It is a saying of the Sumerians, who lived in what is now southern Iraq and goes: "Something which has never occurred since time immemorial; a young woman did not fart in her husband's lap."
A 1600 BC gag about a pharaoh, said to be King Snofru, comes second -- "How do you entertain a bored pharaoh? You sail a boatload of young women dressed only in fishing nets down the Nile and urge the pharaoh to go catch a fish."
The oldest British joke dates back to the 10th Century and reveals the bawdy face of the Anglo-Saxons -- "What hangs at a man's thigh and wants to poke the hole that it's often poked before? Answer: A key."