A group of scientists has now suggested that laughing can be detected in mammalian species as simple as rats. This discussion of laughing animals is discussed at Alan Boyle's blog at MSNBC:

How do you graph the evolution of a laugh? Researchers tickled babies and six different kinds of apes, quantified their giggles, and found that the patterns fit a classic evolutionary tree.

Those patterns hint at the ancient origins of human hilarity and suggest that other social species - including apes, dogs and rats - really, truly laugh as well.

Or check out a laughing gorilla here. Why do we laugh? Mostly, we laugh at things that are not funny. See here, for more information on the psychology of laughing.