How Do We Explain the Evolution of Religion

"Religion is a cross-cultural universal, even though not every human being professes faith in God or some other supernatural being. Those of us who are atheist or agnostic make up 6 percent of the American population. A further 14 percent say they are not affiliated with any particular religion."

"But religiosity is found in every human culture and biologists, anthropologists, and psychologists keenly debate how it arose. Just like language, technology and bipedalism, religion too evolved over time. But how did that happen?"

"In a new paper published online in the journal Animal Behaviour, biologists Bernard Crespi and Kyle Summers ask a specific version of this question: 

'How did religion actually originate and evolve, step by small step, with Darwinian continuity and explicable selective pressures mediating each stage?'"