How about Evolution? First people only appear after some years, as they, well, evolved from other animals or something.
''Rhesus Macaque -> Elves'' anyone?
This would take hundreds of millions of years!
Yeah and we can't have that, it would be ridiculous. I mean, millions of years, right? That would be, like, having rivers run through the world to erode it and mountains rising and oceans appearing due to tectonic activity.... Such a large-scale thing could never happen in worldgen. Ever.
OH NO WAIT
PS. But to be quite frank, I would personally prefer a creation kind of thing for the game, with a world that allows the occasional forgotten god or demon to exist alongside the more mystic "creators" who left the mortal world for one reason or another... and the ones who preferred to stay. It could lead to great quests for adventure mode (apart from the generic "Oh god demons are killing us DO SOMETHING!") when you finally meet "Urdim GoldThroned, God-Emperor of mankind" who tells you to kill a cyclops or something.
Or you place your fortress in an old elven forest witch enrages the ages-old elven god of trees who slept peaceful inside a great oak... until you turned it into a beer-barrel.
But these things should happen very rarely, or else they become too common, two living god should be enough for a middle sized map, maybe some hidden ones, too.
But in the end it's up to Toady to decide and we will (have to
) be able to live what he thinks is best for the game.