kottke.org points to a story by Maggie Wittlin about a recent review article by Matthew Gervais and David Sloan Wilson about the evolutionary development of laughter.

The authors begin their evolutionary tale of laughter well before humor came into the mix, arguing that laughter is a more basic function than even language. "Not only does it precede language developmentally...it probably preceded language in terms of evolution," Wilson said. "So, there was a time in our history when we were laughing before we were talking."

When Leta is really amused by something, she laughs "tee-hee-hee," just like in the comic strips. I don't know how she learned to do this.