Via Scott Adams: Apparently, the Sumerians thought farts were funny:
Academics have compiled a list of the most ancient gags and the oldest, harking back to 1900BC, is a Sumerian proverb from what is now southern Iraq.
"Something which has never occurred since time immemorial; a young woman did not fart in her husband's lap," goes the joke.
Those ancient rubes. We're much more advanced today.