Pukara de Quitor - a 12th century fortification built by the Atacameno people in Chile's Atacama Desert.

Chile’s Atacama Desert in the northern Quebrada Camarones region is notorious for having the highest levels of arsenic in the Americas. Exceeding 1 microgram per liter, these levels are more than 100 times what the World Health Organization considers “safe limits.”

But to the bewilderment of scientists, human populations have continued to survive in the area for more than 7,000 years.

Researchers from the University of Chile in Santiago analyzed the genetic makeup of approximately 150 people from three regions in Chile – the Camarones, the Azapa Valley, also in northern Chile, and San Juan de la Costa in southern Chile, where arsenic levels are historically much lower than the other two locations.

More specifically, they compared the frequencies of four protective gene variants of the AS2MT gene, which is known to be effective in arsenic metabolization, between individuals in the Camarones and the other two populations. Those who can metabolize arsenic better convert it into a less toxic compound known as dimethylarsinic acid (DMA).

The study found that 68 percent of Camarones people had the protective variants, while 48 percent did in Azapa and just eight percent from San Juan de la Costa had them.

The protective variants are called single nucleotide polymorphisms, which alter a single letter of DNA. The changes are so minimal that the researchers are unable to determine how the mutations affect the enzyme molecule and lessen arsenic’s toxicity in the body.

“The higher frequency of protective variants in both northern Chilean populations indicates a long exposure to naturally arsenic-contaminated water sources. Our data suggest that a high arsenic metabolization capacity has been selected as an adaptive mechanism in these populations in order to survive in an arsenic-laden environment,” wrote the authors, in the study published in the American Journal of Physical Anthropology.

The Atacama Desert is the driest non-polar desert in the world, making natural water sources like rivers and wells that much more crucial to survival.

Arsenic is associated with a long list of negative health effects, such as various types of cancers, diabetes, neurotoxicity and developmental effects. The WHO lists Chile, along with China, India, Bangladesh, the U.S. and several other countries as having inorganic arsenic naturally present at particularly high levels in groundwater.