An Unlikely Driver of Evolution: Arsenic

Around 11,000 years ago, humans first set foot in the driest place on Earth.

The Atacama Desert straddles the Andes Mountains, reaching into parts of Chile, Peru, Bolivia and Argentina. Little rain falls on the desert — some spots haven’t received a single drop in recorded history.

But the people who arrived at the Atacama managed to turn it into a home. Some Atacameños, as they are known today, fished the Pacific. Others hunted game and herded livestock in the highlands. They mummified their dead, decorating them with ceremonial wigs before leaving them in the mountains.

Those mummies reveal a hidden threat in the Atacama. When scientists analyzed the hair in 7,000-year-old mummy wigs, they discovered high levels of arsenic. Through their lives, the Atacameños were gradually poisoned.

Arsenic can poison people today through exposure to pesticides and pollution. But arsenic is also naturally present in the water and soil in some parts of the world. The Atacama Desert, sitting on top of arsenic-rich volcanic rock, is one of them. The concentration of arsenic in Atacama drinking water can be 20 times higher than the level considered safe for human consumption.

Now a team of scientists has discovered that the arsenic of the Atacama Desert didn’t just make people sick. It also spurred their evolution.

In a new study in the journal Molecular Biology and Evolution, researchers report that over the years the Atacameños became more resistant to arsenic, thanks to natural selection. It is the first documented case of natural selection in humans for a defense against an environmental poison.

Jonathan K. Pritchard, a geneticist at Stanford University who was not involved in the study, called the results “convincing” and a new addition to “a very small number of known human selection signals.”

The liver defends the body against arsenic by tacking on extra carbon and hydrogen atoms to the element. Those extra atoms make arsenic less toxic and easier to draw out of the bloodstream in the kidneys, so that it can be flushed out of the body with urine.

In the late 1990s, researchers discovered that most Atacameños detoxify arsenic at an unusually high rate. Recently a group of researchers in Sweden went searching for the genes that make the Atacameños so unusual.

The scientists collected urine and blood from women in a village in Argentina called San Antonio de los Cobres. Levels of arsenic in their urine were used to determine how well each woman’s body detoxified the poison.

The scientists also sequenced over a million short segments of DNA in the women’s genomes. They looked for genetic variants shared by the women able to rid themselves of arsenic most efficiently.

These women all shared a distinctive stretch of DNA on chromosome 10, the scientists found. That stretch contains a gene called AS3MT, which encodes a liver enzyme that helps detoxify poisons.

“It’s a confirmation that this gene is really, really important for arsenic excretion,” said Mattias Jakobsson, a professor of genetics at Uppsala University and a co-author of the new study.

Dr. Jakobsson and his colleagues then compared the DNA in people from San Antonio de los Cobres with DNA from people in Peru and Colombia who don’t have to drink arsenic-laced water. For the most part, their DNA was nearly identical. There was only one major difference: the stretch of DNA that contains the AS3MT gene. About 70 percent of people in San Antonio de los Cobres have the variant that lets them resist arsenic.

When people first arrived in the Atacama Desert, the scientists concluded, a few of them carried this mutation. Because there was no way to avoid ingesting arsenic, the mutation immediately became important to their survival.

“If you settle in this area and there is one stream, there aren’t many options for getting water,” said Karin Broberg, a geneticist at the Karolinska Institute and a co-author of the study.

The Atacameños began to suffer from chronic arsenic poisoning, which can lead to cancer, skin lesions, and a weakening of the immune system in babies. The people who carried the protective mutation were able to detoxify the arsenic faster, perhaps by making extra copies of the AS3MT enzyme.

“It’s not a magic cure,” said Dr. Jakobsson. “If you have the protective variant, you’re not going to have a perfect life drinking a lot of arsenic. But the effects are probably smaller.”

That difference meant that people with the mutation survived to have more children than people who lacked it. Over thousands of years, natural selection made it more common.

Scientists have documented several cases in which humans have experienced strong natural selection over the past thousands of years. In some parts of Africa, some individuals evolved resistance to malaria. In northwestern Europe and elsewhere, natural selection favored genes that let adults digest milk. In Tibet, it favored genes for survival at high altitudes.

The new study on the Atacameños, by contrast, shows that toxic chemicals can also drive human evolution.

Understanding how it happened may help guide public health measures to reduce the suffering caused by arsenic poisoning, which threatens an estimated 200 million people worldwide. And it can also help scientists understand how we detoxify chemicals like arsenic, a process that is still fairly mysterious.

“If you find a signal of natural selection, then you know this has been a huge issue for human survival in the past,” Dr. Jakobsson said.