Why is this joke funny?

This is a very interesting NPR article about anthropologist and comedian Robert Lynch's take on why humor may have evolved in humans.

"Lynch thinks evolution may have hardwired a sense of humor into our species because laughter serves as a signal. When you and I laugh at the same joke, we signal to each other that we share the same values, the same beliefs. This may be why people all over the world want friends and romantic partners who share their sense of humor."

Read more here: http://www.npr.org/blogs/health/2012/08/06/157592468/an-anthropologist-walks-into-a-bar-and-asks-why-is-this-joke-funny