>>12370807we evolved bipedalism very quickly. The spine isn't really meant to stand up straight like it does in humans, even other bipedal species like birds don't have vertical spines.
Because apes are already fairly close to bipedal, with hunched postures, it was possible for the full upright human posture to develop relatively quickly. However, there hasn't been enough time for the structural integrity of the body to adapt.
A similar problem is the location of the human genitals, which are placed at a very vulnerable spot. The testes are front and center of the body at perfect height for a predator to bite at, while basically every other animal has well protected testes. This vulnerability is another side effect of rapid bipedalism, an ape obviously does not have the same weakness, with its testes hidden underneath the body. Similarly, women with large breasts often suffer back pain from the weight. If the spine was not vertical, it would be able to bear the load, but the vertical spine requires the female upper back muscles to pitch in far more strength than they're meant for to hold large breasts.
