Early cultures separated into tribes (something many countries still do) in order to keep themselves safe. But they needed to mix with other tribes as well–in order to spread their DNA around, as well as to make friends.
In the March 17th edition of the New York Times, Jeffrey P. Kahn writes: "These lifesaving social instincts didn’t’ readily lend themselves to exploration, artistic expression, romance, inventiveness and experimentation–the human drives that make for a vibrant civilization."
For that, Keller says, "We needed beer." It’s thought that beer brewing started 10,000 years ago.