Harvard Medical School geneticist David Reich says genetics is providing a new way to investigate the past

… Previously, the understanding of what population transformations were like, what the peopling of certain parts of the world was like, what lifestyles were like, was the province solely of archeologists and cultural anthropologists, or biological anthropologists and paleontologists. Now, this new information is coming into the room and is speaking to many of the same questions. In part, there is an interesting friction related to the new type of information that is coming into a field which previously hadn’t had access to that information. It’s a little bit threatening to people who are already in those fields—that there is a new type of information.

Another thing that’s going on is that population geneticists, such as myself, are a bit unschooled. We haven’t gone through graduate school in anthropology, or linguistics, or history, and yet, we’re making very strong statements about these people’s fields. It's a little bit like barbarians are walking into your room, and you can’t ignore barbarians because they have information, weapons, and technology that you didn’t have access to before.

A concrete example of this is work on population history in India that I have been involved in and that I continue to be very intensively involved in. The history of Indian populations is very rich; it’s one of the most incredibly diverse places in the world, with all these ethnic groups.

There are more than 4000 well-defined ethnic groups that practice mutual endogamy and don’t mix with each other in practice. It’s a very complex place. People look very different, they have extremely different cultures and histories and traditions. There’s a great deal of anthropology and anthropological study that has gone on in India to try to understand the population history and the context of the relationships to places outside of India.

In the beginning of 2007, we started studying at the whole genome level, the whole organism level, the DNA from initially twenty-five diverse Indian populations. It’s now more than 200 that we’ve studied. We picked these populations to be as diverse as possible, capturing the linguistic diversity of India. In the south of India, people speak languages called Dravidian, which are not related to languages outside the Indian subcontinent. In the north, people speak Indo-European languages, which are related to the languages of Europe and Armenia and Iran. There are some other language groups, but we picked people to represent the diversity of these language groups, diversity of social status as encoded in the caste system, and we studied the genetic variation.

What we saw was an amazingly simple pattern: The simple pattern was that the great majority of Indian groups today are descended from a mixture of basically just two ancestral populations, one which we call the ancient ancestral North Indian and one which we call the ancestral South Indian. Everybody is mixed in India without exception. Even the most isolated groups, which are hunter-gatherers living in the forest or isolated places, everybody is mixed with at least 20 percent of each of these ancestries.

This is a surprise that comes from the genetics. There is no pure unmixed ancestral population of Indians. People who are Dravidian, who come from the south of India, tend to have more of the ancient South Indian ancestry. The people from the north, the people who speak Indo-European, tend to have more of a North Indian ancestry. But there is variability in proportion: people who have traditionally higher caste status both within Southern and within Northern India tend to have more of the ancient Northern Indian ancestry. So what was this reflecting?