Infant-carrying and bipedalism in humans

Way back at least to H. erectus times, pair bonding and neoteny meant that both parents were needed to care for infants. And they had to travel with them: at least one infant or toddler (interbirth interval may have been longer, like in bush-living !Kung, 4 years, instead of developed-world humans, less than 2 years) and another one or two dependent juveniles. They travelled widely, all the way to Java and China (well, strictly speaking, the population expanded that far, but individuals must have travelled widely for that to happen). They had to cross rough terrain, like canyons, and wade innumerable creeks and streams (how they got across big rivers like the Indus, Ganges and Irrawaddy is another question). So the mother carries the infant or toddler, and the man does what? Go to any urban park and you’ll see: human males universally carry toddlers on their shoulders, leaving one hand free to lead a pre-adolescent. We need a strong vertebral column for this, and a very upright posture. You must have noticed that when carrying anything heavy, your back could support way more weight than your arms, and the weight forced you to walk very erect. Humans are so programmed to carry heavy weights on our backs that we go backpacking for fun. I think the human vertebral column is way stronger than needed for ordinary and infrequent carrying, like meat back from a hunt. We had to be carrying toddlers. I’ve even been tempted to wonder the sexually dimorphic width and strength of men’s shoulder girdles might have been involved in toddler-carrying. Women can’t carry kids on their shoulders, which are too narrow, for any length of time—the kids slip off.