Biological evolution of morality is more reasonable than assuming a supernatural law giver. Indeed hunter gatherer religions frequently do not include the concept of supernatural law givers but such people in societies remain moral without religious underpinning for their morality. 
We evolved our moral sense because moral people look after relatives who share their genes. We also look after apparently unrelated people, in the small hunter gatherer communities where we evolved community members would frequently be related to each other in ways that no one knew as people are sometimes related on their fathers’ side without knowing it. Therefore when hunter gatherers or others living in small communities look after anyone from their own community they may be looking after a relative.
Being recognized as responsible
Those who look after others from their community become trusted and respected. Others look after them and their relatives in turn. People seen as moral are attractive sexual partners. Especially people looking for more than casual sex want a responsible partner. Being moral helps us to pass on our genes. We want to be moral. Modern people know about others outside our immediate community through the media including television and the Internet. That increases the number of people who potentially belong to our community. We can even develop compassion and a sense of responsibility towards sentient animals because we believe that animals feel things in a similar way to the way other people feel things. 
Some types of immoral behaviour obviously reduce the survival of our genes, such as incest so these behaviours have been removed through natural selection. Apparently moral behaviours can become so ingrained in a culture that it appears there must be some supernatural power enforcing it, which links back to the first assumption made by the theistic critics. See one of our most visited articles Christian morality.
- “Religion Functions to Sustain the Moral Order” — Starkly Wrong
- Beyond Selfishness
- This compassion and altruism evolved because it lead to us helping relatives and others who would repay favors but it generalized to other sentient beings.