no known human cultures that engage in polyphasic sleep (not counting biphasic sleep).

Look, our sun forces us into a monophasic pattern because of the day/night cycle that occurs everywhere around the earth but our body's don't naturally fall into it. We sleep at night because our brain is wired to sleep when its dark and that's an evolved mechanism but the behavioral pattern that polyphasic sleep requires isn't evolved into our system its just a natural response to the natural light patterns of our world . In parts of the world where light comes less often sleeping patterns are different than near the equator as evidenced by biphasic sleepers around the world who follow the Siesta pattern naturally.

It was dangerous, to try and guess at evolutionary psychology if you weren't a professional evolutionary psychologist; but when Harry had read about the Milgram experiment, the thought had occurred to him that situations like this had probably arisen many times in the ancestral environment, and that most potential ancestors who'd tried to disobey Authority were dead. Or that they had, at least, done less well for themselves than the obedient. People thought themselves good and moral, but when push came to shove, some switch flipped in their brain, and it was suddenly a lot harder to heroically defy Authority than they thought. Even if you could do it, it wouldn't be easy, it wouldn't be some effortless display of heroism. You would tremble, your voice would break, you would be afraid; would you be able to defy Authority even then?

Harry blinked, then; because his brain had just made the connection between Milgram's experiment and what Hermione had done on her first day of Defense class, she'd refused to shoot a fellow student, even when Authority had told her that she must, she had trembled and been afraid but she had still refused. Harry had seen that happen right in front of his own eyes and he still hadn't made the connection until now...

All i'm saying is that people attribute evolutionary reasons to things that have many separate causes and are unproven because they think they understand it. That's what both these quotes illustrate as far as i know :)

Can anyone confirm that chimps and bonobos are diurnal as well?

I agree with your sentiment, but I find myself constantly reminded by this site that the majority of the world is stupider than we give them credit for.