Friendship and connection in the primate world is quite similar to human behavior, as Dr. Frans de Waal shows. Chimpanzees and bonobos show feelings of empathy, compassion, friendship and even altruistic behavior, but only for their own clan members. They don't care, and might even kill, outsiders vying for resources. Sound familiar?

"In the animal kingdom, we often get a sense of what, in full development, makes us fully human." — Rev. Dr. James A. Forbes, Jr., senior minister of the Riverside Church in New York

A segment from "The Mystery of Love," airing Dec. 13 on PBS (check local listings) — www.themysteryoflove.org