Brown says that many religions have existed without a belief in eternal life, thus undermining the argument that by promising an afterlife, religion evolved as a way for humans to cope with their mortality.
A more plausible explanation, he says, is that religion is a by-product of an aspect of our minds and behaviour that evolved for some other purpose. According to biologist David Sloan Wilson, one such purpose could be coherent and successful group behaviour. Consider how lust inspires us to mate, which has the evolutionarily advantageous knock-on effect of producing babies. Similarly, the pursuit of the sacred inspires us to religion, which has the evolutionary advantageous knock-on effects of causing us to form coherent groups and to follow rules.
Link to Guardian essay.