The evolution of skin pigmentation is a classic example of how local environments can pressure human genes. People with more melanin have darker skin tones and survive better—at least without the modern day application of sunscreen—in sunny environments. On the level of DNA, this means that versions of genes associated with more melanin production were selected for in people living in sunny climates.
Unfortunately, it seems the same genetic adaptations that protected our ancestors from pathogens may now have undesirable consequences in modern societies. Twenty-three of the genes influenced by pathogen-driven selection are involved in the immune system and implicated in autoimmune diseases such as celiac disease, type 1 diabetes and multiple sclerosis. The researchers suggest that versions of SNPs associated with increased risk for autoimmunity may have become relatively common in some populations because they used to protect our ancestors from infection.This study offers new evidence for the hygiene hypothesis, a currently unproven theory suggesting that ramped up immune systems originally evolved to protect people from infectious organisms. As societies devised ways to keep pathogens at bay, however, their immune systems failed to adjust and they began to suffer from immune diseases characterized by too much inflammation.