> Also standing up straight may be inherently unnatural, monkeys don't usually do it, and it might actually be the cause of lower back pain for all anyone knows.

My understanding is that this is well established fact. We are literally the ape that stood up. The resulting curved spine is fine for running around chasing water buffalo with the bros but poor for the kind of chronic load bearing and heavy use required to build and maintain civilization. People who subject their backs to chronic hard use generally wear them out at relatively young ages (middle age or so).