I'm bored, so I'm going to say it's all about the thumbs.
Think about it. Without the thumb we lose our grip on everything. The thumb allowed us to fashion tools more complex than found objects--the right sized rock, the termite stick that is just right. It probably started fairly simply, the chimp with the abnormally long thumbs was better at peeling the termite stick, scored more protein and grew healthier and thereby scored all the chicks.
Imagination simply evolved because we were able to do more with our thumbs--and when I say thumbs I mean the whole physiological mechanism that turned us into the greatest predators on the planet: binocular vision, binocular hearing, hips/ankles/feet that allow for walking upright. Without all those beautiful mechanisms, the big brain is still food for everything else in the Olduvai Gorge. Imagination is not something the scrappy survivor has time for--prey does not have time to dream about a Creator and wonder about tomorrow. Navel gazing and philosophy, creative tool making and manipulating the environment as a means of avocation, that's what makes humans different from the other primates--and it's all thanks to the thumb. The thumb makes leisure time possible.
I think if we want to find ape philosophers, we might look to the ones in zoos where all their needs are met. Someone should ask Koko this question...
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-Amy
without the dark of night we could not see the stars
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