Social Inequality Leaves a Genetic Mark

In humans, the profound biological differences that exist between the sexes mean that a single male is physically capable of having far more children than is a single female. Women carry unborn children for nine months and often nurse them for several years prior to having additional children.¹ Men, meanwhile, are able to procreate while investing far less time in the bearing and early rearing of each child. So it is that, as measured by the contribution to the next generation, powerful men have the potential to have a far greater impact than powerful women, and we can see this in genetic data.

The great variability among males in the number of offspring produced means that by searching for genomic signatures of past variability in the number of children men have had, we can obtain genetic insights into the degree of social inequality in society as a whole, and not just between males and females. An extraordinary example of this is provided by the inequality in the number of male offspring that seems to have characterized the empire established by Genghis Khan, who ruled lands stretching from China to the Caspian Sea. After his death in 1227, his successors, including several of his sons and grandsons, extended the Mongol Empire even farther—to Korea in the east, to central Europe in the west, and to Tibet in the south. The Mongols maintained rested horses at strategically spaced posts, allowing rapid communication across their more than 8,000-kilometer span of territory. The united Mongol Empire was short-lived—for example, the Yüan dynasty they established in China fell in 1368—but their rise to power nevertheless allowed them to leave an extraordinary genetic impact on Eurasia.²

A 2003 study led by Chris Tyler-Smith showed how a relatively small number of powerful males living during the Mongol period succeeded in having an outsize impact on the billions of people living in East Eurasia today.³ His study of Y chromosomes suggested that one single male who lived around the time of the Mongols left many tens of millions of direct male-line descendants across the territory that Mongols occupied. The evidence is that about 8 percent of the male population in the lands the Mongol Empire once occupied share a characteristic Y-chromosome sequence and a cluster of similar sequences differing by just a few mutations. Tyler-Smith and his colleagues called this a “Star Cluster” to reflect the idea of a single ancestor with many descendants, and estimated the date of the founder of this lineage to be 1,300 to 700 years ago based on the estimated rate of accumulation of mutations on the Y chromosome. The date coincides with that of Genghis Khan, suggesting that this single successful Y chromosome may have been his.

Star Clusters are not limited to Asia. Geneticist Daniel Bradley and his colleagues identified a Y-chromosome type that is present in 2 to 3 million people today and derives from an ancestor who lived around 1,500 years ago.⁴ It is especially common in people with the last name O’Donnell, who descend from one of the most powerful royal families of medieval Ireland, the “Descendants of Niall”—referring to Niall of the Nine Hostages, a legendary warlord from the earliest period of medieval Irish history. If Niall was real, he would have lived at about the right time to match the Y-chromosome ancestor.

Star Clusters capture the imagination because they can be tied, albeit speculatively, to historical figures. But the more important point is that Star Cluster analysis provides insights about shifts in social structure that occurred in the deep past that are difficult to get information about in other ways. This is therefore one area in which Y-chromosome and mitochondrial DNA analysis can be instructive, even without whole-genome data. For example, a perennial debate among historians is the extent to which the human past is shaped by single individuals whose actions leave a disproportionate impact on subsequent generations. Star Cluster analysis provides objective information about the importance of extreme inequalities in power at different points in the past.

Powerful males in this period left an extraordinary impact on the populations that followed them—some bequeathed DNA to more descendants today than Genghis Khan.

Two studies, one led by Toomas Kivisild and the other led by Mark Stoneking, have compared the results of Star Cluster analysis on Y-chromosome sequences and on mitochondrial DNA sequences and arrived at an extraordinary result.^5,6 By counting the number of differences per DNA letter between pairs of sequences which reflects mutation that accumulated in a clock-like way over time, these studies estimated the time since different pairs of individuals shared common ancestors on the entirely male (Y-chromosome) and entirely female (mitochondrial DNA) lineages.

In mitochondrial DNA data, all the studies found that most couples living in a population today have a very low probability of sharing a common ancestor along their entirely female line in the last 10,000 years, a period postdating the transition to agriculture in many parts of the world. This is exactly as expected if population sizes were large throughout this period. But on the Y chromosome, the studies found a pattern that was strikingly different. In East Asians, West Africans, Europeans, Near Easterners, and North Africans, the authors found many Star Clusters with common male ancestors living roughly around 5,000 years ago.^5,6

The time around 5,000 years ago coincides with the period in Eurasia that archaeologist Andrew Sherratt called the “Secondary Products Revolution,” in which people began to find many uses for domesticated animals beyond meat production, including employing them to pull carts and plows and to produce dairy products and clothing such as wool.⁷ This was also around the time of the onset of the Bronze Age, a period of greatly increased human mobility and wealth accumulation, facilitated by the domestication of the horse, the invention of the wheel and wheeled vehicles, and the accumulation of rare metals like copper and tin, which are the ingredients of bronze and had to be imported from hundreds or even thousands of kilometers away. The Y-chromosome patterns reveal that this was also a time of greatly increased inequality, a genetic reflection of the unprecedented concentration of power in tiny fractions of the population that became more possible during this time due to the new economy. Powerful males in this period left an extraordinary impact on the populations that followed them—more than in any previous period—with some bequeathing DNA to more descendants today than Genghis Khan.

