Was Lily Potter the first person in the wizarding world to sacrifice herself for someone else?

Probably not, but I could see why they'd want to keep this magic under wraps. Picture an abusive boyfriend and his weak willed girlfriend who does truely love him, even though she shouldn't. He manipulates her into sacrificing herself to save him, and then he's protected. If there were public knowledge of such magic, it could be horribly manipulated.

To be fair, though, so could things like the Polyjuice potion. Wizards seem to be remarkably trusting.