OK ... so I talked to my brother Christopher, who drew this, about this joke because I didn't get it either.
Here is what he said: "It's a drawing of cavemen, who are chasing a pig around. I'm with you on it not being a good gag. This took me a long time to draw because there are a bunch of figures in it. Like four hours.
The Big Mac Blog: Oh my God. Seriously? Dude, I had no idea these took that long.
Christopher: It doesn't have a punch line but I thought it was a funny drawing.
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