Andean Villagers Drank Arsenic-Laced Water for Millennia, and Their Genes May Show It

People in a remote, high-altitude town in northern Argentina may have evolved a genetic edge against arsenic, a poison that naturally seeps into their drinking water, according to ScienceAlert's report on research published in Molecular Biology and Evolution.

The team found a cluster of gene variants that were far more common in the town's residents than in related populations elsewhere in South America. The variants appear to help the body convert arsenic into a form it can flush out more easily.

Why is arsenic such a problem in this part of Argentina?

In the Puna de Atacama plateau, arsenic occurs naturally in volcanic bedrock and leaches into groundwater. ScienceAlert reported that, until a filtration system was installed in 2012, the water in San Antonio de los Cobres contained around 200 micrograms of arsenic per liter. The World Health Organization's recommended limit is 10 micrograms per liter, so the town's water carried about 20 times that amount.

Arsenic is linked to cancer, skin lesions, pregnancy complications and early death. Yet people have lived in the region for at least 7,000 years, and perhaps as long as 11,000.

What puzzled scientists about the local population?

In 1995, scientists noticed that women from the Argentine Andes handled arsenic in an unusual way. The body breaks arsenic down through several chemical forms. One intermediate, monomethylated arsenic, is especially toxic, while a later form, dimethylated arsenic, is easier to excrete in urine. Residents of San Antonio de los Cobres tended to make less of the harmful intermediate and more of the form that leaves the body.

How did researchers look for an explanation?

A team led by evolutionary biologists Carina Schlebusch and Lucie Gattepaille of Uppsala University analyzed DNA from blood samples or cheek swabs taken from 124 women in the town. The women's urine showed a wide range of arsenic metabolite levels. The researchers scanned millions of genetic markers and focused on a gene for an enzyme called AS3MT, which earlier work had tied to arsenic processing.

They found variants near that gene that strongly influenced how the body handles arsenic. To test whether the pattern was unique, they compared the results with public genome data from Peru and Colombia and found the variants were much rarer there.

In their 2015 paper, the authors wrote that the data suggest arsenic tolerance has likely driven protective variants to become more common, calling it the first evidence of human adaptation to a toxic chemical.

Is this only an Andean story?

Not entirely. According to ScienceAlert, a September 2025 University of Chicago study of more than 6,500 arsenic-exposed adults in Bangladesh identified two more gene clusters, FMO and GSTO, that also shape how efficiently people process arsenic. Later research has also pointed to similar genetic signals in other Andean groups that have been exposed for generations, hinting that the adaptation may be more widespread across the region.

Why does it matter?

Arsenic is naturally present at high levels in groundwater in many regions of the world, but few communities have lived with such exposure for so long. That long history may be what allowed natural selection to favor people who were less vulnerable to its effects. The researchers noted that, given the severe health effects of arsenic in children and adults, carriers of the protective variants could have had a very strong survival advantage in such environments.

The finding adds to a small body of evidence on how humans adapt to chemical hazards, an area where little is known compared with adaptation to altitude or climate. It does not mean the residents are immune. The work describes a genetic tendency that shifts how arsenic is processed, and the clean-water system installed in 2012 addressed the exposure itself.

Around Prague, based on reporting by ScienceAlert