What is the conversation mainly about?

M: Well, you seemed to be having fun watching the movie.
W: Yeah, I think it kept me laughing from the start.
M: (4)Whenever I watch comedy, I always like to know why people like to laugh. I mean, why does it feel so good to laugh?
W: (5)I heard from my biology professor that even after centuries of scientific research, no one knows for sure why human beings and just a few primates laugh.
M: (6)I read somewhere that Charles Darwin thought that laughter, which begins with small babies, was like an evolutionary "reward" to the mother and father. Baby’s laughter sounded and felt so different from crying, he believed, that even prehistoric parents must have interpreted it as a sign of well-being, kind of like the purring of a kitten. The parents enjoyed the laughter, which encouraged them to continue caring for the child.
W: Researchers have found it has a positive effect on many patients and it produces certain hormones that actually switch on the body’s immune system and actually help fight off diseases.
M: I’ve heard that apes also like laughing.
W: I heard that too. Apes and a few other primates laugh, but no other animals do.
M: That’s interesting.