The spread of cooking — and the beginning of its visible benefits to pre-human physiology — is thought to have started around 2 million years ago, or perhaps a bit later. It would make sense, the researchers argue, if the genetic adaptation that made us efficient carb eaters cropped up after a few generations of cooking: Raw tubers are really hard for mammals to digest into usable glucose, but once we had access to cooked crops, those plants may have become quite important in our evolution — important enough that only humans with extra copies of the salivary amylase gene survived.