TAKE a close look at the DNA of modern Americans and you see an odd asymmetry. While populations of African Americans contain genes that originated in European men, there is no evidence of genes from African American women in groups of Americans with European ancestry. Relationships between black women and white men have left a clear genetic heritage – but only on the African American side. At first glance this one-sided legacy is puzzling, until you realise that the genes don’t simply reflect biological coupling. They also tell a more subtle story: one about the history of colonialism and racial prejudice in the US.
The slave trade that began in the 15th century brought 12 million Africans to America. Although intimacy between slaves and their owners was strictly taboo, men of white European origin frequently formed sexual relationships with African slave women, often against their will. And if the slave became pregnant, she would usually raise the child single-handedly, within her own community. Such women effectively became conduits for European DNA, channelling it into the slave communities through their children.
Some 140 years after the abolition of slavery in the US, the legacy of this inequality persists in the form of those rogue European genes, as Manfred Kayser of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues found. They made the discovery by analysing samples of DNA from 16 different populations of African Americans and European Americans.
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Kayser’s study is part of a growing trend for combining analysis of mitochondrial DNA (mtDNA) – which is inherited only through the maternal line – with studies of DNA from the Y chromosome – which is passed down from father to son. This approach is revealing patterns of sex-specific inheritance that add a new dimension to the knowledge that we can derive from genes.
We are used to the idea that genes can reveal connections on two very different timescales: connections between species on evolutionary timescales of hundreds of thousands of years, and links between individuals and their immediate ancestors – often ones that help solve puzzles associated with genetic diseases. But Kayser and others are interested in the intermediate past. They are using mtDNA and YDNA to tell us more about human history and prehistory.
Kayser’s findings provide a useful calibration for this kind of analysis, because they prove that documented historical events do indeed create ripples in our gene pools. But most of the research in this field is concerned with investigating incidents that occurred before records existed. It is a very young science and the genetic legacy is not always easy to interpret, but already analyses of mtDNA and YDNA are starting to paint an intriguing picture of prehistoric human behaviour: about migrations, marriages, cultural practices and the interactions between various populations.
In 1998, a pioneering study by Mark Seielstad of Harvard School of Public 91ɫƬ and colleagues compared the worldwide variation of the Y chromosome with that of mtDNA (New Scientist, 31 October 1998, p 11). Such variation is a way of tracking migrations, because each mutation would have arisen randomly in some ancestor and then been passed down to their descendants wherever they went. The researchers took previously published information about variations in mtDNA within populations and between continents and compared it with the YDNA variation in men from 54 populations from Africa, Oceania, Asia, Europe and the Americas. They discovered much less variation between populations in mtDNA than in the YDNA. In other words, the female line is more homogenised than the male. The researchers concluded that throughout our species’ history women have had a higher migration rate than men – eight times as high according to their calculations.
It would be nice to see these findings as an indication that women were historically the adventurers rather than men, travelling to new lands and making their genetic mark. But as Seielstad pointed out at the time, the true cause of the genetic disparity is much more likely to be a patriarchal system in which women were forced to move from their homes to marry into neighbouring or distant clans, while men were able to stay put. This is what anthropologists call patrilocality.
But the story doesn’t end there. Three years later Michael Hammer’s group from the University of Arizona in Tucson did a similar study, this time looking only at variations within mtDNA and YDNA from Africans, and found the opposite result – men migrating, women staying put. Seielstad’s study had looked at 14 populations of farmers from eastern and central Africa. Hammer’s sample was more diverse, including hunter-gathering Pygmies and Bushmen as well as Bantu farmers. Might this be at the root of the discrepancy?
To probe the mystery, geneticists have now carried out a study that is more detailed than either of its predecessors, incorporating 40 populations from all over sub-Saharan Africa. Giovanni Destro-Bisol and Gabriella Spedini of La Sapienza University in Rome, Italy, and a team that includes colleagues from the University of Oxford, compared a range of genetic markers, including several stretches of DNA that mutate rapidly. They also looked at slowly evolving mtDNA and Y chromosome haplogroups – variants defined by unique mutation events.
