Let's face it. Backs suck. It seems that we develop back problems way too early and much more frequently than other animals. Some people believe that this is because we are evolving. Humans basically just learned to stand up straight and our backs haven't caught up, yet.
According to Nick Collins, "Our spines were originally arch-shaped, but standing upright turned the backbone into a weight-bearing pillar, causing the development of the "S" shaped curves which help us balance and walk but also cause lower back pain."
See? Design flaw or are we simply still evolving?