Surprisingly, only 1% of the Earth’s water is suitable for drinking. However, for some in Chile’s Atacama desert, the options for fresh water are even more limited — and dangerous.

In fact, these people have had to deal with poor water conditions for thousands of years. Water laced not only with dirt and debris but with arsenic, a deadly element that can cause a host of health problems.

Settlers came to Atacama desert more than 7,000 years ago, and even back then attaining clean water was quite the challenge. Since the Atacama is one of the world’s largest non-polar deserts, water had to come from deep river wells.

According to the World Health Organization, the soil surrounding these wells contaminates the water with more than 1 milligram of arsenic per liter of water. This is the highest rate of arsenic throughout the entire Americas and is a whopping 100 times over the WHO’s regulated safe limits for arsenic in drinking water.

But despite this mortal danger, the people who live in this desert have no alternative water resources to depend on and have been forced to drink this water for millenia. So how have they survived? Scientists say the answer is simple: evolution.

In fact, scientists who have studied these Chilean people believe that arsenic’s negative effects on the human body has acted as a form of natural selection that made this population of people withstand the poisonous effects, such as miscarriages and birth defects.

Mario Apata of the University of Chile in Santiago looked for the exact gene that has evolved due to high arsenic levels. He found AS3MT, an enzyme that metabolizes arsenic, and saw that in hundreds of people from this specific region, this enzyme works overtime to metabolize the arsenic from the body. Basically, the enzyme works so fast to remove arsenic from the body that there isn’t even time to feel the negative effects on the body.

Not only this, but people from the Camarones region actually had higher frequencies of this enzyme. AS3MT was found in a full 68% of these desert people, compared to 8% in other areas of the country.

“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,” the researchers conclude, publishing their findings in the American Journal of Physical Anthropology.

Even though this study is promising in that it can pinpoint developments in the evolution of the human genome, some scientists believe that it underscores the severe water quality problems many South Americans face. It’s also a reminder not to take access to fresh water for granted. While experts say that the optimum amount of lawn watering is one inch per watering session, the water is so poisonous in some parts of the continent that plants can die within minutes of being watered. This can cause plenty of crop problems, amounting to food shortages, which is a problem all of its own.

So what’s the next step? Apata and his colleagues are planning on sequencing the entire genome around the AS3MT gene as a way to learn more about the fascinating people who have dealt with this evolution for thousands of years.