So we can expect that our ancestors, no matter how curious they were by temperament, did more or less what we all still do today: rely on "what everyone knows." Most of what you (think you) know you just accept on faith. By this I do not mean the faith of religious belief, but something much simpler: the practical, always revisable policy of simply trusting the first thing that comes to your mind without obsessing over why it does so. What are the odds that "everybody" is just wrong to think that yawning is harmless or that you should wash your hands after going to the bathroom? (Remember those "good healthy tans" we used to covet?) Unless somebody publishes a study that surprises us all, we take for granted that the common lore we get from our elders and others is correct. And we are wise to do so; we need huge amounts of common knowledge to guide our way through life, and there is no time to sort through all of it, testing every item for soundness. And so, in a tribal society in which "everyone knows" that you need to sacrifice a goat in order to have a healthy baby, you make sure that you sacrifice a goat. Better safe than sorry.
This feature marks a profound difference between folk religion and organized religion: those who practice a folk religion don't think of themselves as practicing a religion at all. Their "religious" practices are a seamless part of their practical lives, alongside their hunting and gathering or tilling and harvesting. And one way to tell that they really believe in the deities to which they make their sacrifices is that they aren't forever talking about how much they believe in their deities -- any more than you or I go around assuring each other that we believe in germs and atoms. Where there is no ambient doubt to speak of, there is no need to speak of faith.
-- Chapter 6, Breaking the Spell: Religion as a Natural Phenomenon, by Daniel Dennett
It is amazing, isn't it, how much religious folks have to assure each other that God exists? I mean, if it was so obvious, why the constant reinforcement? Sometimes I think they doth protest too much... especially when the missionaries come to MY door on MY time trying to convince me of something that you'd think wouldn't need convincing. Strange.