Now please don't ask me where I got this one from (because I don't remember... maybe someone can verify): I saw on some documentary that the evolution of primate vision can indirectly give thanks to snakes. Primate vision obviously isn't the greatest (as mentioned), but it is unique nonetheless in areas of depth perception. Had we, while swinging in the trees, not passed on the visual capability of spotting dangerous serpents in the trees (you need good depth perception to spot a green mamba or boomslang!), there might have been less of us around to have prospered as a species. Just a thought...