Symbol “As” on the Periodic Table of Elements, arsenic, is poisonous to humans and animals. An ingredient in rat poison and chemical weapons, arsenic causes vomiting, digestive upset, shock, and death. Chronic exposure to even low levels of arsenic brings about liver and kidney damage, skin lesions, nerve damage, and cancer.

However, one group of humans seems to be immune to arsenic’s deleterious effects. Villagers in San Antonio de los Cobres in northwest Argentina have been drinking water with arsenic levels as much as 20 times higher than the World Health Organization’s safe limit. They live in the Andes Mountains, 12,500 feet above sea level, where arsenic seeps through volcanic bedrock into the water that they drink. These villagers and their ancestors, the indigenous Atacameño people, have been drinking this poisonous water for 11,000 years, without any apparent harm to their health.

Consuming miniscule amounts of arsenic is actually not uncommon, even in the United States. Naturally-occurring arsenic in water can result from erosion of sediment and mineral ores, or from industrial waste runoff. Plants, rocks, and soil can also contain arsenic. Rice, for example, has small amounts of arsenic that it absorbs from soil. But in the U.S., the Environmental Protection Agency (EPA) requires water treatment plants to purify potable water so that the highest allowable level of arsenic—the maximum contaminant level (MCL)— is 0.01 mg/L (milligrams per liter).

So how have these Argentinian villagers been able to survive arsenic exposure at many magnitudes higher than the MCL? Led by geneticist Dr. Karin Broberg, a group of Swedish scientists collected urine and blood samples to study the genomes of 124 females from the village, as well as from related populations in other villages around the Andes. They discovered that the women's DNA contained variants in AS3MT, the main gene responsible for arsenic metabolism in humans. When people drink arsenic-heavy water, their bodies break down the chemical (into monomethylarsonic acid and dimethylarsinic acid) very quickly. It’s known that MMA is more toxic than DMA, and DMA is also readily removed from the body in urine. But the villagers show “uniquely low urinary excretion of MMA” (PDF), meaning more is being metabolized into the less toxic DMA than is true for other populations. Although we don’t know exactly how the villagers' variants in metabolism protect them from arsenic, this increased DMA production allows them to consume much higher quantities of arsenic than other related groups

Evolution at Work

Naturally, over the course of thousands of years, the Andean villagers have adapted to their environment. People with the AS3MT gene variant survived and reproduced, while people without it died from arsenic exposure. Almost 70% of 6,000 villagers have the AS3MT variant that allows their bodies to quickly break down and excrete arsenic. Interestingly, some people in other parts of the world also have the AS3MT gene mutations, but a much higher percentage of the Argentinian villagers have it than other populations (such as Native Americans and Asians) do.

In March 2015, the Swedish scientists published their study, theorizing that natural selection has allowed the villagers to evolve the ability to effectively metabolize arsenic. Although this study only looked at 124 people, and we don’t know what the long-term negative health effects of the arsenic on people with the AS3MT variant might be, this discovery raises questions about how DNA can protect us against chemicals. Broberg and her team’s study establishes the first instance of humans successfully adapting to a toxic chemical.