For thousands of years, humans living high in the Argentine Andes have relied on drinking water, which exposes most to fatal diseases.
There, arsenic naturally occurring in the volcanic bedrock seeped into groundwater, contaminating local water supplies with toxic metalloid levels that pose serious health risks to most people.
But for one group in northern Argentina, natural selection may have provided an unusual genetic advantage.
People in the Argentine Andes carry a genetic variant that may help them metabolize arsenic more safely, according to DNA analysis of people in western South America.
“Adaptation drives genomic change; however, evidence for human-specific adaptations remains limited,” a team led by evolutionary biologists Carina Schlebusch and Lucie Gattepaille of Uppsala University wrote in a 2015 paper.
“Our data show that adaptation to the environmental stressor arsenic may lead to an increase in the frequency of protective variants of AS3MT, providing the first evidence of human adaptation to toxic chemicals.”
Given enough time and gentle enough exposure to danger, life has demonstrated a remarkable ability to adapt to a wide range of wild conditions—from extreme heat, to complete lack of oxygen, to dangerous radiation levels.
However, little is known about how humans adapt to toxic chemicals in the environment. Arsenic is highly toxic and has been linked to cancer, skin lesions, birth defects and premature death. It is also widespread, occurring naturally in high levels in groundwater in many parts of the world.
The current recommended limit for arsenic in drinking water by the World Health Organization is 10 micrograms per liter.
Before a filtration system was installed in 2012, arsenic levels in drinking water in San Antonio de los Cobres, a remote high-altitude town in Argentina’s Atacama plateau, were about 200 micrograms per liter, about 20 times the recommended limit.
However, the area has been inhabited for thousands of years—at least 7,000 years, and perhaps as long as 11,000 years.
This apparent ability to escape dangerously high arsenic levels has puzzled scientists for decades. In 1995, scientists noted that women from the Argentine Andes had a “unique” ability to metabolize arsenic, as evidenced by metabolites in their urine.
Groundwater in the Cabo Atacama plateau in northern Argentina naturally contains high concentrations of arsenic. (jarcosa/iStock/Getty Images Plus)
When arsenic enters the body, enzymes convert it into several chemical forms. One of the intermediate forms, called monomethylated arsenic (MMA), is particularly toxic. The later form, dimethylarsenic (DMA), is more easily excreted by the body in the urine.
People in San Antonio de los Cobres tended to produce less toxic intermediates and more easily excreted forms, suggesting that their bodies were unusually efficient at processing arsenic.
Out of curiosity, Schlebusch, Gatpel and their colleagues wanted to solve this conundrum at the genetic level.
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The team used cheek swabs to collect DNA from 124 women in San Antonio de los Cobres, whose urine samples showed the same arsenic metabolite signature as in the 1995 study. They then analyzed millions of genetic markers in the genome.
To determine whether the genetic variant was unique to the Argentinian population, the researchers compared their results with publicly available genome data from Peru and Colombia from the international 1000 Genomes Project.
Previous research suggested that an enzyme called arsenic (+3 oxidation state) methyltransferase (AS3MT) may play a key role in arsenic metabolism, so that’s where the researchers focused their work.
They found that a group of genetic variants near the AS3MT gene strongly affects how the body processes arsenic. These variants are more common among the people of San Antonio de los Cobres than among genetically similar Peruvian and Colombian populations.
These variants appear to enable the body to more efficiently convert arsenic into a form that can be safely excreted in urine, thereby reducing the accumulation of the most toxic intermediate compounds—a result that is entirely consistent with earlier studies of arsenic metabolites in urine.
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While arsenic contamination is common around the world, few communities live with such high levels for long periods of time.
In San Antonio de los Cobres, people have lived with arsenic in groundwater for thousands of years, long enough for natural selection to favor traits that reduce arsenic’s toxic effects.
Later research showed that similar genetic signals may also be present in other Andean populations exposed to arsenic over generations, supporting the finding that long-term arsenic exposure can increase genetic tolerance and suggesting that such adaptations may be more widespread across the region.
“Given the severe adverse health effects of arsenic in children and adults, individuals carrying arsenic-tolerant haplotypes… may have a very strong selective advantage in high-arsenic environments,” the researchers wrote.
The study was published in Molecular Biology and Evolution.
