The heart. Not the physical one that pumps blood, it is a metaphysical thing, with its own powers and flaws. All living things have a heart, although some hearts are artificial and some are natural.

You have likely had a soul ever since you were born. Or hatched, or created or decanted, or whatever is appropriate to your species before you became human. If you were capable of acting in your own self-interests and self-preservation, then you had a soul even then. If you were a, for lack of a better word, if you were a mindless automaton receiving orders and making no decisions for yourself then perhaps you did not have a soul.