Campus & Community

Women on the move

6 min read

Female genes outtraveled those of males, research shows

Mark Seielstad checks refrigerated DNA samples collected from around to world in order to track the migration rates of men and women. (Staff photo by Kris Snibbe)

For most of human history, men have traveled as explorers, warriors, hunters, and traders, so it’s logical to assume that they have spread their genes more widely than women. But what seems logical isn’t always right.

Since a Harvard graduate student published his Ph.D. thesis three years ago, evidence has been accumulating that women are the real movers of society, spreading their genes as they married and moved in with their husband’s families.

“Differences in migration rates between men and women can be measured by studying their genes,” says Mark Seielstad, who wrote the thesis and is now a research associate at the Harvard School of Public Health’s Program for Population Genetics. “Such measurements reveal that, when it comes to exchanging genes among populations, women’s movements have been more important than those of males.”

Sure, men traveled great distances in their search for fortune and fame, but they generally had children near their own birthplaces. The women they married moved from their own family groups to those of their husbands. The predominance of the latter process in dispersing genetic seeds can be traced through so-called mitochondrial DNA, or genes that have been passed only between females since the time of Eve. This type of DNA shows significantly greater distribution and change worldwide than do Y chromosomes, genetic markers passed only from male to male.

“Every male with the same last name should have identical Y chromosomes which can be traced back to Adam, or the original male,” Seielstad notes. “And that’s what we found in our study of men in Europe.”

Seielstad and colleague Nadia Singh have also used this technique to date the arrival of the first people in the Americas. From tracking genes that appear in Native Americans only, they conclude that migrants reached North America from Asia not more than 20,000 years ago, as much as 20,000 years later than the maximum date determined by earlier and less accurate analyses.

Genes from Eve and Adam

Seielstad’s thesis, published in 1998, presented the first detailed evidence that women have played a more important role than men in spreading new genes around the world. He looked at people living on all continents (excluding Antarctica) and found Y chromosome patterns that tied men to specific locations. Women’s mitochondrial DNA, in contrast, showed no such limits. According to this evidence, women have spread their genes not only from region to region but to other continents.

Most genes are shuffled like a deck of cards when people have children; offspring carry a combination of genes from their mother and father. Not so with mitochondrial DNA which is passed unchanged from mother to daughter. Therefore, every woman alive today can trace her mitochondrial DNA back to a single woman – to a genetic Eve.

The term “Eve” is somewhat misleading, Seielstad cautions. It does not mean there was a time when only one woman existed. There were other women alive at the same time, but they did not pass their DNA forward to the present. A woman’s mitochondrial lineage goes extinct if she has no daughters.

The same holds true for the Y chromosome. If you inherit that chromosome, you become a male. If a male has no sons, he doesn’t pass on that chromosome, so all males today can trace their DNA to a single Adam.

That’s what Seielstad did, and he came to the conclusion that variations of the male chromosome are much smaller than those of female DNA. The conclusion brought him lots of attention. “I did a lot of interviews with the media,” he recalls.

Anthropologists weren’t as surprised by the idea of women as the movers, if not the shakers, of societies for thousands of years. Those scientists are aware that, when people married in most traditional societies, the women moved in with their husband’s clan.

There are societies, however, where a man moves in with his wife’s family. “It’s not common, but you do see it in Africa and Southeast Asia,” Seielstad says. He’s now looking for a reverse pattern, that is, for situations where the Y chromosome has spread more widely than matrilineal DNA.

So far, Seielstad has found good evidence of a higher male migration rate in such groups. Now, he’s working on mitochondrial DNA among women in such societies in Thailand and Indonesia. If he and his colleagues find less genetic variety, as he expects, that should provide a final proof of his point.

These relationships no longer occur in modern developed countries, Seielstad notes. “Few married adults move in with the wife’s or husband’s patents; they go where jobs take them,” he says. “However, in less developed nations you still see the pattern of women making the most moves.”

The first Americans

Close study of Y chromosomes from all over the world have revealed a particular mutation, or variation of a gene on the chromosome, which is present in nearly all Native American males. This serendipitous finding enabled Seielstad and Nadia Singh to date the arrival of the first humans in the Americas.

There’s a lot of debate about this date. Other types of genetic studies trace the oldest migration back 30,000-40,000 years. But the earliest physical evidence of humans in America comes from archaeological digs in Chile that date to 12,000-15,000 years ago. What happened between the two sets of dates?

To find out, Seielstad and Singh checked blood samples from different parts of Asia, looking for Y chromosomes that possess the Native American mutation or marker. They found it in Siberia as expected, but it is most common in Uzbekistan, Kyrgyzstan, and other countries west of China. By studying other parts of the Y chromosomes, the researchers calculated the rate at which mutations took place, and thus how many years ago these Asians entered North America.

“We calculate 20,000 years,” says Seielstad. “We don’t believe people have been living in the Americas for longer than this.”

That figure narrows the range between genetic and archaeological dates, and gives the new Americans about 5,000 years to get from Western Alaska to Southern Chile.

The 30,000-40,000 year estimates came from small studies of variations in things like blood types, Seielstad points out. “We believe our estimate is more accurate,” he says.

That will not be the last word, of course. Scientists continue to argue over the technical details of Seielstad’s analysis. However, later dates for the settlement of America probably will win general acceptance, just as the surprising conclusion that women have been more responsible for gene migrations than men. The results demonstrate that genetic information can be used, not just for identifying individuals, but for tracking the migrations of entire societies and civilizations.