The Journey of Man: A Genetic Odyssey (28 page)

In 1998 Mark Seielstad, then a graduate student working with Luca Cavalli-Sforza and Dick Lewontin, published a paper that proposed a solution to the Y mystery. Seielstad studied Y-chromosome markers in fourteen African populations, finding that the fraction of variation distinguishing between populations was much greater than that seen for other genetic markers. In a sample of European populations, the divergence between populations as a function of geographic distance increased at a much higher rate for the Y than for other genetic systems, such as mtDNA. Seielstad’s interpretation of these two patterns was that women moved more than men, dispersing their mitochondrial lineages among neighbouring populations, producing a relatively homogeneous mtDNA distribution. The men, meanwhile, stayed at home – and their Y-chromosomes diverged independently in the different populations. The finding led Cavalli-Sforza to quip that Verdi was right when he wrote
‘la donna e mobile’
(the woman moves).

Seielstad’s publication created quite a stir, even attracting the attention of activists such as Gloria Steinem, who requested a copy. It seemed to undermine the ancient notion of peripatetic Lotharios wandering the globe, sowing their wild oats and dispersing their Y-chromosome lineages. What the activists failed to take into account, though, was that it actually reinforced the notion that women make a minor contribution to group identity. In a patrilocal society, it makes little difference who your mother was – it is your father who gives you your family or clan affiliation, and your inheritance. What Seielstad had found was that human culture has had a significant effect on the pattern of genetic variation in our species. Simple, local decisions about marriage and property, summed over hundreds of generations, had produced profound differences in the pattern of genetic variation on the male and female sides. Hindu castes show clear evidence of this pattern, with much greater Y-chromosome than mtDNA divergence
between the castes, suggesting that women could move between castes while men were locked into theirs.

The real test of this theory, as Seielstad pointed out, was to examine the pattern of variation in matrilocal societies. The prediction is that these would show greater divergence for mtDNA, with the Y lineages tending to be homogenized among them. This was finally done in 2001, when Mark Stoneking and his colleagues published a study on the Karen, as well as a sample of patrilocal Thai tribes from the same area. They found Seielstad’s predicted pattern of greater Y diversity in the Karen, providing strong evidence that patrilocality had produced the geographic clustering in Y-chromosome variation seen in most human societies.

While this helped to explain the localization of Y-chromosome lineages, it skirted round another odd observation. As we saw in
Chapter 3
, the coalescence time – the time elapsed since our common ancestors, Adam and Eve – is much more recent for the Y-chromosome than it is for mtDNA. Patrilocality can explain the high degree of Y divergence between populations, but the overall coalescence time should still be the same for Y and mtDNA. In effect, the Y pattern should be fragmented into many deeply divergent populations, all of which trace their ancestry to a single African man who lived around 150,000 years ago. Instead, we see many fairly divergent populations, all of which seem to coalesce to a common ancestor as soon as they are traced back to Africa: the data points to an African Adam who lived only a few thousand years before humans started to leave the continent. This result suggested another factor at work.

The rate of genetic drift – random changes in marker frequency due to small population size – depends on the actual size of the population, as we saw earlier. In large populations drift is negligible, while in small populations the effects of drift are significant. In the smallest populations, such as those first Beringeans who colonized the Americas, tiny population sizes can lead to a few lineages reaching frequencies of 100 per cent in a very short period of time. This is the explanation for why Native Americans are almost uniformly blood group O – types A and B were lost during their journey through the Siberian ice age.

This same pattern can be used to explain the recent dates for our
Y-chromosome ancestor. If there are fewer men than women in a population, then the rate at which Y-chromosome lineages are lost will be greater. But this can’t be true, you might be saying – the birth ratio is 50 : 50. Surely there are the same number of men and women in every population? Surprisingly, while this is true in terms of numbers, it is not true for the number that pass on their genes by leaving offspring. In the genetic sense, those who don’t reproduce don’t count, and should be excluded from the equation. What we are interested in, then, is what is known as the
effective
population size – the number of breeding men and women. This is where we see the difference.

The likely explanation for why there is a greater rate of lineage loss for the Y-chromosome is that a few men tend to do most of the mating. Furthermore, their sons – who inherit their wealth and social standing – also tend to do most of the mating in the next generation. Carried through a few generations, this social quirk will produce exactly the sort of pattern we see for the Y-chromosome: a few lineages within populations, and different lineages in neighbouring populations. It will also produce a very recent coalescence time for the Y, since the lineages that would have allowed us to trace back to an Adam living 150,000 years ago were lost while our ancestors were still living in Africa. The definitive proof of this hypothesis will come only from careful studies of traditional societies, where the same social patterns have been practised for hundreds or thousands of years, but my prediction is that it will be confirmed by the data. As with the search for the language of Adam and Eve, the study of the effects of culture on human genetic variation promises to be one of the most exciting areas of enquiry in anthropology over the next few decades. Unfortunately, we may be racing against the clock, as we’ll see in the next chapter.

