The highly toxic arsenic-contaminated groundwater in the Andes Mountains could kill anyone, but an indigenous community has lived peacefully for millennia. This extraordinary survival secret has just been decoded by science through an extremely unique genetic mutation.

In the middle of the desolate Puna de Atacama plateau in the Andes mountains of Argentina, the town of San Antonio de los Cobres lies precariously at an altitude of 3,775 meters above sea level. This place contains a biological paradox that has shocked the global medical community for decades.

Due to geological characteristics, arsenic minerals (often called arsenic) from volcanic rock layers continuously seep into groundwater. Until a modern water filtration station was installed in 2012, local residents consumed water with arsenic concentrations of up to 200 micrograms per liter. This number is 20 times higher than the safety limit of 10 micrograms per liter recommended by the World Health Organization (WHO).

For ordinary people, absorbing arsenic at high concentrations for a long time is a death sentence, leading to a series of incurable diseases such as cancer, serious skin damage, pregnancy complications and early death. However, archaeological evidence confirms that humans have continuously settled in this land for at least 7,000 years, possibly even up to 11,000 years. They drank that poisonous water for many generations without being destroyed as usual.

The beginning of the journey to decode the mystery took place in 1995, when experts recognized that women in the Andes region possessed the strange ability to transform arsenic. Normally, when arsenic enters the body, the enzyme system will transform it into different chemical states.

Among them, the intermediate metabolite called monomethylated arsenic (MMA) has extremely dangerous toxicity. In the next stage, this compound is transformed into dimethylated arsenic (DMA) - a less toxic form and easily excreted through urine. Clinical analysis shows that the bodies of San Antonio de los Cobres people produce very little MMA toxins, while optimizing the production of DMA to quickly eliminate toxins.

In order to find answers at the molecular biology level, a team of evolutionary ecology experts led by Carina Schlebusch and Lucie Gattepaille of Uppsala University conducted a large-scale survey. The results of the work were published in the scientific journal Molecular Biology and Evolution.

Researchers collected and analyzed DNA from blood and cheek mucosa samples from 124 women in San Antonio de los Cobres, and compared millions of genetic markers with data from similar populations in Peru and Colombia from the international 1000 Genomes Project.

The focus of the search is on the AS3MT gene - responsible for encoding the enzyme arsenic methyltransferase, a key link in the toxic degradation cycle. The authors discovered a cluster of genetic variations located close to the AS3MT gene that appeared at a high density in residents of an Argentine highland town, completely surpassing that of neighboring communities. This is the first real evidence that humans can change their genes to adapt to a toxic chemical in their environment.

This evolutionary mechanism does not stop in South America. In September 2025, another groundbreaking study by the University of Chicago on 6,500 arsenic-exposed adults in Bangladesh discovered two more gene clusters, FMO and GSTO. Together with AS3MT, these gene clusters work together to help the body improve the efficiency of arsenic purification and prevent the accumulation of dangerous intermediate compounds.

According to scientists, thousands of years of continuous exposure to contaminated groundwater has created an extremely fierce natural selection pressure. In that harsh environment, lucky individuals possessing the arsenic-tolerant variant held a remarkable survival advantage to maintain the species. This genetic imprint not only opens up new perspectives on the human body's infinite adaptive capacity, but also proves that life always finds the most miraculous way to overcome dangers.