This was seriously not a joke:

Ha-Ha! Ape study traces evolution of laughter
Jun. 4, 2009 12:34 PM
Associated Press

NEW YORK - When scientists set out to trace the roots of human laughter, some chimps and gorillas were just tickled to help. Literally.

That’s how researchers made a variety of apes and some human babies laugh. After analyzing the sounds, they concluded that people and great apes inherited laughter from a shared ancestor that lived more than 10 million years ago.

Experts praised the work. It gives very strong evidence that ape and human laughter are related through evolution, said Frans de Waal of the Yerkes National Primate Research Center at Emory University in Atlanta.