“Fast and slowly evolving polymorphisms provide two different and in part complementary ways to view the genetic structure of populations,” says Destro-Bisol. While rapidly mutating regions reflect recent evolutionary events, the mutation in these areas is somewhat chaotic and repetitive, so even if two individuals have the same mutation, it doesn’t necessarily mean they share an ancestor. Slowly evolving DNA, on the other hand, says nothing about recent events but is a far more reliable indicator of relatedness, because the same mutation is highly unlikely to occur twice by chance.
The distribution of these markers suggested an overall male migration rate that was nearly twice that of females, corroborating Hammer’s study. But when Destro-Bisol and his team divided their populations into hunter-gatherers and farmers, two distinct patterns emerged: male migration was higher for hunter-gatherers, while female migration was greater among farmers. Their findings suggest that the invention of farming ushered in a new matrimonial system and associated pattern of migration. And since our ancestors have been farming for no more than 12,000 years, patrilocality seems to be a relatively recent innovation among humans.
Population geneticist Luigi Luca Cavalli-Sforza of Stanford University in California, who contributed to the Seielstad study, is not surprised that Destro-Bisol’s more detailed analysis reveals distinctions that his group was not able to detect. The Bantu people of the Nigeria-Cameroon border learned to farm around 3000 years ago, after which they expanded east and south, encountering first the Pygmies and then the Bushmen of southern Africa. They probably exchanged genes with all the hunter-gatherers they met en route, says Cavalli-Sforza, and had got as far as 300 kilometres north of what is now Cape Town when the Dutch landed and blocked their advance. “Their migration took a very long time,” he says. “Males and females obviously migrated together, but the story as read in the mtDNA and [Y chromosome] is likely to be quite different.”
And so it is. When Destro-Bisol’s team looked more closely at their data, they saw traces not only of the different migratory practices of hunter-gatherers and farmers, but also of complex social interactions between the two. By mapping the distribution of a Y chromosome mutation called M2, which originated with the Bantu around the time of their expansion, and other mutations associated with Pygmies, they saw a one-way flow of YDNA from Bantu to Pygmy. The highest frequency of M2, at 65 per cent, occurred among the Western Biaka Pygmies, with the lowest among the Ju/’hoansi or !Kung Bushmen, who have largely avoided intermarriage with Bantu speakers. Meanwhile, the distribution of a single mtDNA haplogroup associated with Pygmies suggested that mtDNA flowed only in the other direction: from Pygmy to Bantu.
That asymmetry reflects the social hierarchy that farming imposed on the continent, says Destro-Bisol. It also fits very well with what Cavalli-Sforza and others have written about the relationship between these two peoples. Pygmies once roamed over a vast area of equatorial rainforest. Then the Bantu arrived, armed with a superior metallurgic knowledge that enabled them to scythe a path through the jungle, gradually pushing the Pygmies into smaller and smaller pockets. With their range restricted, and separate Pygmy groups cut off from one another, gene flow between those groups was reduced. They began to breed with the Bantu instead, but in a way that was controlled by social inequalities between the two – inequalities that still exist.
“Pygmy women are accepted as wives by Bantu communities for the low price of the bride and for the fame of their great fertility,” explains Destro-Bisol. Along with their children, and in contrast to African-American slave women, Pygmy women entered Bantu society and brought their mtDNA with them. Bantu women did not move the other way, though, because the Pygmies were regarded as inferior, and social taboos forbade Bantu women from marrying “beneath themselves”. Meanwhile the flow of Bantu YDNA into the Pygmy population could be explained by extramarital affairs between Bantu men and Pygmy women, by Pygmies adopting the orphans of mixed marriages, and by the return of Pygmy women and their children to their own communities after divorce.
What such studies reveal, says Destro-Bisol, is that cultural practices can shape the human gene pool. On a global scale, this has been happening at least since the invention of agriculture. Before that, he admits, culture may have acted more as a genetic buffer, providing our ancestors with technologies and practices that allowed them to adapt to diverse environments and so spread across the world without requiring specific genetic adaptations. But Destro-Bisol describes agriculture as “the driving factor for the establishment of more complex societies with social inequalities within and between populations”. In this sort of world, culture can leave its mark on our genes.
