[DtheP47]

As peoples lives got better through time they had the time to worry about death.
With the invention of heaven came greater optimism and it enabled people to look beyond their lives.
Interesting too that the peoples of the Americas came up with the self same ideas as Eurasia and Africa.
i.e. If crops failed there were the same kind of conclusions, the same kind of gods and the same kinds of people trying to please the gods.