...consult Steven Johnson's marvelous 2006 book "The Ghost Map: The Story of London's Most Terrifying Epidemic -- and How It Changed Science, Cities, and the Modern World." ....
"The search for unpolluted drinking water is as old as civilization itself. As soon as there were mass human settlements, waterborne diseases like dysentery became a crucial population bottleneck. For much of human history, the solution to this chronic public-health issue was not purifying the water supply. The solution was to drink alcohol."
...."Dying of cirrhosis of the liver in your forties was better than dying of dysentery in your twenties." Besides, alcohol, although it is a poison, and an addictive one, became, especially in beer, a driver of a species-strengthening selection process.
Johnson notes that historians interested in genetics believe that the roughly simultaneous emergence of urban living and the manufacturing of alcohol set the stage for a survival-of-the-fittest sorting-out among the people who abandoned the hunter-gatherer lifestyle and, literally and figuratively speaking, went to town.
To avoid dangerous water, people had to drink large quantities of, say, beer. But to digest that beer, individuals needed a genetic advantage that not everyone had -- what Johnson describes as the body's ability to respond to the intake of alcohol by increasing the production of particular enzymes called alcohol dehydrogenases. This ability is controlled by certain genes on chromosome four in human DNA, genes not evenly distributed to everyone. Those who lacked this trait could not, as the saying is, "hold their liquor." So, many died early and childless, either of alcohol's toxicity or from waterborne diseases.
The gene pools of human settlements became progressively dominated by the survivors -- by those genetically disposed to, well, drink beer. "Most of the world's population today," Johnson writes, "is made up of descendants of those early beer drinkers, and we have largely inherited their genetic tolerance for alcohol."