In an Earth-like planet filled with humanoid creatures (upright, two legs, two arms, head, etc.) the inhabitants have evolved to grow bowlegs.
The question is why? What advantage would it bring them?
From what I have read, bow-legs predispose you to arthritis (due to uneven stresses on bones: https://en.wikipedia.org/wiki/Genu_varum), as well as make you a bit shorter.
Note: This is genu varum - the bones themselves are straight, it is their alignment which is wrong. No bones are curved.