An isolated group of people living in the Andes have developed a tolerance to arsenic in their water supply. The discovery is not only the first time humans have been found to have adapted to arsenic, but it is also "the first known case of human selection for tolerance to a toxic chemical,” according to a paper published in Molecular Biology and Evolution.
Senior author Karin Broberg of Sweden's Karolinska Institute conducted a study of residents in the northern Argentinean town of San Antonio de los Cobres (SAC). The area is dry and volcanic eruptions have left an arsenic residue that has entered the local water supply. The residents have inadvertently responded by evolving the capacity to methylate arsenic, reducing its toxicity and making it easier to excrete. Broberg found that there was a strong association between the capacity of people in the area to process arsenic in this way and the expression of the AS3MT gene.
AS3MT's role in producing an enzyme that enables arsenic metabolism has been known for a decade. However, this is the first time a population has been found with such high rates of AS3MT expression. Those studied were also particularly likely to have unusual variations in genes located near the AS3MT gene.
In wealthy countries, a tolerance for arsenic is probably only handy if your enemies like Agatha Christie stories, but in much of the world, it is a very different matter. Tens of millions of people—primarily, but far from exclusively, in India and Bangladesh—are slowly dying from arsenic in tube wells. Much work has gone into finding a cost-effective solution, but so far nothing has been widely adopted.
Exposure to arsenic is a relatively new phenomenon, caused by the desire to stop using surface water prone to bacterial pollution. However, in some places the arsenic is closer to the surface and, as the authors note, residents have “lived for thousands of years with drinking water contaminated with arsenic.”
Most of the residents of SAC are Native Americans whose ancestors have been in the region for an estimated 11,000 years. A comparable village in Peru, where arsenic levels are much lower, showed dramatically lower, but also more varied, levels of AS3MT expression. A 7,000 year old mummy found near SAC had concentrations of arsenic in its hair high enough to suggest tolerance to the heavy metal. Broberg estimates people with high AS3MT expression were 0.03-0.05% more likely to have offspring that survived in the area, a small effect but enough to transform the population in this timespan.
The work is important for what it tells us about human evolution and gene expression, but the authors have deferred discussion of the public health implications to future papers. Populations suffering from arsenic now will certainly not be satisfied with the idea that over a few thousand years their descendants will gain immunity to the condition. Arsenic removal may be more expensive or time-consuming than we would like, but options exist that are far cheaper than gene therapy.