People of the Pacific: The Genetic Evidence

Jacqueline Eng

Writer’s comment: Anthropology 153 (Human Biological Variation) was the last required and most challenging class for my physical anthropology degree. We studied the many variations in human populations and their possible biological explanations. As in other anthropology classes, we utilized knowledge from other disciplines, including statistics, pathology, physiology, and genetics. When I chose the topic for my paper, I knew almost nothing about the Pacific islands and just a bit more about genetics. But I love learning new things and, through my research, I gained a lot of knowledge about the region and theories on people’s origins. Writing the paper was satisfying because I could employ the anthropological holistic approach, using linguistic, archaeological, and biological support for each hypothesis. In particular, I learned that human populations can be much alike and yet remain plastic to adapt to different environments and physical conditions.
- Jacqueline Eng

Instructor’s comment: Jacqueline’s essay on the origins of the peoples of “remote Oceania” strongly reflects her UC Davis training in the “four field” anthropological approach. In it, she brings evidence pertaining to the origin of Austronesian languages and the prehistoric Lapita culture, both believed to have accompanied Polynesians to their present homeland, to bear on genetic evidence gained from recently developed molecular genetic methods. She clearly explains and documents data supporting the hypotheses in a well-organized, clearly written text. Ms. Eng’s essay is imaginative, well reasoned, and scientifically rigorous in its attempt to account for all pertinent data, all indicative of a graduate level of scholarship.
- David Glenn Smith, Anthropology Department

Introduction

    The Pacific Ocean spans one-fourth of the earth’s surface and is dotted with hundreds of islands. These islands were some of the last places on earth to be settled by anatomically modern humans; thus scholars have long been curious about the origins and affinities of the Pacific islanders. From whence did these people come, when and how? There have been three approaches studying these questions: 1) linguistics—the origin and distribution of two primary languages found in the Pacific, Papuan and Austronesian; 2) archaeology—the origin and distribution of the Lapita culture; and 3) biology—specifically, the genetic evidence. The biological approach utilizes research on mitochondrial DNA (mtDNA) variation, restriction fragment length polymorphisms, frequency of alpha-thalassemia, and hypervariable regions on mtDNA of the Pacific islanders.
     Pawley and Green (1973) divide the waters between Asia and the Americas into two parts, Near Oceania and Remote Oceania. Near Oceania comprises all islands on the west side of the Pacific from New Guinea to as far eastward as the island of San Cristobal at the end of the Solomon archipelago. The islands, many of them relatively large, are separated from each other by narrow gaps of water, and as one travels eastward, each island is usually visible from the last one behind. Remote Oceania comprises all islands separated from the islands of Near Oceania by water gaps greater than 350 km wide. There are many imposing water barriers dividing islands and archipelagos. The distances between several islands and the nearest neighbors are sometimes vast.
     Linguistic and archaeological evidence points to two different stages of colonization of the Pacific. The first stage involved Papuan-speaking people colonizing the Melanesian areas of New Guinea at least 40,000 years ago (Groube et al.,1986), the Bismarck Archipelago by 33,000 ya (Allen et al.,1988), and the Northern Solomons by 29,000 ya (Wickler and Spriggs, 1988). The second stage involved the colonization of Remote Oceania in the remaining areas of Melanesia and all of Polynesia and Micronesia. These colonizers were Austronesian speaking and are associated with the Lapita culture, the earliest sites of which are dated around 3,500 ya (Spriggs, 1997). This paper will focus on the origin and relationships of these Austronesian-speaking people; it will present the three major hypotheses proposed for the origin of the Pacific peoples, namely the origin of Polynesians, and will use genetic evidence to evaluate the validity of the hypotheses.

