For thousands of years, humans living high in the Argentine Andes relied on clean water that made most people fatally ill.
There, arsenic naturally present in volcanic rock seeps into groundwater, contaminating the local water supply with levels of the toxic metalloid that would pose serious health risks to most human populations.
But for a group from northern Argentina, natural selection may have provided an unusual genetic advantage, research suggests.
A population in the Argentine Andes carries a genetic variant that likely helps them metabolize arsenic more safely, according to a DNA analysis of people in western South America.
“Adaptation drives genomic changes; however, evidence for specific adaptations in humans remains limited,” researchers wrote in a 2015 study by evolutionary biologists Carina Schlebusch and Lucie Gattepaille of Uppsala University.
“Our data show that adaptation to tolerance to the environmental stressor arsenic likely led to increased frequencies of protective AS3MT variants, providing the first evidence of human adaptation to a toxic chemical.”
Given enough time and gentle enough exposure to danger, life has shown a remarkable ability to adapt to all kinds of wild conditions – from extreme heat to complete lack of oxygen to dangerous levels of radiation.
However, relatively little is known about how human populations adapt to toxic chemicals in their environments. Arsenic is highly toxic and associated with cancer, skin damage, pregnancy complications and premature death.
It is also widespread, naturally occurring at high levels in groundwater in many parts of the world.
The current recommended limit for arsenic in drinking water, set by the World Health Organization, is 10 micrograms per liter.
Until a filtration system was installed in 2012, the isolated town of San Antonio de los Cobres, on Argentina’s Puna de Atacama plateau, had drinking water containing about 200 micrograms of arsenic per liter, about 20 times the recommended limit.
Yet the region has been inhabited for thousands of years – at least 7,000 years, and possibly as long as 11,000 years.
This apparent ability to ignore dangerously high arsenic levels has intrigued scientists for decades.
In 1995, scientists noted that women in the Argentine Andes had a “unique” ability to metabolize arsenic, as evidenced by metabolites in their urine.
When arsenic enters the body, enzymes convert it into several chemical forms.
One of these intermediate forms, called monomethylated arsenic (MMA), is particularly toxic. A newer form, dimethylated arsenic (DMA), is easier to excrete in urine.
People in San Antonio de los Cobres tended to produce less of the toxic intermediate and more of the easily excreted form, suggesting that their bodies were exceptionally efficient at processing arsenic.
Intrigued, Schlebusch, Gattepaille and their colleagues wanted to solve the enigma at the genetic level.
The team analyzed DNA from blood samples or cheek swabs, taken from 124 women in San Antonio de los Cobres, whose urine showed a wide range of levels of arsenic metabolites.
They then looked at millions of genetic markers across the genome.
To determine whether the genetic variant was unique to the Argentine population, the researchers compared their results with publicly available genomic data from Peru and Colombia, taken from the international 1000 Genomes Project.
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Previous research has shown that an enzyme called arsenic methyltransferase (+3 oxidation states) (AS3MT) could play a key role in arsenic metabolism, so that’s where the researchers focused their efforts.
What they discovered was a group of genetic variants near the AS3MT gene that strongly influenced how the body processes arsenic.
These variants were much more common among residents of San Antonio de los Cobres than in genetically similar populations in Peru and Colombia.
It’s not just the Andes.
A study carried out in September 2025 by the University of Chicago, involving more than 6,500 adults exposed to arsenic in Bangladesh, identified two other groups of genes: FMO And OSG – which also determine how efficiently a person metabolizes arsenic, in addition to the AS3MT variant.
The variants appear to make the body more efficient at converting arsenic into forms that can be safely excreted in urine, thereby reducing the accumulation of the more toxic intermediate compounds – a result that fits well with previous studies of arsenic metabolites in urine.
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Although arsenic contamination is common around the world, very few communities have lived with such high levels of exposure for extended periods of time.
In San Antonio de los Cobres, people have lived with arsenic in their groundwater for thousands of years – long enough for natural selection to favor traits that reduce vulnerability to arsenic’s toxic effects.
Subsequent research suggests that similar genetic signals may also appear in other Andean populations exposed to arsenic for generations, confirming findings that long-term exposure can lead to genetic tolerance and suggesting that adaptation may be more widespread in the region.
“Given the serious deleterious health effects of arsenic in children and adults,” the researchers wrote, “individuals carrying the arsenic tolerance haplotype…may have a very strong selective advantage in arsenic-rich environments.”
The research was published in Molecular biology and evolution.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. Although we are proud of our process, we are only human. If you spot an error, please let us know.
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