, lighter-skinned and more mongoloid-looking, inhabiting the whole of Micronesia and Polynesia, forming a continuum with the peoples of island southeast Asia.

MRC Molecular Haematology Unit, Institute of Molecular John Radcliffe Hospital, Headington, Oxford OX3 9DU (J B Clegg PhD)

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Medicine,

Tracing the genetic

record

Not all

opinions are so clearly polarised. But it is into this minefield that geneticists have warily chosen to tread, perhaps more in hope than expectation. Genetic analysis can only make inferences, often very sketchy, about the distant ancestry of living people. A genetic reconstruction, just as in archaeology, is necessarily reliant on extant material. Genes long since lost, artifacts still buried or thought of but never made, are of no help. So a genetic account of the colonisation of the Pacific, though valid in itself, will still be only one version of events. Moreover, because of the effects of population and molecular processes, such as genetic drift and recombination, different genetic loci may unduly emphasise certain aspects of the story. But, perhaps surprisingly, the genetic evidence-an amalgam of protein, antigen and, more recently, DNA data-is remarkably consistent in its interpretation of Pacific prehistory.’’ First, the genetic record provides no evidence for population movement from South America to Polynesia. There has undoubtedly been contact-the Andean sweet potato and one or two other American plants are widespread throughout the Pacific-but it seems any links that existed were ephemeral and probably Polynesiandriven. Polynesians and Amerindians have major 5

differences in blood HLA types, and groups, mitochondrial (mt)DNA lineages that argue against gene flow between the two populations: Heyerdahl’s theories find no support here. Archaeological and linguistic evidence points to a long and continuous occupation of New Guinea (and Australia) stretching back at least 50 000 years, a date corroborated by analysis of mtDNA sequence variation. But both nuclear genes (globin, HLA) and mtDNA reflect major differences between present-day southeast Asian populations and Melanesians. Here genetics begins

Melanesia after travelling there through the north Pacific Micronesian archipelagoes. Although not a new idea, not many scholars of Pacific prehistory would support this hypothesis at present. But it is not inherently implausible and it is an illustration of just how difficult historical reconstruction can be, even with the best resources available.

Pacific

Figure: Regions of the Pacific

part company both with those who argue for a lengthy pan-Oceanic differentiation of Pacific peoples and with the express train supporters. Many of the genetic traits characteristic of island and mainland southeast Asia can be followed through the islands of Micronesia to the far reaches of the east Pacific: Polynesians (and Micronesians) undoubtedly have a substantial southeast Asian component to their genetic make-up. But the rest is Melanesian. One can even begin to see where these Melanesian genes may have come from (the island archipelagoes, not New Guinea). On the face of it, then, the genetic data lean towards the express train hypothesis but not quite so decisively as its supporters would hope. The original proto-Polynesian migrants must have done more than merely flirt with Melanesia; a few beachside encounters seem hardly sufficient to account for the 30% Melanesian globin genes in present-day to

5

Polynesians.

Genetic tales of dead

men

There is another twist to the story. If, 10 years ago, anyone had suggested that biological molecules, DNA even, could survive for thousands of years in buried bones, they would have been accused of reading too much science fiction. But the truth is indeed stranger and dead men can tell genetic tales.6 mtDNA from the bones of prehistoric islanders from Polynesia-Tahiti, the Marquesas, Easter Island-looks remarkably like its modern counterpart with its very characteristic southeast Asian sequences. But in Fiji and Tonga in the central Pacific there is a different story. There, Polynesian mtDNA lineages disappear as one goes back in time, being replaced by a less specific type that could well be Melanesian. If so, the express train might be derailed because its hallmark, the characteristic Lapita pottery that spread so rapidly throughout the southwest Pacific and which seems to have accompanied the first inhabitants of Fiji, may have been developed and carried there by indigenous Melanesians already used to inter-island trading (a differentiator argument) and not by proto-Polynesians rushing through. Perhaps the proto-

