by David Reich
The next day, the full group reconvened in Singh’s office. We sat together and came up with new names for ancient Indian groups. We wrote that the people of India today are the outcome of mixtures between two highly differentiated populations, “Ancestral North Indians” (ANI) and “Ancestral South Indians” (ASI), who before their mixture were as different from each other as Europeans and East Asians are today. The ANI are related to Europeans, central Asians, Near Easterners, and people of the Caucasus, but we made no claim about the location of their homeland or any migrations. The ASI descend from a population not related to any present-day populations outside India. We showed that the ANI and ASI had mixed dramatically in India. The result is that everyone in mainland India today is a mix, albeit in different proportions, of ancestry related to West Eurasians, and ancestry more closely related to diverse East Asian and South Asian populations. No group in India can claim genetic purity.
Ancestry, Power, and Sexual Dominance
Having come to this conclusion, we were able to estimate the fraction of West Eurasian–related ancestry in each Indian group.
To make these estimates, we measured the degree of the match of a West Eurasian genome to an Indian genome on the one hand and to a Little Andaman Islander genome on the other. The Little Andamanese were crucial here because they are related (albeit distantly) to the ASI but do not have the West Eurasian–related ancestry present in all mainland Indians, so we could use them as a reference point for our analysis. We then repeated the analysis, now replacing the Indian genome with the genome of a person from the Caucasus to measure the match rate we should expect if a genome was entirely of West Eurasian–related ancestry. By comparing the two numbers, we could ask: “How far is each Indian population from what we would expect for a population of entirely West Eurasian ancestry?” By answering this question we could estimate the proportion of West Eurasian–related ancestry in each Indian population.
In this initial study and in subsequent studies with larger numbers of Indian groups, we found that West Eurasian–related mixture in India ranges from as low as 20 percent to as high as 80 percent.22 This continuum of West Eurasian–related ancestry in India is the reason for the Indian Cline—the gradient we had seen on our principal components plots. No group is unaffected by mixing, neither the highest nor the lowest caste, including the non-Hindu tribal populations living outside the caste system.
The mixture proportions provided clues about past events. For one thing, the genetic data hinted at the languages spoken by the ancient ANI and ASI. Groups in India that speak Indo-European languages typically have more ANI ancestry than those speaking Dravidian languages, who have more ASI ancestry. This suggested to us that the ANI probably spread Indo-European languages, while the ASI spread Dravidian languages.
The genetic data also hinted at the social status of the ancient ANI (higher social status on average) and ASI (lower social status on average). Groups of traditionally higher social status in the Indian caste system typically have a higher proportion of ANI ancestry than those of traditionally lower social status, even within the same state of India where everyone speaks the same language.23 For example, Brahmins, the priestly caste, tend to have more ANI ancestry than the groups they live among, even those speaking the same language. Although there are groups in India that are exceptions to these patterns, including well-documented cases where whole groups have shifted social status,24 the findings are statistically clear, and suggest that the ANI-ASI mixture in ancient India occurred in the context of social stratification.
The genetic data from Indians today also reveal something about the history of differences in social power between men and women. Around 20 to 40 percent of Indian men and around 30 to 50 percent of eastern European men have a Y-chromosome type that, based on the density of mutations separating people who carry it, descends in the last sixty-eight hundred to forty-eight hundred years from the same male ancestor.25 In contrast, the mitochondrial DNA, passed down along the female line, is almost entirely restricted to India, suggesting that it may have nearly all come from the ASI, even in the north. The only possible explanation for this is major migration between West Eurasia and India in the Bronze Age or afterward. Males with this Y chromosome type were extraordinarily successful at leaving offspring while female immigrants made far less of a genetic contribution.
The discrepancy between the Y-chromosome and mitochondrial DNA patterns initially confused historians.26 But a possible explanation is that most of the ANI genetic input into India came from males. This pattern of sex-asymmetric population mixture is disturbingly familiar. Consider African Americans. The approximately 20 percent of ancestry that comes from Europeans derives in an almost four-to-one ratio from the male side.27 Consider Latinos from Colombia. The approximately 80 percent of ancestry that comes from Europeans is derived in an even more unbalanced way from males (a fifty-to-one ratio).28 I explore in part III what this means for the relationships among populations, and between males and females, but the common thread is that males from populations with more power tend to pair with females from populations with less. It is amazing that genetic data can reveal such profound information about the social nature of past events.
Population Mixture at the Twilight of Harappa
To understand what our findings about population mixture meant in the context of Indian history, we needed to know not just that population mixture had occurred, but also when.
One possibility we considered is that the mixtures we had detected were due to great human migrations at the end of the last ice age, after around fourteen thousand years ago, as improving climates changed deserts into habitable land and contributed to other environmental change that drove people hither and yon across the landscape of Eurasia.
