The theory that I was taught (read my signature) is that we evolved to walk on two legs when we moved from heavily forested areas to open plains. We are more suited to walking long distances and standing for long periods (check out your knees locking out when you stand up) this allowed us to see predators at a greater distance. If this is true then when the previous ice age began the forests would have receded meaning that there would have been fewer primary carnivores for the two legged secondary carnivores to consume, the numbers of their food source diminish so do they, and then die out. And up we pop. this may all be rubbish, it's not really my subject.
About the Neanderthals though, is there any evidence that they tried farming. I thought they were only hunter/gatherers until they died out (either though competition or incorporation with us).
