It's more accurate to say that the vast majority of tetrapods evolved their spines to support use parallel to the ground, and that even those which occasionally walk around bipedally spend lots of time either brachiating (in trees) or quadrupedally.
There were no designs involved.
Humans are the largest bipedal walkers, but not the only ones. Here's an interesting survey:
I'd say (and I am a biologist) there is truth to it, but we also didn't evolve sitting all day. There would be much less back problems if would walk and stand more because our muscles would be trained constantly. Our bodies are things that wear out when not using them.
There were no designs involved.
Humans are the largest bipedal walkers, but not the only ones. Here's an interesting survey:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571302/