In relation to what dannye mentions, babies laugh long before they can talk. A study at the University of Maryland has found that laughter may actually be a remnant of the earliest primate communication and not related to speech at all. "Laughter is a tool to study vocal evolution", as chimpanzees laugh too, though in a manner quite different from the universal human "ha".1 Their predominantly four-legged locomotion prevents the control over their breath which is necessary for human-sounding laughter as well as vocal speech.

For further reading, see "A serious article about laughter" by Sara Abdulla in Nature Science Update:

1Robert R. Provine, University of Maryland, Baltimore County. p. 30, "Speaking of Laughter". National Geographic. May 2001, Vol. 1999, No. 5.