We’ve been through a tour of how culture, from the development of agriculture to local marriage patterns, has had an effect on human genetic diversity. We are now ready to re-evaluate the Hawaiians who were ‘discovered’ by Captain Cook in the late eighteenth century.
Where did they come from, and why had they conquered the Pacific in the last few thousand years?

The first question we can ask is whether there is a linguistic relationship among the Polynesian languages that suggests a source population. The answer is that there is. While Thor Heyerdahl favoured a South American origin for the Polynesians, their languages are more closely related to those spoken in south-east Asia. As early as the nineteenth century, scholars had linked the languages of Polynesia to those spoken in Taiwan (then Formosa) and Malaysia. Today, Taiwan is inhabited by Han-speaking Chinese, but prior to the seventeenth century it was home to aboriginal groups speaking completely different languages. All of these languages were united into one family, Malayo-Polynesian, which became known as Austronesian in the early twentieth century. So, there is clear linguistic data tracing from Hawaii back to Asia, rather than the Americas.

The overlap between the Austronesian languages and the spread of agriculture in east Asia is striking, and the theory which emerged for the peopling of Polynesia is that agriculturalists who had perfected the art of sailing simply hopped from island to island through south-east Asia, eventually heading into the open ocean. The ‘Express Train’ model, as it became known, predicted a close genetic link between aboriginal Taiwanese and the Polynesians. MtDNA seemed to support this model, although its resolution – as we have seen elsewhere – is often limited. Recent results from the Y-chromosome, though, have suggested that the theory needs to be modified.

The pattern seen for the island south-east Asians is that, while agriculturalists of (ultimately) Chinese origin did have a significant impact on the gene pool, there are a substantial number of indigenous lineages (particularly M130) found throughout Indonesia and Melanesia. These are also present at high frequency in the Polynesians. What this suggests is that after agriculture was introduced to island south-east Asia, it went through a maturation phase as it was adapted to local crops that were better suited to the environment there. Instead of flying past on their express train, the agriculturalists dawdled and dabbled, gradually adapting their culture to its new home. Archaeologist Peter Bellwood has pointed out that the crop yield of Chinese rice strains drops significantly if they are planted near the equator, since
they need the variation in day length found only outside the tropics in order to mature. These sorts of pressures would have encouraged agriculture to change as it passed through south-east Asia, in some cases replacing millet and rice with other crops. The Polynesian taro root, ubiquitous throughout the Pacific and used to make Hawaiian
poi
, reflects this change. The genes also show evidence of a sojourn in south-east Asia before heading out to sea.

The answer to our question of timing, then, can be found in the maturation phase of agriculture. It was only after a fully mature tropical variant of agriculture had taken root that the proto-Polynesians were able to set sail for undiscovered lands. They took with them their crops, confident in their ability to survive wherever they came ashore. Hunter-gatherers would never have been able to make this leap into the unknown ocean – repeatedly – because they had no idea what lay beyond the horizon. The Polynesians, though, as inheritors of a well-adapted agricultural tradition, were in control of their own destinies. They may have been encouraged to set sail by an expanding population at home (another consequence of agriculture), but their unique solution was only possible because they had the
choice
of sailing into the unknown. And it was the pursuit of an ever-increasing spectrum of choices that would produce the final Big Bang of human evolutionary history.

Figure 9 Genealogical tree showing the relationship among the Y-chromosome markers discussed in the text. All trace their descent from M168, who lived in Africa.

Figure 10 The spread of Y-chromosome lineages around the world.

A couple of years ago, I was asked to perform a genetic analysis as part of a television programme. The goal was to show, using genetic data, that all humans trace back to a recent African ancestor. Initially I was hesitant, since it would involve revealing personal genetic results in front of a television camera for the whole world to see. But after being reassured by the producers and the people who donated samples, I went ahead with the analysis. Four men living in London volunteered to have their Y-chromosomes tested, and I analysed the markers we have encountered in this book – M168, M130 and so on. When the work was complete, the data showed the expected pattern for three of the four men. The man with Irish/Scottish ancestry had a Y-chromosome defined by M173, the highest frequency Y lineage in north-western Europe. The Japanese man had M122, in common with around 20 per cent of his fellow countrymen. The Pakistani had the M89 lineage, found throughout the Middle East and central Asia. The final man, though, had a surprising pattern. An Afro-Caribbean, he was hoping for a genetic link to the Zulus of southern Africa, with whom he felt a strong cultural bond. The DNA revealed a more complicated story.

This man turned out to have an M173 Y-chromosome, the canonical European lineage. M173 has never been identified in hundreds of samples from indigenous sub-Saharan Africans, so the obvious question was how did he come to have such an anomalous result? The other, non-Y markers we tested revealed him to be otherwise
genetically African – including the presence of a marker I had first identified in a Zulu man in the mid-1990s. Clearly, the Y was telling a different story – one that helps to illustrate the main theme of this chapter.