Polynesians were "aceramic",

and the

only direct

traces we

have of their passage are their genes. Some of these they could have acquired in Fiji rather than western island

light on western ills

What has all this to do with medicine? Anthropological genetics has often relied (sometimes inadvertently) on medical projects, and Oceania is no exception. Some of the key information that clearly links Polynesians with Melanesians emerged from a genetic epidemiological study on a-thalassaemia and malaria in the southwest Pacific.7 And important HLA data have been gathered during investigations into disease susceptibility,s an issue of major practical importance in the Pacific. We accept that lifestyles affect health. Perhaps less obvious is the notion that living conditions in the distant past may have serious present-day repercussions. For almost all of our existence Homo sapiens has been genetically adapted to a hunter-gatherer way of life that is now confined to a few populations in the more remote areas of Africa and South America. Agriculture and other technologies, and through them the ability to support higher population densities and more affluent lifestyles, are very recent innovations on the human evolutionary scene. However, the genetic compositions of populations change relatively slowly in response to environmental change. In effect, we are now running on old genes in a new environment. For the most part this may not matter, but there is increasing suspicion that many of the common "western" illnesses-hypertension, heart disease, and so on-have an underlying genetic basis that in some ways reflects our past. Polynesians and their distant Amerindian cousins (they are indeed related, but from a common Asian origin tens of thousands of years ago, not by a recent Pacific canoe journey) are vivid cases in point. Many of these populations have extremely high incidences of type II (non-insulin-dependent) diabetes,8 a disease known to have substantial genetic and environmental components. The latter include a high calorie "junk food" diet, little exercise, and obesity; in fact, a not atypical western lifestyle but a relative newcomer to much of Oceania. Switching to this lifestyle (a transition aptly described as coca-colonisation)9 seems to be the trigger for diabetes epidemics in many developing countries-it has been estimated that in China and India alone there will be 50 million diabetics by the year 2000. In a prescient paper10 written over 30 years ago, the geneticist J V Neel suggested that (type II) diabetes might be "an untoward aspect of a ’thriftiness’ genotype which is less of an asset now than in the feast-or-famine days of hunting and gathering cultures". Neel’s idea was that people living for millennia in generally impoverished conditions would be genetically adapted to storing any sporadic surplus food calories that came their way as fat. In effect, the thrifty genotype was a genetic protection mechanism starvation. Now, against periodic unfortunately, affluence has subverted that mechanism and diabetes is the result. In the normal course of events we might now expect the operation of natural selection to put the process of 1071

maintaining thrifty

leading to their susceptible populations. A drop in the genes into reverse,

eventual loss from incidence of type II diabetes in the worst affected parts of the Pacific suggests this may indeed be the case and it may explain why, paradoxically, type II diabetes is much less common in the rich industrialised countriesselection has already removed some of the offending genes during the past few hundred years of relative affluence. But the intervention of modern medical management of diabetes is now likely to retard that process, perhaps indefinitely. Which may be just as well, since there is no certainty that the world can sustain existing levels of consumption for ever. Meanwhile, it is probably no comfort to the peoples of Oceania that, after tolerating for so long the attentions of western anthropologists, they are now in the genetic limelight as a paradigm of some of the industrialised world’s most serious diseases. And despite all this, they would still like to know where they came from.

References 1

Diamond J. Express train

to

Polynesia. Nature 1988; 336:

307-08.

Bellwood P. Prehistory of the Indomalaysian archipelago. Sydney: Academic Press, 1985. 3 Bellwood P. The Austronesian dispersal and the origin of languages. Sci Am 1991; July: 70-75. 4 Terrell J. Prehistory in the Pacific Islands. Cambridge: Cambridge 2

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6

7

University Press, 1986. Hill AVS, Serjeantson SW. The colonisation of the Pacific: a genetic trail. Oxford: Oxford Science Publications, 1989. Hagelberg E, Clegg JB. Genetic polymorphisms in prehistoric Pacific Islanders determined by analysis of ancient bone DNA. Proc R Soc Lond (B) 1993; 252: 163-70. Flint J, Hill AVS, Bowden DK, et al. High frequencies of &agr;thalassaemia are the result of natural selection by malaria. Nature 1986;

321: 744-49. Zimmet P. Challenges in diabetes epidemiology—from West to the rest. Diabetes Care 1992; 15: 232-52. 9 Zimmet P. The epidemiology of diabetes mellitus and associated disorders. Diabetes Annual 1991; 6: 1-19. 10 Neel JV. Diabetes mellitus: a ’thrifty’ genotype rendered detrimental by ’progress’. Am J Hum Genet 1962; 14: 353-62. 8

BOOKSHELF

often ineffective means by which adults teach their own children about Ronald 0 Valdiserri. New York: Cornell University Press. 1994. Pp 160.$16.95 sex. Valdiserri comments that our ISBN 0-801429811. society is certainly more open about Ronald 0 Valdiserri is a pathologist: essays with a relevant quotation. A sexuality and that we use words pubengaged in HIV prevention research Thomas Hardy quote on the Titanic, licly that would have been impossible at the Centers for Disease Control for example, is used to introduce an to imagine only a generation or two and Prevention. He is also the twin ofF’., essay, "Icebergs", that addresses our ago. Yet, he points out, in other someone who has died of AIDS. Furfaith in technology-excessive in the respects we still have far to go. In a ther, he is an accomplished writer,: case of HIV prevention-and how we . manner typical of his work, the who in this thin volume of 26 concise allocate health care resources. As in author asks for a more "adult" toleressays has given us a very special: all of this work, one feels the author’s ance of homosexuality, expressing the offering. In his book, Valdiserri honesty and hears his plea for tolerconcern that many of the problems weaves a subtle, yet complex, and ance and understanding as essential experienced by HIV-infected people intensely personal vision of HIV and attributes in addressing the complex and much of the stigma of the disease AIDS as it affects our society. Not: social issues raised by the HIV . stem from this basic lack of undersurprisingly, the author addresses the; epidemic. ; standing. Although these concerns While all Valdiserri’s essays are have been difficulty of HIV prevention and in :. expressed many times by nearly every essay the reader is aware effective some may be more so for a other authors, Valdiserri is able to of Edwin, the author’s twin brother. given reader. I was particularly . reframe them in a context of his This book is not, however, either a moved by "Potsherds and dinner family and the maturation of a child textbook of AIDS epidemiology and plates". Valdiserri starts by remarking into an adult. Thus, a complex social prevention or a history of Edwin’s on the way that archaeologists often problem is rendered more approachdeath. Rather, Valdiserri muses on learn much by the observation of proable and human in scale. the meaning of all of this in essays; saic items like potsherds cast off by a In the final title essay, Valdiserri that are controlled and spare-almost prior civilisation. He then reflects on considers the patience needed to devotional in feeling. The lack of a "personal archaeology" wherein he improve an inhospitable clay soil to overt rage in this work is itself inter- : surveys his own kitchen cabinets, become a productive garden. Withesting-the author quietly conveys noting for example a dinner plate out so stating, the author asks us to his immense grief at the death of this remaining from his and his brother’s .,’ compare this to the need we have for brother and other close friends from; childhood. He comments on the difeven greater patience in reducing AIDS. Yet there remains a strong ficulty of separating his own memothose behaviours that place individuemotional undercurrent revealed in ries and identity from that of his als at risk for HIV infection. twin. An old gravy boat reminds him the difficulty the author has in underGardening in Clay is an important standing those who would pre-judge; of large family dinners and, thus, on contribution to the literature of : the diversity of family and on his AIDS. It is a quietly intelligent and persons infected with HIV. in need to separate from his brother. addresses Gardening Clay, again, profound work and clearly written several recurring themes: the author’s Finally, he notes a set of glasses left .,’ from the heart. It allows us to see own career-his choice of pathology him by Edwin which remain to keep more easily the importance of AIDS and later prevention research, the scihis brother’s presence visible. Thus, and is a model of tolerance and ence and politics of prevention, and in one short essay Valdiserri describes understanding. the loss of his brother. Each essay the cycle of his relationship with one : tends to focus on one of these conwho has died of AIDS. Paul A In another essay, "Down there", UCSF AIDS Volberding cerns and the pattern is similar: Program, San Francisco General Hospital, throughout. Valdiserri begins histhe author addresses directly the San Francisco, CA 94110, USA

Gardening

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in

clay