Hypotheses

    Many researchers (e.g. Bellwood, 1987, 1989; Serjeantson and Hill, 1989; Lum and Cann, 1998) support the hypothesis of a Southeast origin. It goes as follows: 1) Australia, New Guinea, and the Solomons were settled between 40,000-35,000 ya by populations of ultimate Southeast Asian origin who must have transmitted some of their genes to modern Australian and Papuan-speaking Melanesians; 2) about 3,500 ya, horticultural Austronesian-speaking populations of Southeast Asian origin colonized the rest of the Pacific starting around the New Guinea coast; 3) then, proceeding eastward though Melanesia, they rapidly expanded into Tonga and Samoa, and from there to western Polynesia; and 4) after 1,300 years of consolidation, eastern Polynesia was settled by a series of migrations (from first settled to last): Marquesas Islands, Samoa, Hawaiian Islands, and Easter Island by 800 ya, and in the first millennium AD, people reached New Zealand to the south, and from there, Chatham Islands by 1400 AD. Ultimately, this first hypothesis proposes that Melanesians are more closely related to aboriginal Australians than to other Asians, and that Polynesians and Micronesians have retained phenotypes and genetic histories more similar to those of the original migrants from Southeast Asia, from whom they are descended.
     The second hypothesis, championed by archaeologists such as Terrell (1986), and Terrell et al.(1997) states that: 1) the immediate homeland of the people who invented the Lapita culture, and therefore the ancestors of Polynesians, must have been located somewhere in Irwin’s (1992) easy voyaging corridor, not in Asia; and 2) based on archaeological evidence, the homeland was most likely somewhere in the north New Guinea-Bismarck Archipelago, in the western part of Melanesia. Thus, according to this hypothesis, all Pacific island populations evolved within Oceania from founder populations originating in western Melanesia.
     The third hypothesis, periodically revived (e.g. Heyerdahl, 1952; Langdon, 1982), states that the initial settlement of eastern Polynesia was by South American Native Americans prior to the arrival of Polynesians.

Genetic Evidence

Mitochondrial DNA
     Studies of mitochondrial DNA (mtDNA) variation have been valuable for understanding the evolutionary genetics of modern populations. Stoneking and Wilson (1989) reviewed the properties of human mtDNA which make it useful for analysis of variation: 1) mtDNA consists of 16,569 nucleotide base pairs (bp) and only seven percent of that is noncoding; 2) a typical somatic cell has thousands of mtDNA molecules that appear to be identical within an individual; 3) mtDNA is strictly maternally inherited with no recombination, so any observed differences between mtDNA types in different individuals are due to mutation alone; and 4) mtDNA evolves more rapidly than nuclear DNA, so local population differentiation will be accelerated, adding to the application of mtDNA in studying closely-related populations.
     Using restriction enzyme analysis, Cann and Wilson (1983) identified nine regions (I-IX) of length polymorphism in mtDNA that involve insertions or deletions of a few base pairs. Their phylogenetic analysis suggests that in most of the polymorphic regions, a given length mutation has independently arisen several times in human lineages. Wrischnik et al.(1987) have shown that there is a length mutation in region V due to the deletion of a 9 bp tandem repeat sequence (CCCCCTCTA) in a small noncoding region between the genes for cytochrome oxidase II and lysil transfer RNA. They suggest that this deletion is a good anthropological marker for people of east Asian origin because they detected it in mtDNA types of people of Asian origin (nearly 20% of east Asians and Japanese), and not in Europeans or Africans. Thus, the 9 bp deletion has been called “Asian-specific” (e.g. Hertzberg et al.,1989). The phylogenetic analysis of Wrischnik et al.(1987) suggests that this deletion occurred only once during the evolution of modern types of human mtDNA. Recently, however, the deletion was detected in other populations, including Africans and Australian aborigines, but phylogenetic and mismatch-distribution analyses of mtDNA control region sequences in individuals with the deletion show that it occurred independently in several human lineages, rather than a single African origin, with migration from Africa to Asia (Soodyall et al.,1996). The 9 bp deletion is still widely used as a marker for Pacific populations.
     Hertzberg et al.(1989) studied the presence of the 9 bp deletion in Polynesians from five different island groups and in New Guineans and Australian aborigines. They found a high frequency in Polynesians (93%), including 100% of Samoans, Maoris, and Ninueans. In contrast, the deletion was absent in a group of 30 Papuan New Guineans and found in only one individual in a sample population of 31 Australian aborigines. Their data support the hypothesis that the independent group of pre-Polynesian ancestors who colonized the Pacific were ultimately derived from east Asia.
     Melton et al.(1995) describe the distribution of the “Polynesian motif,” where there are three substitutions (CGT) in the control region at positions 16217, 16247, and 16261. This motif was observed in 20% of east Indonesians with the 9 bp deletion. The authors found that mtDNA types related to the motif have the highest frequency in the corridor between Taiwan south through the Philippines and east Indonesia. They argue their results are consistent with linguistic evidence of a Taiwanese origin for proto-Polynesian expansion, which spread throughout Oceania via Indonesia.
     Lum et al.(1994) found that Polynesians show at least three distinct mtDNA groups that probably shared a common maternal ancestor more than 85,000 ya. Major group I lineages have a high frequency in Remote Oceania, and are found in non-Polynesians, such as Indonesians, Native Americans, Micronesians, Malaysians, Japanese, and Chinese. There is a low frequency of group II Polynesians who do not have the region V deletion, and this group may represent the predominant lineage group of Papuan Melanesia. Lineage group III tentatively links Samoa to Indonesia. Lum et al.(1994) shows at least a minor contribution of Melanesian mtDNA to contemporary Polynesians, and that mainland Asians mixed with Melanesians during their colonization of the Pacific. Likewise, Sykes et al.(1995) used mitochondrial lineage analysis on a large sample of Polynesians, people from the western Pacific and Taiwan, and they identified two, possibly three, distinct groups. The predominant group, representing 94% of Polynesian mtDNA, has the 9 bp deletion. In Polynesia, the diversity of this group is limited, while related lineages in Indonesia, the Philippines, and Taiwan are increasingly diverse. The authors posit that this trend in diversity suggests a relatively recent major eastward expansion into Polynesia, possibly originating in Taiwan, but which underwent one or more severe population bottlenecks. The second group, representing 3.5% of Polynesians, does not have the 9 bp deletion, and is fairly diverse in Melanesia. The third group is very low in representation: two Polynesians had unrelated haplotypes matching sequences from native South Americans, which may be evidence of prehistoric contact between Polynesia and South America. But while the 9 bp deletion (Lorenz and Smith, 1994) and transitions at 16,189 and 16,217 are also common in Native Americans, the predominant Polynesian haplotype (11) has not been found in Native Americans. Therefore, Sykes et al.(1995) support the model that there was a relatively rapid expansion from Southeast Asia.
     Combining the two approaches of genetic studies and linguistics also adds support to the hypothesis for a Southeast Asian origin. Chen et al.(1995) did a worldwide analysis of genetic and linguistic relationships of human populations. They found that linguistic differences are at least partially reflected by genetic distances. Lum and Cann (1998) analyzed five different mtDNA region V polymorphisms from a sample representing Oceanic and Asian populations. They state that the frequency cline of a common deletion and the distributions of a rare expanded length polymorphism support the origin of Micronesians and Polynesians in Island Southeast Asia. They observed significant correlations between genetic and linguistic distances and between genetic and geographic distances, but not between linguistic and geographic distances. They interpret these results as support for the notion that after the rapid colonization by Southeast Asians, extensive gene flow occurred between these migrants and neighboring people from Island Southeast Asia and Papuan-speaking Melanesians without much impact on the established languages.

