The root causes of complex diseases such as type-2 diabetes and obesity have been difficult to identify because the diseases are, well, complex. They occur at the dicey biological intersection of genes and environment, and, because they arose in our relatively recent past, it’s not easy to simply compare DNA sequences from “then” and “now” to pinpoint likely genetic culprits. Now researchers have identified genetic variations in a hormone involved in the secretion of insulin — a molecule that regulates blood sugar levels — that occur more frequently in some human populations than others.