Lots of theories, but one that never gets mentioned is, to carry infants on your back. If you're a tree-dweller that's the norm. Then when you go on the ground

Lots of theories, but one that never gets mentioned is, to carry infants on your back. If you're a tree-dweller that's the norm. Then when you go on the ground you turn from two-handed climbing to two-handed foraging. Need the hands free; can't be crawling on all fours. To carry infants on the ground, you want to be balanced, have your weight on your hips. We see hips change once primates started spending time on the ground - they moved under their torso, as if to carry weight. Typically, ethnographers and anthropologists of the past utterly neglected child-care or any other female activity. Because history.