Over a hundred years ago, Engels pointed out that upright posture marked the transition from ape to man, a completely materialist explanation. This view has been confirmed by the more recent researches of anthropologists such as Leakey. Upright posture liberated the hands for gripping with an opposable thumb. This enabled tools to be used and developed.
Upright posture also allowed early humans to rely more on the eyes, rather than the other senses, for sensing the world around. The use of the hands developed the powers of the brain through the medium of the eyes.