Third option: whatever force created her made her look human in anticipation of something she was meant to do during this brief period of time in which her form would make communication and the forming of emotional bonds important.

You know- the whole 'billions of years old' thing makes some of her earlier (well, okay, chronologically later) behavior a lot less creepy. If she's existed for billions of years she has little or no reason to bond with men, or help tribesmen with pesky buffalo, or work in the school for that matter. That she does speaks volumes that, despite her ageless nature, she has a sense of connection with humanity- even if her poise does not make this evident.

Or maybe I'm just reaching and she's just killing time until we all die out and she can go back to not having to deal with these irritating monkeys who think that just because we can communicate with her, she's interested in what we have to say.