Well, Noah - your grandson? I think? I'm sorry, it's been a long time since I really thought about any of this. But I grew up being told that when God flooded the world, he instructed Noah to build an ark and take aboard his family and two of every animal on Earth, so that they could survive until the waters receded and rebuild the world.

I don't think it literally happened that way, because it doesn't make a whole lot of sense from a, you know, genetic standpoint - but that's the story, and from what I picked up from Castiel about how these things got written down, the real history is probably something a lot like that.