A second possibility is that the mixtures reflected movements of farmers of Near Eastern origin into South Asia, a migration that could be a possible explanation for the spread of Near Eastern farming into the Indus Valley after nine thousand years ago.
A third possibility is that the mixtures occurred in the last four thousand years associated with the dispersal of Indo-European languages that are spoken today in India as well as in Europe. This possibility hints at events described in the Rig Veda. However, even if mixture occurred after four thousand years ago, it is entirely possible that it took place between already-resident populations, one of which had migrated to the area from West Eurasia some centuries or even millennia earlier but had not yet interbred with the ASI.
All three of the possibilities involve migration at some point from West Eurasia into India. Although Singh and Thangaraj entertained the possibility of a migration out of India and into points as far west as Europe to explain the relatedness between the ANI and West Eurasian populations, I have always thought, based on the absence of any trace of ASI ancestry in the great majority of West Eurasians today and the extreme geographic position of India within the present-day distribution of peoples bearing West Eurasian–related ancestry, that the shared ancestry likely reflected ancient migrations into South Asia from the north or west. By dating the mixture, we could obtain more concrete information.
The challenge of getting a date prompted us to develop a series of new methods. Our approach was to take advantage of the fact that in the first generation after the ANI and ASI mixed, their offspring would have had chromosomes of entirely ANI or ASI ancestry. In each subsequent generation, as individuals combined their mother’s and father’s chromosomes to produce the chromosome they passed on to their offspring, the stretches of ANI and ASI ancestry would have broken up, with one or two breakpoints per generation per chromosome. By measuring the typical size of stretches of ANI or ASI ancestry in Indians today, and determining how many generations would be needed to chop them down to their current size, Priya Moorjani, a graduate student in my laboratory, succeeded in estimating a date.29
We found that all Indian groups we analyzed had ANI-ASI mixture dates between four thousand and two thousand
years ago, with Indo-European-speaking groups having more recent mixture dates on average than Dravidian-speaking groups. The older mixture dates in Dravidian speakers surprised us. We had expected that the oldest mixtures would be found in Indo-European-speaking groups of the north, as it is presumably there that the mixture first occurred. We then realized that an older date in Dravidians actually makes sense, as the present-day locations of people do not necessarily reflect their past locations. Suppose that the first round of mixture in India happened in the north close to four thousand years ago, and was followed by subsequent waves of mixture in northern India as previously established populations and people with much more West Eurasian ancestry came into contact repeatedly along a boundary zone. The people who were the products of the first mixtures in northern India could plausibly, over thousands of years, have mixed with or migrated to southern India, and thus the dates in southern Indians today would be those of the first round of mixture. Later waves of mixture of West Eurasian–related people into northern Indian groups would then cause the average date of mixture estimated in northern Indians today to be more recent than in southern Indians. A hard look at the genetic data confirms the theory of multiple waves of ANI-related mixture into the north. Interspersed among the short stretches of ANI-derived DNA we find in northern Indians, we also find quite long stretches of ANI-derived DNA, which must reflect recent mixtures with people of little or no ASI ancestry.30
Remarkably, the patterns we observed were consistent with the hypothesis that all of the mixture of ANI and ASI ancestry that occurred in the history of some present-day Indian groups happened within the last four thousand years. This meant that the population structure of India before around four thousand years ago was profoundly different from what it is today. Before then, there were unmixed populations, but afterward, there was convulsive mixture in India, which affected nearly every group.
So between four thousand and three thousand years ago—just as the Indus Civilization collapsed and the Rig Veda was composed—there was a profound mixture of populations that had previously been segregated. Today in India, people speaking different languages and coming from different social statuses have different proportions of ANI ancestry. Today, ANI ancestry in India derives more from males than from females. This pattern is exactly what one would expect from an Indo-European-speaking people taking the reins of political and social power after four thousand years ago and mixing with the local peoples in a stratified society, with males from the groups in power having more success in finding mates than those from the disenfranchised groups.
The Antiquity of Caste
How is it that the genetic marks of these ancient events have not been blurred beyond recognition after thousands of years of history?
One of the most distinctive features of traditional Indian society is caste—the system of social stratification that determines whom one can marry and what privileges and roles one has in society. The repressive nature of caste has spawned in reaction major religions—Jainism, Buddhism, and Sikhism—each of which offered refuge from the caste system. The success of Islam in India was also fueled by the escape it provided for low-social-status groups that converted en masse to the new religion of the Mughal rulers. Caste was outlawed in 1947 with the birth of democratic India, but it still shapes whom people choose to socialize with and marry today.
A sociological definition of a caste is a group that interacts economically with people outside it (through specialized economic roles), but segregates itself socially through endogamy (which prevents people from marrying outsiders). Jews in northeastern Europe, from whom I descend, were, prior to the “Jewish emancipation” beginning in the late eighteenth century, a caste in lands where not all groups were castes. Jews served an economic function as moneylenders, liquor vendors, merchants, and craftspeople for the population within which they lived. Religious Jews then as now segregated themselves socially through dietary rules (kosher laws), distinctive dress, body modification (circumcision of males), and strictures against marrying outsiders.