Genetic anthropology can also help settle questions about how culture spreads. One of the challenges facing those trying to understand our prehistoric past is to work out whether innovations such as agriculture, new technologies, languages and religious practices have spread as a result of migration and intermarriage or by diffusion of ideas. “I expect that word of mouth will turn out to have spread culture far more effectively than [intermarriage],” speculates Destro-Bisol, pointing to the success of the ancient Romans in imposing their culture on the peoples they conquered, despite scarcely interbreeding with them. But other experts put greater emphasis on migration and intermarriage, or even displacement of local groups by incomers, to explain the occurrence of certain artefacts in the archaeological record or shared cultural practices among groups of modern people. Undoubtedly, both people and ideas have spread throughout human history, but analysis of mtDNA and YDNA can now help anthropologists distinguish which has been the main force in spreading some particular element of culture.
Take the Balinese. They are the only Indonesians to practise the Hindu religion and to maintain the Hindu caste system. Bali lies near the centre of the southern chain of islands in the Indonesian archipelago, so it provided a handy stepping stone for the first human migrations from Asia to Melanesia, Australia and the Pacific, and later became a pit stop on trade routes. “There are several hypotheses but in general it was assumed that Bali’s ‘Indianisation’ has been driven by trade and the spread of ideas, not genes,” says Tatiana Karafet of the Genomic Analysis and Technology Core at the University of Arizona.
To test this idea, Karafet and her colleagues analysed the Y chromosomes of around 500 Balinese men from 27 populations, and compared them with YDNA from nearby Asian and Pacific populations, as well as men from the Indian subcontinent and the Middle East. They looked at two different markers, and both gave pretty much the same result: around 20 per cent of the Balinese gene pool was contributed by Indian male ancestors. They came, bred with the locals, and left behind not only their genes but also their entire belief system.
Even more intriguing is a group called the Mbugu, who live in the Usambara mountains of eastern Tanzania. The Mbugu are among the very few peoples who speak a language that combines two others. In fact they speak two separate languages: the first is a dialect of the Bantu language of the neighbouring and socially dominant Pare tribe, which the Pare can understand; the second, the mixed language, is “inner Mbugu”, which is incomprehensible to their Pare neighbours. Inner Mbugu combines a Bantu grammar with a Cushitic vocabulary of Ethiopian origin. The question is, did the mixing of those language elements occur alongside a gradual blending of genes between Bantu and a Cushitic-speaking people, the ancestors of the Mbugu?
Maarten Mous of the University of Leiden in the Netherlands points to the creativity – verging on linguistic games – in the lexicon of inner Mbugu. This, he says, suggests the language arose spontaneously and rapidly. Work by Sarah Tishkoff from the University of Maryland seems to support this theory. Her analysis of mtDNA shows that there has been very little gene exchange between the Mbugu and the surrounding Bantu people, and she believes inner Mbugu was invented as a sort of cultural barrier.
“The Mbugu can speak the Bantu language if they want to,” she says, “But they keep the mixed language almost more as a secret code, for special ceremonial, religious practices.” Superstition could also have something to do with it, she suggests. The Mbugu, like some other African peoples, may believe that if others learn about their rituals and witchcraft, they will be used against them. This, Tishkoff argues, is an example of people actively and consciously using their language as a shield to protect their culture and identity – thereby shaping their genome.
As genetic anthropology flourishes, it will undoubtedly increase our knowledge of human history. By exposing the subtle interplay between genetic change and cultural practices, studies of mtDNA and YDNA are proving more revealing than many people had thought possible. And they have the potential to shed new light on knotty questions such as the peopling of the Americas, the spread of agriculture across the globe and the origins of languages and religions.
But there is a catch: it takes time for social change to feed through to the gene pool. This means that short-term interactions between various human populations will not show up in the genes. It also explains why the gene pool of African Americans tells the story of social interactions between black slave women and their masters, but says nothing about marriages between African-American men and Caucasian women – the most common interracial relationship in the US since 1970. That will only become apparent to the genetic anthropologists of the future.