Of course, the position that Mario and I take in
The Spiritual Brain is that the driver of religion is humankind's spiritual nature. How it came about is currently unknown, but there is some reason to believe, based on prehistoric
cave paintings and other
art works, that it happened rather suddenly.
On that view, diversity is the outcome of different personality types, mind sets, moral development, and such. Pioneer psychologist William James recognized this long ago, when he pointed out that cheerful people tend to adopt different spiritualities than depressive ones.
The silliness of the current theory-of-the-month lies principally in the fact a stroll down any street in downtown Toronto, where I live, will show that most major religions thrive on evangelization, - the very opposite of avoiding disease by forming a closed little group. Mormons, Jehovah's Witnesses, evangelical Christians, Muslims, Buddhists, Hare Krishna, secular humanists - whoever they are, they want you to come in, germs and all, attend their services, read their literature, go through their initiation - and then go out and bring in more people.
Was religion ever any different? Ancestor worship and shamanism are different, because, by their very nature, they do not encourage evangelism. But, as we pointed out in
The Spiritual Brain, religions of that type are bound up with magic - best understood as a primitive attempt at technology, an attempt to control the world without really understanding it. So we will not understand much about the evolution of religion if we focus on that kind of thing.
Religion, separated from magic (which is often forbidden in major religions), tends to be evangelistic by nature. People who have apprehended what they believe to be a spiritual truth usually want to communicate it, to help others. Religion has probably spread as many epidemics than it has stopped, for that very reason - not that anyone could do anything about that until the existence and role of viruses and bacteria were understood.)
One thing I find intriguing about materialist theories-of-the-month about religion or spirituality is the way they typically begin by ignoring the most obvious facts about the subject. Thus, the best theory of religion becomes a fact-free theory.
See also: Religion: It got started to avoid the spread of
disease?