7000 years of arsenic resistance
The town of San Antonio de los Cobres in Argentina is isolated on a high mountaintop that has been trying to kill them for the last 7000 years. The volcanic rock has been leaching arsenic into their drinking water well above “safe” levels, and yet the town lives on.
Looking at mummies from the area dating back 400 to 7000 years, high arsenic concentrations have been found in many of the populaces’ ancestors. It seems that over this time, a gene has been understandably growing in popularity that methylates the arsenic so that it can be safely peed out. While it’s unknown if this is enough to protect against all arsenic damage, it does seem to be helping avoid the well-known, immediate effect of death.
Many other populations carry the AS3MT gene, but this isolated population has had time to propagate mutations to create more copies of it, which seem to be expressed most effectively in the liver. So the basic concept for arsenic protection was already there, but in this more extreme environment it was selected for much more heavily.
Source: Arsenic Antidote Hidden In Our Genes by Michaeleen Doucleff, Goats and Soda