I think that the current hypothesis is that at some point the human brain reached an evolutionary tipping point. Add in human speech, toolmaking & symbolic manipulation (writing) and you have the beginnings of an intergenerational repository of knowledge. So while not all human beings are as bright as Isaac Newton or Albert Einstein, the sum of all human knowledge continues to expand.
Apes have the ability to stand on their hind legs. Doing so free's up the hands for other tasks. Some time ago in West Africa, an extended warm period forced some apes out of the trees and onto the savannah, where walking erect probably allowed the brain to remain cooler (and gave certain advantages in predator early-warning). So human intelligence may have rapidly evolved as a consequence of a behavioral adaptation brought about by a sudden environmental shift.
I'll bet it was MUCH 'cooler' up in the trees!
intergenerational repository of knowledge - see Morphogenetic Resonance
Hmm. I was under the impression that the main reason for environmental pressure toward bipedalism was that the spread of the savannah favored animals that could see over the grasses and spot predators, hence the selective pressures in favor of bipedalism (but that's as much a conjecture as any other, I suppose). Intelligence, I thought, was more a byproduct of a shift to a high protein meat diet after the development of hunting skills that then allowed for the deveopment and maintenance of a larger brain.