Actually, there are at least two good clues: 1) 9,000 years ago, the date of first evidence of humans fermenting grapes, or whatever it was... and 2) about 10 million years ago, the time when primates who could digest alcohol split apart from those who couldn't.
ClearCase_guy: "the fact that humans COULD metabolize ADH4 does not in any way provide evidence that we were consuming alcohol, right?
It has been suggested here that developing the capacity for booze didn't mean our ancestors necessarily fell off their wagons... er, I mean trees.
But evolution is very unforgiving of capabilities which don't get used much -- if we don't use it we lose it, eventually.
So the fact that we still have the ability to digest alcohol must mean we needed that ability, at least sometimes.
Thanks!