Any one who understands the “hope” that religion can impart on a community understands that cavemen with “a religion” (on average) have a greater edge on survival than caveman groups whose brains were more skeptical.
Is it more likely that the human brain evolved to believe in gods given that God or gods exist?
Sure God(s) exists as an abstraction in minds of a group of people. There has been thousands of Gods in various names in nearly every society and culture going back at least tens of thousands of years. But I am sure that all those other Gods are fake Gods and just your God is the one true God. 
And among other things, it made humans more effective at organizing for warfare.
This is one, but not the only point made in this book: