... the rapid spread of a gene which lets us reap the benefits of milk shows evolution in action. ... In order to digest milk, adult humans need to have a gene which produces an enzyme called lactase to break down lactose, one of the main sugars it contains. Without it, a drink of milk proves an uncomfortable experience, causing bloating, stomach cramps and diarrhoea. Today, more than 90% of people of northern European origin have the gene.