Caste in India is organized at two levels, varna and jati.31 The varna system involves stratification of all of society into at least four ranks: at the top the priestly group (Brahmins) and the warrior group (Kshatriyas); in the middle the merchants, farmers, and artisans (Vaishya); and finally the lower castes (Shudras), who are laborers. There are also the Chandalas or Dalits—“Scheduled Castes”—people who are considered so low that they are “untouchable” and excluded from normal society. Finally, there are the “Scheduled Tribes,” the official Indian government name for people outside Hinduism who are neither Muslim nor Christian. The caste system is a deep part of traditional Hindu society and is described in detail in the religious texts (Vedas) that were composed subsequent to the Rig Veda.
The jati system, which few people outside India understand, is much more complicated, and involves a minimum of forty-six hundred and by some accounts around forty thousand endogamous groups.32 Each is assigned a particular rank in the varna system, but strong and complicated endogamy rules prevent people from most different jatis from mixing with each other, even if they are of the same varna level. It is also clear that in the past, whole jati groups have changed their varna ranks. For example, the Gujjar jati (from which the state of Gujarat in northwest India takes its name) have a variety of ranks depending on where in India they live, which is likely to reflect the fact that in some regions, Gujjars have successfully made the case to raise the status of their jati within the varna hierarchy.33
How the varna and jati relate to each other is a much-debated mystery. One hypothesis suggested by the anthropologist Irawati Karve is that thousands of years ago, Indian peoples lived in effectively endogamous tribal groups that did not mix, much like tribal groups in other parts of the world today.34 Political elites then ensconced themselves at the top of the social system (as priests, kings, and merchants), creating a stratified system in which the tribal groups were incorporated into society in the form of laboring groups that remained at the bottom of society as Shudras and Dalits. The tribal organization was thus fused with the system of social stratification to form early jatis, and eventually the jati structure percolated up to the higher ranks of society, so that today there are many jatis of higher as well as of lower castes. These ancient tribal groups have preserved their distinctiveness through the caste system and endogamy rules.
An alternative hypothesis is that strong endogamy rules are not very old at all. The theory of the caste system is undeniably old, as it is described in the ancient Law Code of Manu, a Hindu text composed some hundreds of years after the Rig Veda. The Law Code of Manu describes in exquisite detail the varna system of ranked social stratification, and within it the innumerable jati groups. It puts the whole system into a religious framework, justifying its existence as part of the natural order of life. However, revisionist historians, led by the anthropologist Nicholas Dirks, have argued that in fact strong endogamy was not practiced in ancient India, but instead is largely an innovation of British colonialism.35 Dirks and colleagues showed how, as a way of effectively ruling India, British policy beginning in the eighteenth century was to strengthen the caste system, carving out a natural place within Indian society for British colonialists as a new caste group. To achieve this, the British strengthened the institution of caste in parts of India where it was not very important, and worked to harmonize caste rules across different regions. Given these efforts, Dirks suggested that strong endogamy restrictions as manifested in today’s castes might not be as old in practice as they seem.
To understand the extent to which the jatis corresponded to real genetic patterns, we examined the degree of differentiation of each jati from which we had data with all others based on differences in mutation frequencies.36 We found that the degree of differentiation was at least three times greater than that among European groups separated by similar geographic distances. This could not be explained by differences in ANI ancestry among grou
ps, or differences in the region within India from which the population came, or differences in social status. Even comparing pairs of groups matched according to these criteria, we found that the degree of genetic differentiation among Indian groups was many times larger than that in Europe.
These findings led us to surmise that many Indian groups today might be the products of population bottlenecks. These occur when relatively small numbers of individuals have many offspring and their descendants too have many offspring and remain genetically isolated from the people who surround them due to social or geographic barriers. Famous population bottlenecks in the history of people of European ancestry include the ones that contributed most of the ancestry of the Finnish population (around two thousand years ago), a large fraction of the ancestry of today’s Ashkenazi Jews (around six hundred years ago), and most of the ancestry of religious dissenters such as Hutterites and Amish who eventually migrated to North America (around three hundred years ago). In each case, a high reproductive rate among a small number of individuals caused the rare mutations carried in those individuals to rise in frequency in their descendants.37
We looked for the telltale signs of population bottlenecks in India and found them: identical long stretches of sequence between pairs of individuals within the same group. The only possible explanation for such segments is that the two individuals descend from an ancestor in the last few thousand years who carried that DNA segment. What’s more, the average size of the shared DNA segments reveals how long ago in the past that shared ancestor lived, as the shared segments break up at a regular rate in each generation through the process of recombination.
