Saturday, February 25, 2017

Desert Dwellers Evolve to Drink Poison

Quebrada Camarones
Desert people evolve to drink water poisoned with deadly arsenic
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.
OK, bad journalism warning.  1 microgram per litre (liter for us 'Muricans) is a perfectly acceptable level for arsenic in drinking water. The US upper limit is 10. I'm quite sure the scientists gave them the right number (probably as 1 mg/L or 1000 micrograms/liter) and the journalist mistranslated it for the article. Seawater, which you wouldn't want to drink much of, because of toxic levels of perfectly normal salts, contains about 1 microgram per liter
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.
I was aware of this because I did a lot of arsenic research back in the day. For example, some phytoplankton have a problem with arsenic when phosphorus is low (phosphate and arsenate are chemical analogues), and have evolved efficient systems for methylating and excreting the As as MMA and 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,
Just another example of how humans are constantly evolving to adapt to their environment. Even such traits we take for granted as blue eyes, blond hair and fair skin, are relatively recent additions to the human tool kit.

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