Exactly when humans first started to work with yeast remains an open question. The first unambiguous evidence dates from around nine thousand years ago in China, but microscopic starch grains have been unearthed on stone tools in Kenya that date from a hundred thousand years ago. The shape of the starch grains suggests that the tools had been used to process the African wine palm, Hyphaene petersiana, which is still used to make liquor. Given that any sugary liquid left for longer than a day will start to ferment by itself, it is probable that humans have been brewing for far longer.
