We've also evolved the ability to simply 'pay it forward': I help you, somebody else will help me. I remember hearing a parable when I was younger, about a father who lifts his young son onto his back to carry him across a flooding river. 'When I am older,' said the boy to his father, 'I will carry you across this river as you now do for me.' 'No, you won't,' said the father stoically. 'When you are older you will have your own concerns. All I expect is that one day you will carry your own son across this river as I no do for you.' Cultivating this attitude is an important part of Humanism--to realize that life without God can be much more than a series of strict tit-for-tat transactions where you pay me and I pay you back. Learning to pay it forward can add a tremendous sense of meaning and dignity to our lives. Simply put, it feels good to give to others, whether we get back or not.
This is not a book about whether one can be good without God, because that question does not need to be answered --it needs to be rejected outright. To suggest that one can't be good without belief in God is not just an opinion, a mere curious musing -- it is a prejudice.