Around 1500 B.C., light-skinned invaders swept into India from the north. Calling themselves "Aryans" (a Sanskrit word meaning nobles), they eventually seized control of the subcontinent.
Now geneticists at the University of Utah and their colleagues in India have uncovered evidence of that ancient invasion, preserved in the genes of today's Indians. They also discovered that the invaders:
Were warriors from eastern Europe or western Asia.
Were men who arrived without families but fathered offspring in India.
Either set up the caste system that held sway in India for thousands of years, or stepped into an existing caste system and installed themselves at the top.
For most of its history, caste rules held India in a rigid grasp. A person lived and died in the same caste. Status, wealth and opportunities were determined by caste. People were supposed to marry within their caste.
Brahmans lived in luxury. Members of the lowest caste, Harijans or "Untouchables," did all the filthy work and were forbidden even to come into contact with anyone of another caste.
While officially abolished in recent years, India's caste system still resonates through the culture. It affects nearly one-sixth of the world's population.
The new research seems to show that genetic differences exist among the castes, and that European or Eurasian ancestry is the driving force behind the social divisions.
The ancient invasion is recorded in the genes of living people.
"We can look at DNA and we can deduce history," said Lynn B. Jorde, professor of human genetics at the U. and one of the study's principals. "It gives us a marvelous new tool to figure out histories of population."
The Utah scientists have been working on the project for the past seven years with colleagues from Andhra University and about nine other Indian universities. They include, among others, Jorde; Michael J. Bamshad, an assistant professor of pediatrics; and senior laboratory specialist W. Scott Watkins, all of the U.'s Eccles Institute of Human Genetics.
Interviewed in their laboratory on the seventh floor of the Eccles Institute, Jorde and Bamshad explained that the two types of DNA most important to the research were the Y chromosome, which is handed down intact through the male line, and mitochondrial DNA, which is inherited only from one's mother.
In the study, they examined DNA extracted from blood samples donated by nearly 1,000 individuals. Sampling cut across caste lines and was carried out both in the northern and southern sections of India.
They compared specific DNA markers that vary throughout the world according to ethnic background.
The maternal DNA of all Indian caste groups "showed the greatest affinity genetically with Asians," Jorde said.
"So if we only looked at mitochondrial DNA, the maternal DNA, we'd say that, well, all of the castes are Asians."
In the Y chromosome, the DNA handed down directly from father to son, the samples from lower-caste Indians showed they had close connections with the rest of Asia. This was consistent with the mitochondrial DNA samples.
However, the Y chromosomes obtained from upper-caste Indians "have a greater affinity with Europeans than they do with Asians," Jorde said.
"The middle castes are just about right in between. They're about as Asian as European. So we see this real gradient."
What this suggests is that male European-types arrived at some point and took over the highest positions. Because the maternal DNA remained Asian, they did not take women with them to India.
Instead, they married into the Asian population and set themselves and their offspring apart as "superior."
"Now, it may be that the caste system was already in place and that they just . . . said, 'OK, we're the invaders so we're taking the high positions.' " Or possibly they set up the system themselves.
Perhaps that is why, among the three upper castes, the one with the most connections to Europeans is the warrior caste. "That's consistent with the idea that it was male warriors that came over 3,500 years ago and occupied those high strata," he said.
Before now, Bamshad said, "most of the evidence for a shared, recent common ancestry between many Indian people as well as many Europeans has been because of shared linguistics." This has been called the Indo-European language.
Where did the invaders come from? "Sort of west Asia, east Europe, that general geographic region," said Jorde.
However, nobody has been able to use genetic tools to pinpoint the Europeans who are the descendants of the root stock of invaders who went to India, Bamshad said.
Indians have tremendous differences of opinion about the origins of their culture, he added.
"There are many people within India who believe that there was no invasion, that Indian castes arose in India and there's never been any influence from populations migrating from the north, or from west Eurasia. But there are many scholars who believe that archaeological records and data, as well as clues from some of the written records about the establishment of the Hindu caste system, suggest that there was an external influence and that external influence came from western Eurasia."
The strongest European genetic findings came from northern India. Heading south, there are fewer and fewer influences. "That's what you'd expect if people with European affinities came in through the north primarily and gradually their genes flow to southward," Jorde said.
Another type of DNA called the autosomes -- from chromosomes 1 through 22 -- also showed patterns like that found in the maternal DNA. "So we're seeing consistencies. This isn't a fluke," Jorde said.
What surprised him was the big difference between the Y chromosome and mitochondrial DNA, he said. When it first surfaced, team members would scratch their heads and try to figure out its significance. "In fact, we had a lot of arguments about what it meant," he said.
"We finally came to a consensus that it was telling us that females were moving up and down the caste system -- primarily up -- and males weren't." A man was locked into the caste into which he was born, but a woman could marry into a higher caste.
Bamshad was struck by the consistency of the patterns they discerned, and by the fact that the findings were consistent with "some of the stories at least about how India was populated and how the caste system was developed."
He is quick to add that the work so far is preliminary, "even though we've done an exhaustive amount of work and analysis."
Jorde noted that the evolutionary geneticist Theodosius Dobzhansky wrote about 50 years ago that the Indian caste system is one of the great natural human genetic experiments.
"People have been interested in this for decades," Jorde added.
"The problem is that we didn't have the genetic tools to really look at this with a fine degree of resolution until just the last 10 years or so. So now we can really look at history, and what excites and reassures us is that history really does make its mark on our genes."