Why would a chimpanzee attack a human?: Scientific AmericanI don't think it's so much that chimps are exceptionally strong for a primate, it's that modern humans are really weak.
... The chimpanzee has strength for a human that is utterly incomprehensible. People watch pro wrestlers on TV and think they are strong. But a pro wrestler would not be able to hold a chimpanzee still if they wanted to. Chimpanzee males have been measured as having five times the arm strength as a human male. Even a young chimpanzee of four or five years, you could not hold it still if you wanted to. Pound-for-pound, their muscles are much stronger...
Why are we such runts? We look pretty big, and we still run pretty well, but in hand-to-hand combat we're hopeless.
My guess it's so that we don't damage each other when we fight. We're not a particularly peaceful primate (though we're becoming far gentler than we were), and we're very expensive to rear to adulthood. Maybe it's better for our hand to hand combat to be relatively harmless; a means to establish dominance rather than do harm.
For harm we have tools, fire, pits and cliffs, and we've killed anything that was ever remotely threatening.
We're the Potemkin primate -- at least so far as muscles go.
The rest of the article is worth a read. Briefly, chimps are not, shall we say, domesticated. Dogs are domesticated. Cats are sort of domesticated. Humans are domesticated. Chimps are not.
Tigers and chimps are playful prior to puberty, but unlike dogs or humans they're not playful as adults. Adult chimps are very aggressive, and, to us, unpredictably vicious.
Stay far away from chimps.
Update 2/20/09: Another explanation would be that chimp muscles take a lot of energy to run and produce a lot of heat. We need to feed and cool our hungry brains, tools mean our main problems are microbes, entropy and each other, so we sacrificed our muscles. I couldn't find any publications on this topic, but the energy balance idea isn't novel.
Update 3/1/2009: Aha! Chimpanzees aren't as strong as we've been thinking! They're stronger than we are, but not fantastically stronger. On the other hand, we're clearly wimpy primates. John Hawks mentions a couple of genes that may have been important in the loss of human strength. He doesn't mention his thoughts on what's going on, perhaps, based on his past practice, because he has relevant research underway. From what Hawks has taught us about evolution, I suppose some of those genes might have been knocked out in the cause of re-purposing them for something related to human cognition.