Birth of Religion
To discuss the birth of religion, we must first understand the preexisting human structure.
During the Paleolithic and Mesolithic period human existence was extremely dependant and fragile. The acquisition of food was solely predatory, meaning the success of a hunt was largely dependant on the vacilattions of prey populations. These would insignificantly rise and fall annually, but would more significantly rise and fall as the population systems naturally play against each other.
There wasn't enough food on hand to support a group larger than around 15 people. The growth and development of human society depended on the ability to collect and store food.
Women would be gathering food, and it can be imagined that the first step toward agriculture would have been small but powerful -- possibly the simple act of leaving behind grains after gathering in an area.
Men would be gathering into small hunt parties and going on excursions to find meat. As hunter populations rose, prey populations would naturally fall. Possibly due to some mythical explanation, humans could have decided to stop hunting smaller herds and let them regrow. This would be very similar to the replanting of seeds.
The Greater Forces Explained
As man begins to depend on both herd and crop, allowing for larger and more delicate populations, natural disasters impeding the harvest of either would be devestating.