Humans were just another ape, and not necessarily the smartest. We did have the most sophisticated communication system. In particular, our ability to believe in "collective myths" allowed us to organize in groups larger than 150. That was the main thing that set us apart. Our ability to believe in something that does not exist in the physical world, such as our belief in things like states, countries, the value of money, etc. Religion is one such thing. (Example, the state of New York, USA does not exist in a physical sense. If all humans everywhere ceased to exist, there would no longer be such a thing as a "state" because it is an artificial social construct that everyone either agrees to believe in, or is forced to act as if it exists on punishment of death) If two tribes of humans agree to the belief, they are able to work together even though they don't know one another. For example, if 3 tribes of humans all believe in the same tree spirit, they can work on a giant monument that they could not do independently. This also meant that if tribes that did not know one another believed they would be smitten for a transgression, say their crops would be cursed if they stole a neighbor's crops, they would all abide by the same code of behavior. At the same time, humans who rebelled too strongly against the system would have been pushed out of popular culture. Example: you don't give offering to the earth spirits? You gotta go so we don't have a bad harvest. Fast forward a few thousand years and eventually the more aggressive religions consume less aggressive ones and spread like wildfire. A lot of that was due to "convert or die" religious crusades, like the early followers of the Abraham faiths. Once Catholicism/Christianity really hit the scene, they forced out everyone who didn't accept their religion. Either those people were physically run out of town or killed. The result is a population of humans who are largely not capable of thinking differently from how they were raised. Most people do not believe what they believe because their particular religion is correct. They believe it because evolution has, for better or worse, spread mostly the DNA of people who are programmed to get along with the majority of their culture. My tentative guess is that greater than 70% of humans will never believe differently than how they were raised. Think back on your past. Do you believe what you believe because someone around you believed it? Did you make an active and educated decision to research all of the religions before choosing yours, or is it what your parents believed so it's what you believed?

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Sources:

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Sapiens: A Brief History of Humankind by Yuval Noah Harari

I'll have to dig up the other book from storage, but most of the info comes from the above.