Additional Evidence
     Chen et al.(1992) performed RFLP and haplotype analysis on 14 ethnic groups in the western Pacific region. They divided the populations into two clusters: 1) Australian aborigines, Island Melanesians, and Melanesians; and 2) East Asians, Southeast Asians, Micronesians, and Polynesians. Their results indicate that Micronesians and Polynesians are derived from Southeast Asian origin, not Melanesian. Analysis of HLA-DR and -DQ DNA polymorphisms by Serjeantson et al.(1987) also supports this view.
     Flint et al.(1986) summarized the main alpha-thalassemias caused by gene deletions, and found a high frequency of ?-thalassemia correlating with malaria endemicity throughout Melanesia, yet an -alpha3.7 III thalassemia deletion was also found in regions of malaria-free Polynesia. This suggests that the ancestors of Polynesians may have spent time in settled malarial regions of Melanesia where the deletion was selected for (Harding and Clegg, 1996). This would be consistent with the hypothesis that Melanesians colonized Polynesia. Hagelberg (1997) also supports this hypothesis. She analyzed mtDNA from ancient bones of prehistoric Pacific islanders, and failed to detect the 9 bp deletion or the Polynesian motif in individuals associated with the Lapita culture in island Melanesia and presumed to be the ancestors of modern Polynesians.
     Zago et al.(1995) studied alpha-globin gene haplotype distributions in native South Americans and found similarities to distributions observed in Southeast Asia and Pacific Island populations, indicating genetic affinities. However, the absence of several features of the alpha-globin gene cluster in the South Americans that are consistently present among Pacific Islanders suggests that the similarity of haplotypes between the South Americans and the people of Polynesia, Micronesia, and Melanesia is more likely due to ancient common ancestry rather than from recent direct genetic contribution.

