We're obviously designed for some kind of sitting. Our bodies bend very comfortably that way. If you walk uphill for ten miles, your body really wants to sit down on the nearest log or rock to take a rest (though how this was supposed to be comfortable back when we used to be naked is a mystery to me). I have no doubt that our ancestors used to sit down to rest. Probably to eat, and to drink water, and to talk, and to weave those loincloths which would be necessary to make sitting down on a log much less uncomfortable.
Our backs and feet suck for standing all day. Mind you, our backs and feet suck for pretty much everything.