Though one country politically, the genetics of indigenous Mexicans shows that their ancestors were very distinct groups that mixed remarkably little. A study published today in Science found more genetic isolation than expected among these populations.

"You can clearly differentiate each of the native American groups one from the other," said Carlos Bustamante, a professor of genetics at Stanford who led the research. It was "kind of surprising," he said, that this "plays out even though there's been 500 years of admixture, huge amounts of population growth, and lots of migration and movement."

The study marks the first time that researchers looked at the genetic history of Mexico, taking samples from more than a thousand people representing 20 indigenous and 11 mestizo (a person of combined European and Native American descent) groups. The map they made from that data shows nine distinct groups—including Maya, Lacandon, Tojolabal, and Zapotec—with very little intermingling among them.

Medical Implications

The research helps better explain the settlement patterns of early Mexico and has medical implications for Mexicans and people of Mexican heritage, said Bustamante, who is also co-founding director of Stanford's Center for Computational, Evolutionary and Human Genomics, whose team also included researchers from the University of California, San Francisco, and the National Institute of Genomic Medicine in Mexico.

Tiny genetic changes can lead to medically relevant differences, putting some ethnic groups at more or less risk for different diseases. For instance, Bustamante and his team looked at a standard measure of lung function, in which "normal" is defined differently based on a person's ancestry.

The diversity of Mexico's heritage showed up in the lung functions of the mestizo people Bustamante studied—those with both European and native heritage—some of whom would have been defined as diseased when they were actually healthy.

"The fact that you have Maya versus northern Mexican ancestry actually impacted the measurements of lung function," Bustamante said.

Mexican Americans, by contrast, generally have more of a mix of ancestry, he said, including indigenous Mexican, European, and a bit of African.

But doctors need to be cautious, he said, about lumping people together based on a box they checked on a survey. The Mexican migration patterns are similar to ones all over the world, said Dr. Harry Ostrer, a professor of pathology, genetics, and pediatrics at the Albert Einstein College of Medicine in the Bronx. A few people venture far away from their homes and settle in a new area where they are isolated from others, he said. The next generations intermarry and stay put, so any genetic mutations they carry spread through the group.

"People lived very simple lives," he said. "There weren't cities. There was not necessarily a drive for migration to improve one's economic life."

Bustamante said he was surprised to find that so many of these groups remained distinct, though. In Europe, he said, genetics show the waves of powerful invaders who took over vast swaths of land; in Mexico, by contrast, the indigenous groups retained their distinct territories—at least until the Europeans arrived.