system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people (myself included) slouch our backs?

And why does it feel so unnatural to keep a 'correct posture' after noticing I'm slouching and adjusting accordingly - is it more muscular, genetic...? Not trying to go hunchback by 30!
gpt
Because from an evolutionary perspective the human spine is a piece of garbage.

Our spines are curved into an S to begin with because we're upright and that places a strain on bones that evolved to be horizontal to the ground. Then we (as modern people) tend to spend all day sitting on our asses in one position getting fatigued. Our spines take effort to keep upright, and flopping backwards goes against the existing curve of the spine - so there's only one direction left to go.