Lactic acid bacteria
Adult humans originally could not digest milk products. Then, all humans were lactose intolerant (as, for example, cats are after weaning). Lactase persistence is the phenotype where genes prolonging the activity of lactase beyond infancy. Humans are unusual in having lactase persistence. We know it evolved fairly recently (in the last 10,000 years) among some populations. Still, most people worldwide are lactase nonpersistent. They cannot digest lactose products after weaning. The evidence is that lactose persistence grew in human groups who herded cattle. To them it was a great advantage: they could digest milk throughout life.