What makes the world’s first documented bar joke funny? No one knows.

In a tweet that garnered thousands of responses in March, the Twitter account @DepthsOfWiki posted about a 4,000-year-old proverb written on a clay tablet. The line, which experts believe is a joke from the ancient civilization of Sumer, starts with the set-up, “A dog walks into a tavern.” But the punchline has left scholars and online commenters scratching their heads. The joke’s meaning has been lost, and finding it could reveal something unique about early human civilization.

In this episode, the first of two parts, Endless Thread journeys back in time, attempting to deconstruct the origins of humor and explain an unexplainable joke from the forgotten tablets of the past.