a great piece of anthropomorphic conjecture. Perhaps the real answer lies in the clues of our best friends. We are similar to cats and dogs. Cats are good ambush hunters and scavengers. Dogs are pack hunters and also good scavengers. We combined all these skills to (beat) our rivals; dogs, cats & hyenas. We adapted as part of our environment. Our bipedal nature is efficient and it allowed us to range far and wide in large territories. We learned what was where and when in relation to food sources. Perhaps our masterstroke above all ? omnivorous ability. We could and still can pretty much eat anything. Our dog and cat rivals on the other hand can't and couldn't exist or forage for all the other things we eat. And really to have that endurance we needed to store a lot of starch rich tuber in our muscles and in extra fuel tanks as widely distributed fat. Carbohydrates are maybe one of the keys to our survival, evolution. Interestingly, I have been living on a severely reduced carbohydrate intake (under doctors orders) for about 9 months and my endurance is very low at the moment. I have also eaten up 30lbs in bodyfat reserves !