From ancient DNA combined with archaeology, we are beginning to build a picture of what this inequality might have meant. The period around 5,000 years ago north of the Black and Caspian seas corresponds to the rise of the Yamnaya, who took advantage of horses and wheels to exploit the resources of the open steppe for the first time.⁸ The genetic data show that the Yamnaya and their descendants were extraordinarily successful, largely displacing the farmers of northern Europe in the west and the hunter-gatherers of central Asia in the east.^9,10

Archaeologist Marija Gimbutas has argued that the Yamnaya were an unprecedentedly sex-biased and stratified society. They left behind great mounds, about 80 percent of which had male skeletons at the center often with evidence of violent injuries and buried amidst fearsome metal daggers and axes.¹¹ Around 80 percent of the satellite graves in Yamnaya burial mounds also contained males, often with evidence of great wounds. Gimbutas argued that the arrival of the Yamnaya in Europe heralded a shift in the power relationships between the sexes. It coincided with the decline of “Old Europe,” in Gimbutas’s reconstruction a society with little evidence of violence, in which females played a central social role as is apparent in the ubiquitous Venus figurines. In her reconstruction, “Old Europe” was replaced by a male-centered society, evident not only in the archaeology but also in the male-centered Greek, Norse, and Hindu mythologies of people who speak the Indo-European languages probably spread by the Yamnaya.¹²

There were extraordinary hierarchies and imbalances in power at work in the Yamnaya expansions.

Gimbutas’s reconstruction has been criticized as fantastical by her critics, and any attempt to paint a vivid picture of what a human culture was like before the period of written texts needs to be viewed with caution. Nevertheless, ancient DNA data has provided evidence that the Yamnaya were indeed a society in which power was concentrated among a small number of elite males. The Y chromosomes that the Yamnaya carried were nearly all of a few types, which shows that a limited number of males must have been extraordinarily successful in spreading their genes. In contrast, in their mitochondrial DNA, the Yamnaya had more diverse sequences.⁹ The descendants of the Yamnaya or their close relatives spread their Y chromosomes into Europe and India, and the demographic impact of this expansion was profound, as the Y-chromosome types they carried were absent in Europe and India before the Bronze Age but are predominant in both places today.¹³

This Yamnaya expansion also cannot have been entirely friendly, as is clear from the fact that the proportion of Y chromosomes of steppe origin in both western Europe¹⁴ and in India¹⁵ today is much larger than the proportion of the rest of the genome. This preponderance of male ancestry coming from the steppe implies that male descendants of the Yamnaya with political or social power were more successful at competing for local mates than men from the local groups. The most striking example I know is from Iberia in far southwestern Europe, where Yamnaya-derived ancestry arrived suddenly at the onset of the Bronze Age between 4,500 and 4,000 years ago. Daniel Bradley’s laboratory and my laboratory independently produced ancient DNA from individuals of this period.¹⁴ We find that in the first Iberians with Yamnaya-derived ancestry, the proportion of Yamnaya ancestry across the whole genome is almost never more than around 15 percent. However, around 90 percent of males who carry Yamnaya ancestry have a Y-chromosome type of steppe origin that was absent in Iberia prior to that time. It is clear that there were extraordinary hierarchies and imbalances in power at work in the Yamnaya expansions.

The Star Cluster work rests on Y chromosomes and mitochondrial DNA. What can whole-genome analysis add? When whole-genome data are used to reconstruct the size of the ancestral population of most agricultural groups in the last 10,000 years, they document population growth throughout this period, with no evidence of the Bronze Age population bottlenecks detected from Y chromosomes.¹⁶ This is not what one would expect from averaging the mitochondrial DNA and Y chromosomes. Instead, it is clear that the Y chromosome was a nonrepresentative part of the genome where certain genetic types were more successful at being passed down to later generations than others. In principle, one possible explanation for this is natural selection, whereby some Y chromosomes gave a biological advantage to those who carried them, such as increased fertility. But the fact that this genetic pattern manifested itself around the same time in multiple places around the world—in a period coinciding with the rise of socially stratified societies—is too striking a pattern to be explained by natural selection at multiple independently occurring advantageous mutations. I think a more plausible explanation is that in this period, it began to be possible for single males to accumulate so much power that they could not only gain access to large numbers of females, but they could also pass on their social prestige to subsequent generations and ensure that their male descendants were similarly successful. This process caused the Y chromosomes these males carried to increase in frequency generation upon generation, leaving a genetic scar that speaks volumes about past societies.

It is also possible that in this period, individual women also began to accumulate more power than they ever had before. Yet because it is biologically impossible for a woman, even a very powerful one, to have an extremely large number of children, the genetic effects of social inequality are much easier to detect on the male line.

David Reich is a professor of genetics at Harvard Medical School and a Howard Hughes Medical Institute Investigator. Awards he has received include the Newcomb Cleveland Prize from the American Association for the Advancement of Science and the Dan David Prize in the Archaeological and Natural Sciences for his computational discovery of intermixing between Neanderthals and Homo sapiens.

References

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Lead Image Credits: Pixabay; Djomas / Shutterstock

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