In San Antonio de los Cobres the water arrived from the springs and underground with the most reassuring appearance in the world. Clear, cold, necessary. All around, the dry landscape of the Argentine Puna, a plateau more than 3,700 meters high, volcanic earth, thin air, low houses, mountains still in the background. However, arsenic also passed through that water for generations. Invisible, natural, released by the rocks in the underground path, present enough to transform a daily gesture into a continuous exposure.
The World Health Organization indicates a guideline value of 10 micrograms per liter for arsenic in drinking water. In San Antonio de los Cobres, until a filtration system was installed in 2012, the water contained about 200 micrograms per liter, twenty times that limit. Inorganic arsenic is linked to cancers, skin lesions, cardiovascular disease, diabetes, respiratory problems, developmental damage and pregnancy complications. The truly effective measure always remains the same: safe water. The rest comes later, if it comes.
Yet in this area of the Argentine Andes the indigenous communities have lived for thousands of years, they had children, they built countries, habits, lineages. Right there a research group found a rare signal: some genetic variants, concentrated around the AS3MT gene, seem to make the way in which the body transforms arsenic into forms that can be eliminated in urine more efficient. The study, published on Molecular Biology and Evolutionis considered one of the clearest evidence of human adaptation to a toxic substance present in the environment.
The body that learns from poison
Arsenic has a reputation as an industrial poison or an old crime novel, but in many areas of the planet it is already found in groundwater. Argentina, Bangladesh, Chile, China, India, Mexico, Pakistan, United States, Vietnam: the list of countries involved is long, and concerns millions of people. The WHO estimates that around 140 million people in at least 70 countries have drunk water with arsenic above the guideline value of 10 micrograms per litre.
In the body, inorganic arsenic is modified through a series of chemical steps. Among the forms produced there is MMA, monomethylarsonic acid, associated with greater toxicity. Then there is DMA, dimethylarsinic acid, which is excreted more easily in urine. The difference, therefore, also lies in the individual ability to transform a greater portion of arsenic into the more disposable form. People in the Andean communities studied showed precisely this pattern: low share of MMA, high share of DMA. Translated without a coat: their organism seemed quicker to push out the poison.
To test this, researchers analyzed 124 women exposed to arsenic in San Antonio de los Cobres. They measured arsenic metabolites in urine and scanned over a million filtered genetic markers, after much more extensive initial genotyping. The clearest signal appeared on chromosome 10, near AS3MT, a gene already known for its role in arsenic methylation.
The interesting data comes from the comparison. The Argentine Andean population was compared with genetically similar groups in Peru and Colombia, areas considered less historically exposed to arsenic. In San Antonio de los Cobres, the region around AS3MT showed stronger genetic differentiation and the sign of a so-called selective sweep: when a useful variant increases rapidly in the population, it also brings with it nearby pieces of DNA, leaving a recognizable signature. That signal appeared in the Argentine group, while it was absent in the Peruvian and Colombian comparison groups.
Partial protection
The protective variant reconstructed by the researchers coincided with 58.4% of the haplotypes analyzed in the population of San Antonio de los Cobres. In Peruvians the value dropped to 29.1%, while in a Vietnamese population used as a comparison it reached 26.8%. This suggests an important detail: natural selection may have worked on genetic variability already present in the ancestors of those communities, without waiting for the appearance of a brand new mutation.
The story then expands to Bolivia. A subsequent study on the indigenous Aymara-Quechua and Uru populations of the Bolivian Andes found signals of positive selection near the same AS3MT gene and very high frequencies of variants associated with more efficient arsenic metabolism. In the communities around Lake Poopó, arsenic concentrations in the water varied widely: in Aymara-Quechua villages the median was 130 micrograms per liter, with values up to 571, while in Uru villages the median was 46 micrograms per liter. There, too, a consistent biological signature appeared: less MMA and more DMA in the urine, especially among the Uru women.
This, however, must be kept with your feet firmly planted on the ground. Genetic adaptation means relative advantage, not immunity. Drinking contaminated water remains dangerous. AS3MT variants can reduce some of the damage, make elimination more efficient, shift the chemical balance towards less problematic forms. Arsenic continues to affect skin, lungs, liver, blood vessels, pregnancy, infant development and oncological risk. A biological defense built over thousands of years lessens the blow. Water reclamation avoids this.
When the environment enters the genes
The Andean story is part of a broader story: human beings change together with the places they inhabit. In Tibet, for example, some variants linked to EPAS1 help people live at altitude, where oxygen is scarce. In many people, the high mountains push the body to produce more red blood cells, resulting in thicker blood and greater cardiovascular risks. In Tibetans this response is more controlled, a valuable form of adaptation above 4,000 metres.
In Greenland, Inuit showed genetic signals linked to the traditional diet rich in marine fats, especially in the FADS gene cluster, involved in fatty acid metabolism. In north-western Kenya, however, recent research on the Turkana has linked some genetic variants to the ability to live in extremely hot and dry environments, with a diet based mainly on milk, meat and blood from livestock. In that case, adaptation to water scarcity and a pastoral diet can become fragile when the living environment changes: city, different diet, sedentary lifestyle, new metabolic risks.
Arsenic in the Andes adds a different, rougher piece. Here the environment to be endured was in the form of a toxin in the cup, in the pot, in the bodies of mothers and children. Natural selection favored those who could transform that poison with a little more efficiency. A small difference, repeated across generations, was probably enough: a few more children, a few pregnancies that reached full term better, a few years gained, enough to increase the frequency of protective variants.
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