" In short, many lines of evidence suggest that climate change spurred selection for bipedalism in order to improve early hominins’ ability to acquire the fallback foods they needed to eat when fruit was not available. More evidence is needed to test this scenario fully, but whatever its cause, the shift to standing and walking upright was the first major transformation in human evolution. But "

― Daniel E. Lieberman , The Story of the Human Body: Evolution, Health, and Disease