What do human spit, baker’s yeast, and fly sex have in common? Together, they illustrate a way in which new kinds of genes evolve.

Scientists published a paper in Nature Genetics Sunday in which they studied an enzyme called amylase that’s produced in saliva and breaks down starch. Human amylase genes share a common ancestry with the amylase gene found in our close relative, the chimpanzee. But they are different in some important ways. Instead of one amylase gene, we have several. Human amylase genes range from 2 to 15 copies, averaging three times as many as chimpanzees. But our extra copies not sprinkled randomly across the world’s population. European Americans and tuber-eating African hunter-gatherers known as the Hadza have