Breaking the Spell: Chap 05 - The Evolution of Stewardship
Some ideas like language just pass themselves along with no conscious effort. Some ideas, like calculus, require dedicated cultivation. In a similar way, the next important step for the evolution of religions was to go from a spontaneous and self directed phenomena to one that was consciously cultivated.
Just as folk music is different from classical music, religion becomes a very different beast when there are people consciously tending it and directing it's evolution. As a consciously distinct human enterprise there are now different evolutionary pressures in place which are affected by the interests of the special class of people who have assumed a role of authority in the religion.
The move to organized religion most likely occurred when agriculture allowed large numbers of us to move into cities. This started the formation of religion "guilds". Possibly it was also at this time that religions started affiliating themselves with the "kleptocracies" and formed a mutually beneficial relationship that would enforce the kleptocrats rule and keep the leaders of the various religions in power.