As described in the previous three posts, evolution wired humans to be moral, to form supernatural beliefs, and to sustain those beliefs. Yet, we are not genetically predisposed to believe Jesus is the son of God or Muhammad spoke to the angel Gabriel. In other words, religious traditions are a product of cultural evolution, not biological evolution. As societies went from tribal to modern, and as cultures interacted with one another, supernatural beliefs and religious traditions morphed to better suit the needs of their host civilizations. As time went on, only the most useful religions were left standing.
Hunter Gatherer Religions
For hundreds of thousands of years, our ancestors lived in small bands hunting and gathering to survive. We can glean a sense of what their religious life might have been like by looking at the traditions of modern isolated tribes in places like Brazil, Africa, and New Guinea. These societies hold beliefs in invisible agents who are responsible for diseases, weather, and other natural events. These agents tend to be quite amoral and whimsical, and most tribal societies don't use them to inspire proper moral behavior. Given that humans evolved to be cooperative and compassionate within small groups, these tribes tend to be good at maintaining social harmony on their own without religious inspired moral doctrines. Another common element in these societies is the presence of an individual (e.g. a shaman) whose sole function is to communicate with these invisible agents to ask favors on the tribe's behalf. As the scholar Mircea Eliade wrote, "What is fundamental and universal is the shaman's struggle against what we could call 'the powers of evil.' ... It is consoling and comforting to know that a member of the community is able to see what is hidden and invisible to the rest and to bring back direct and reliable information from the supernatural worlds."
While most hunter gatherer societies had very little political or religious organization, as they evolved to chiefdoms and state societies, politics and religion needed to become more sophisticated in order for these cultures to function. Oftentimes, those who spoke on behalf of the gods ended up gaining political control of these societies. They would use their special connection to the gods to legitimize their right to rule, and some would even claim to be gods themselves. For example, the pharaohs of ancient Egypt were believed to be gods, and many Roman emperors were also deified. Since these societies tended to have far more than 150 constituents (the maximum size for a cohesive social unit,) formalized moral and ethical codes were essential for maintaining social harmony. In addition, incorporating divine retribution for moral indiscretions was an effective way of ensuring citizens would follow the rules, even when no one was looking. These divine laws were generally only applied to fellow adherents of any particular religion. Given that these larger societies were often in conflict with neighboring chiefdoms/states, religious intolerance was a useful way to inspire nationalism and motivate citizens to kill political/cultural enemies. In short, as human civilizations became increasingly complex, religion functioned as a mechanism to maintain social harmony and to further the aims of the political elite.
As I define it, mythology encompasses the stories behind the gods, spirits, and/or agents who are venerated by a particular religious tradition. In the book When They Severed Earth from Sky: How the Human Mind Shapes Myth archaeologist Elizabeth Wayland Barber and her historian husband Paul T. Barber argued that these stories were the method by which ancient humans would transmit vital information over many generations. As time went on, only a few elements of the original stories would remain, thus obfuscating their initial meanings. For example, what does a one-eyed monster that bellows from on top of a mountain and hurls boulders at people sound like? According to the Barbers, the original story of the Cyclops may have actually been a tale to warn people about the dangers of a volcano. In ancient societies, this morphing of mythology became exacerbated as different cultures interacted with one another and merged. Oftentimes, the character of certain gods would change as the cultural contexts shifted. For example, Poseidon, the Greek god of the sea, was originally an Indo-European horse god that assimilated the qualities of several Near Eastern aquatic deities. As the culture took on more of a seafaring role, his aquatic qualities overtook his original equestrian ones. Eventually, when Rome became the dominant culture of the west, they used Poseidon as the template for their own god Neptune.
Survival of the Fittest Religion
The survivability of a religion depended on its ability to spread its message, to adapt to changes in cultural/historical contexts, and its compatibility with human psychology. For example, some religions were successful because of their endorsement of militarism, they required members to have many children and proselytize to the masses, or because they promoted a worldview that was very comforting. Ultimately, religions have to be infectious, persistent, and useful to the success of their host society in order to survive.
For many religious people, the fact that their religion survived while many others failed is proof that their god is the real God. However, all this really shows is that their religion was better at adapting, spreading, and surviving than other belief systems. For many spiritual but not religious types, the similarity between religions and the proliferation of supernatural beliefs are evidence that, as Cee Lo Green put it, "all religion's true." What this really means is that all religions were created by humans for humans, and many stem from the same or similar cultural traditions.
Great Lecture by Jared Diamond on the Evolution of Religions:
Good Article on the Survivability of Religions:
Great Book on Mythology:
Good Article on the Cultural Evolution of Religion: