Andean Community's Genetic Variant May Help Them Safely Process Arsenic in Drinking Water

Until 2012, the tap water in San Antonio de los Cobres, a remote town on the high Puna de Atacama plateau in northern Argentina, carried about 200 micrograms of arsenic per liter. The World Health Organization recommends a limit of 10. That put the town's water at roughly 20 times the guideline, and people had been drinking it for a very long time.

The arsenic is natural. It leaches out of volcanic bedrock into the groundwater, and the toxic metalloid has long been tied to cancer, skin lesions, birth defects, and early death. It is also widespread, present at high levels in the groundwater of many regions around the world. What sets this corner of the Andes apart is the length of the exposure. People have lived in the region for at least 7,000 years, and possibly as long as 11,000.

That history puzzled scientists for decades. In 1995, researchers reported that women in the Argentinian Andes had what they called a unique ability to metabolize arsenic, visible in the metabolites found in their urine. When arsenic enters the body, enzymes convert it through several chemical forms. One intermediate, monomethylated arsenic (MMA), is particularly toxic. A later form, dimethylated arsenic (DMA), is easier to excrete in urine. People in San Antonio de los Cobres tended to produce less MMA and more DMA, which suggested their bodies handled arsenic unusually efficiently.

A team led by evolutionary biologists Carina Schlebusch and Lucie Gattepaille of Uppsala University set out to find the genetic explanation. They collected cheek-swab DNA from 124 women in the town, all of whom had the same urinary arsenic profile as in the 1995 work, and analyzed millions of genetic markers across the genome. To test whether the pattern was unique to this population, they compared the results with public genome data from Peru and Colombia, drawn from the international 1000 Genomes Project.

They concentrated on a gene for the enzyme arsenic (+3 oxidation state) methyltransferase, known as AS3MT, which earlier research had flagged as important to arsenic metabolism. Near that gene they found a cluster of variants with a strong influence on how the body processes the metal. These variants were far more common in San Antonio de los Cobres than in genetically similar populations in Peru and Colombia. They appear to help the body convert arsenic into forms that can be excreted in urine, reducing the buildup of the most toxic intermediates, which fits the earlier urine studies.

The researchers published their work in Molecular Biology and Evolution. They wrote that adaptation drives genomic changes, but that evidence of specific adaptations in humans remains limited. Their data, they said, show that adaptation to arsenic has likely raised the frequency of protective AS3MT variants, which they described as the first evidence of human adaptation to a toxic chemical. Given arsenic's severe health effects in children and adults, they added, people carrying the tolerance haplotype could have a very strong selective advantage in high-arsenic environments.

Later research suggests similar genetic signals may appear in other Andean populations exposed to arsenic for generations. That would support the idea that long-term exposure can drive genetic tolerance, and it hints the adaptation may be more widespread across the region. A filtration system installed in 2012 has since removed the pressure from the town's water, but the genetic record of those millennia remains. Whether this holds up as a general case of human adaptation to a toxin is the question further studies in the Andes will have to answer.