Apparently the joke is as follows:
"Something which has never occurred since time immemorial; a young woman did not fart in her husband's lap,"
It's not that funny. I guess the Sumarians were the Germans of their day humour wise. Or maybe I'm being too German here because I don't seem to get it.
I guess a Marsh Arab might get it, or maybe the funny is lost in translation. Or time. Probably time.
Read the article. It talks about the early days of British humour, with a joke from the Anglo-Saxon period, and I especially like the Roman joke.