"In a community lacking pure-water supplies, the closest thing to "pure" fluid" was alcohol. Whatever the risks were posed by beer (and later wine) in the early days of agrarian settlements were more than offset by alcohol's antibacterial properties. Dying of cirrhosis of the liver in your forties was better than dying of dysentery in your twenties... To digest large quantities of (alcohol), you need to be able to boost production of enzymes called alcohol dehydrogenases, a trait regulated by a set of genes on chromosome four in human DNA. Many early agrarians lacked that trait, and thus were genetically incapable of "holding their liquor". Consequently, many of them died childless at an early age, either from alcohol abuse or from waterborne diseases... Most of the world's population is made up of descendants of those early beer drinkers, and we have largely inherited their genetic tolerance to alcohol."
BTW, I strongly recommend the book. It provides an amazing account of the role of scientific preconceptions, and it tells the story of the map that started Spatial Analysis. Spoiler: it is a myth that John Snow discovered the source of cholera after drawing his famous map. In fact, he draw it to convince the others that water, and not, miasma was responsible for the spreading of the disease.