Genes are never perfectly conserved; they accumulate mutations through generations. The rate of mutations can act as a clock, telling us how far back was the c

Genes are never perfectly conserved; they accumulate mutations through generations. The rate of mutations can act as a clock, telling us how far back was the common origin of two genes that are similar today. When we see modern populations that carry neanderthal genes that are better conserved within the population than some human-specific genes are with other human populations, we can infer that the neanderthal intermixing happened after humans became... human. At large scale, many such inferences can be combined to form a historical map of population mixing and the migrations implied... That's what the research in the article is about.