Discussion

    The majority of the genetic evidence supports the “Southeast Asian” hypothesis that Polynesia was colonized by people of Southeast Asian origin. Mitochondrial DNA analysis has proven a useful tool in exploring the colonization of the Pacific. Wrischnik et al.(1987) described the presence and distribution of a 9 bp deletion in the noncoding region V of mtDNA and first pointed out its potential as a marker for detecting east Asian origins in human populations. Hertzberg et al.(1989) found that the deletion was fixed or near fixation in Polynesian populations, but absent in Australian aborigines and Papua New Guinea Highlanders. Melton et al.(1995) described the Polynesian motif in mtDNA and they also support a Southeast Asian origin, namely for a possible Taiwanese origin, which coincides with linguistic evidence. Lum et al.(1994) found three distinct mtDNA groups for Polynesians. There was a high frequency of group I, which includes Indonesians, Native Americans, Micronesians, Malaysians, Chinese and Japanese. Lum et al.(1994) support the view of colonization from east Asia. They also suggest some admixture between Asian migrants and Melanesians before the former colonized the Pacific. Sykes et al.(1995) also detected three groups: group I includes 94% of the Polynesians sampled. Their data support an east Asian origin for colonization, possibly from Taiwan, which suffered a bottleneck, accounting for the decreased diversity in Polynesians. For example, Murray-McIntosh et al.(1998) studied the hypervariable 1 region of mtDNA and noted the decreasing variability in Polynesians, from west to east. They estimated a founding population size of about 70 women for New Zealand Maori.
     The “Melanesian” hypothesis, which proposes a largely Melanesian origin with indigenous development of characteristic traits (Terrell, 1986) is called “untenable” when considered with all the genetic evidence (Serjeantson and Hill, 1989). There is some genetic support given by Hagelberg (1997) who studied ancient bone DNA from people associated with the Lapita culture and believed to be the ancestors of Polynesians. She detected no 9 bp deletion in these individuals. Also, the presence of -alpha3.7 III deletion in Polynesian suggests that the Polynesian ancestors may have spent time in Melanesia, supporting the view that Melanesian expansion into the central Pacific preceded the colonization of the Pacific (Harding and Clegg, 1996). But the results from Hertzberg et al.(1989), whereby the 9 bp deletion, fixed or near fixation in Polynesians but not found in Australian aborigines nor in Papuan New Guineans, undermine this hypothesis. The RFLP and haplotype analyses of genetic loci also support the view that Polynesians and Micronesians are derived from populations in Southeast Asia, having originated independently of the Melanesian population (Chen et al.1992). Serjeantson et al.(1987) reach the same conclusion by their analysis of HLA-DR and -DQ DNA polymorphisms. Genetic and linguistic analysis also supports this hypothesis (Lum and Cann, 1998). Thus Serjeantson and Hill (1989) conclude that it is quite unlikely that a group evolving within Melanesia could have acquired, by chance, so many non-Melanesian genes. There is, however, evidence of genetic admixture between the Asian colonizers and Melanesians before the former colonized the Pacific (Lum et al.1994).
     The “Native American” hypothesis suggests that the prevailing westerly direction of both wind and water currents supports a settlement from the Americas. There is evidence for possible cultural contacts between South America and eastern Polynesia (Bellwood, 1989), but there is little genetic support for the view that South Americans originally colonized Polynesia. Sykes et al.(1995) state that in the third lineage group in their analysis of Polynesian mtDNA, two Polynesians had two unrelated haplotypes matching sequences of South Americans, which is evidence of prehistoric contact between the two populations. However, the authors support the view of Southeast Asian origin, and data for group I are consistent with its introduction into Polynesia from Southeast Asia rather from than the Americas. Also, the predominant Polynesian haplotype 11 has not been found in Native Americans. Zago et al.(1995) also suggest that the similar distribution of alpha-globin haplotypes between South Americans and Southeast Asians and Pacific Island populations is likely due to ancient common ancestry rather than direct genetic contribution through immigration.

Conclusion

    Three hypotheses have been proposed for the origins of Polynesians. One hypothesis suggests a Southeast Asian origin, a second hypothesis suggests a Melanesian origin, and a third hypothesis suggests a South American origin. This paper has reviewed the genetic evidence provided thus far, namely evidence garnered through analysis of mtDNA. The majority of research gives support to the first hypothesis, that Southeast Asians were the original colonizers of Polynesia, while there is little genetic support for the other two hypotheses. While the aim of this paper was to review primarily the mtDNA evidence, there are other forms of genetic evidence as well, such as studies of red cell antigens, immunoglobins, and hemoglobins, that explore this problem. In addition to other biological studies, such as that of cranial and dental features, knowledge may still be gained through studies of linguistics and archaeology. To guarantee a more accurate picture of Polynesian origins, future studies should utilize all lines of genetic evidence and all three approaches.

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