The dose makes the poison. However, that dose may not be the same for everyone. A recent study shows that a human population of the Argentinean Andes has genetically adapted to a polluted environment and increased its resistance to arsenic toxicity.
Natural selection is one of the mechanisms driving evolution and is central to the notion of adaptation: individuals that possess characteristics beneficial to their survival and reproduction in a given environment are likely to produce more offspring than other individuals; as they pass on the beneficial traits to their descendants, these characteristics become more and more common in the population.
There are a few well-known examples of recent selection in humans: lactase persistence (which enables adults to digest lactose), resistance to malaria, increase in the number of copies of the amylase gene (which improves the capacity of humans to digest starch-rich diets), or skin pigmentation (which is lighter in regions where UV radiations are less intense). Now, Swedish researchers have uncovered evidence of human adaptation to an arsenic-rich environment in a population of the Argentinean Andes.
Arsenic is metabolized in the body by enzymes that transform it into two slightly different chemical forms, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) – they do so by attaching one or two methyl groups to arsenic, respectively (a methyl group is a chemical group made of one atom of carbon and three atoms of hydrogen). DMA is more readily excreted in the urine and is less toxic than MMA. The efficiency of arsenic metabolism and the proportions of the two metabolites MMA and DMA in the urine vary in different human populations. Certain indigenous populations in the Andes, including the one the researchers studied, are known to metabolize arsenic efficiently and to mostly excrete the less toxic metabolite DMA rather then MMA.
By analyzing genetic variations present in the genome of 124 women living in a village of the Argentinean Andes with high environmental arsenic exposure, the researchers identified a region of the genome that seemed to particularly relate to efficient arsenic metabolism. In particular, they found important genetic differences in a region containing AS3MT, a gene that codes for an enzyme involved in arsenic methylation, when they compared the Argentinean population to a closely related Peruvian population living in a region with lower levels of environmental arsenic. This indicated to the researchers that AS3MT likely played a major role in the adaptation of human populations to high levels of arsenic in the environment. Using other methods to compare the genetic variation between the Argentinean Andes population and Peruvian and Colombian populations, the researchers found further evidence for positive selection of genetic variants associated with efficient arsenic metabolism in the Argentinean Andes population.
Arsenic is naturally present in the bedrock in many places, and in certain regions of the world humans have been exposed to high levels of arsenic in drinking water for thousands of years. The northern Argentinean Andes is one such region. It is unclear when the selective pressure related to arsenic exposure would have started, but it is likely to be some time between about 11,000 years ago (suggested date for a human population settlement in this region) and about 7,000 years ago (estimated age of a mummy found in this region that had high levels of arsenic in hair).
This study is the first to show that human populations have been able to adapt, in a relatively short time (evolutionarily speaking), to a local environmental pollutant they encountered when settling into a new part of the world. Perhaps this should come as good news to a species that has now become so adept at polluting its own environment. Or perhaps not. After all, a relatively short time in evolutionary terms is still a long time compared to the speed at which humans are now able to contaminate their environment with toxic pollutants.
Human Adaptation to Arsenic-Rich Environments. Schlebusch CM, Gattepaille LM, Engström K, Vahter M, Jakobsson M, Broberg K. Mol Biol Evol. 2015 Mar 3. doi: 10.1093/molbev/msv046
Schlebusch CM, Gattepaille LM, Engström K, Vahter M, Jakobsson M, & Broberg K (2015). Human Adaptation to Arsenic-Rich Environments. Molecular biology and evolution PMID: 25739736