PEOPLE in a south American desert have evolved to detoxify potentially deadly arsenic that laces their water supply.
For settlers in the Quebrada Camarones region of Chile’s Atacama desert some 7000 years ago, water posed more than a bit of a problem. They were living in the world’s driest non-polar desert, and several of their most readily available water sources, such as rivers and wells, had high levels of arsenic, which can cause a variety of health problems.
The arsenic contamination here exceeds 1 microgram per litre: the highest levels in the Americas, and over 100 times the World Health Organization’s safe limits. There are virtually no alternative water sources, and yet, somehow, people have survived in the area. Could it be that arsenic’s negative effects on human health, such as inducing miscarriages, acted as a natural selection pressure that made this population evolve adaptations to it? A new study suggests this is indeed so.
The body uses an enzyme called AS3MT to incorporate arsenic in two compounds, monomethylarsonic (MMA) acid and dimethylarsinic (DMA) acid. People who metabolise arsenic more efficiently convert more of it into the less toxic, more easily expelled DMA.
Mario Apata of the University of Chile in Santiago and his colleagues looked at variations in the gene coding for AS3MT in nearly 150 people from three regions of the country. They found higher frequencies of the protective variants in people from Camarones: 68 per cent there had them, as opposed to just 48 and 8 per cent of people in the other two. “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 (American Journal of Physical Anthropology, doi.org/bz4s).
The variants that protect the Camarones people are called single nucleotide polymorphisms – changes in a single DNA letter of the genetic code. Anthropologist Lorena Madrigal of the University of South Florida in Tampa says these are such tiny mutations that they aren’t telling us exactly how the changes affect the enzyme molecule and its detoxifying effects.
Previous studies found similar mutations in the AS3MT gene that contribute to improved arsenic metabolisation in Vietnam and Argentina. Sequencing the entire chromosomal region around this gene could reveal more, but there’s still a long way to go before we fully understand the molecular mechanism for how arsenic resistance works.
Though it’s a fascinating example of what appears to be contemporary evolution in humans, it also underscores the water quality problems that many populations face, says Madrigal. And many may not be able to evolve to deal with it.
Another notable example of recent human evolution is lactose tolerance. A mutation which allowed adults to keep producing the enzyme lactase to digest milk emerged around 7000 years ago, alongside dairy farming, and now 35 per cent of adults carry it and can digest milk as a result.
“I would say [the rise in arsenic tolerance] is comparable to the rapid spread of lactose tolerance. Certainly the timescales we are looking at for both cases are comparable,” says Aaron Miller at Northwestern University in Evanston, Illinois.