I don't think you can honestly include anyone whose contributions were strictly evolutionary. The discovery of fire or weaponry or really any scientific principle is only a matter of time. The discoverer is almost irrelevant; if not him/her, then it would have been someone else. You could even categorize most of the intellectuals under this heading.
Likewise, anyone who became "great" by following the natural human instinct to acquire wealth, security, and dominance over his fellow man really shouldn't qualify either. I just don't see it as terribly impressive, being enslaved to your primitive, genetically-determined impulses. You conquered an entire continent? Good for you... tool.
So I'm going with a humanitarian - someone who bettered the world for a large number of people with no substantial material gain to show for his or her efforts. Unfortunately, I don't know enough about such people to offer up any nominations.
But hey, my criteria... slick, huh?