Because there aren't any written records, (excluding cave art if we expand the definition) all history before (circa) 4000 BC is speculative. It is similar to you sitting in your house and hearing the squeal of tires followed by the sound of metal on metal. Without actually seeing the car accident, you can deduce that there HAS been a car accident. It is the specifics that must be generalized. By the same token, if you come upon the accident 5 minutes after the event, SEEING certain things (i.e. the extent of the damage to the cars, if a fatality was involved) other information can be generalized. (relative speed of one or both of the vehicles) So it is with us moving from the Neolithic period (10,000-4000 BC) to the beginnings of recorded history. (Uruk; 4100-3800 BC)
One deduction is, during the Neolithic Age (if not much earlier) man formulates a worldview (religion) of "animism." (anima = soul; Latin) It is the idea that everything (including inanimate objects) have a spiritual essence. "Most animistic belief systems hold that this spirit survives physical death. In some instances, the spirit is believed to pass into a more leisurely world of abundant game and ever ripe crops, while in other systems...the spirit remains on earth as a ghost, often becoming malignant (evil) in the process." Further; "From the belief of the survival of the dead arose the practice of graveside rituals such as the offering of food or of lighting fires to honor the dead...(that) later became an act of full fledged ANCESTOR WORSHIP." ~New World Encyclopedia
If you think I paint with too wide a brush, there is this: "Animism was originally the religion of all Hunter-Gathers, our ancestors for most of the last 200,000 years. How do we know that? Through archaeological traces of our ancestors and comparative anthropological studies of present day hunter gatherers ALL of whom are animists. ~Introduction to Animism Lanie Johnson, M.A. and Ken Fischman Ph.D.
So what can we state with some assurance? Homo sapiens believed from the beginning they would have an afterlife. The concept of heaven (abundant game and ever-ripe crops) and devils/demons (a ghost, often becoming malignant) emerges. Totemism (early form of statue or idol veneration) and Shamanism, (the priesthood) both "off-shoots" of animism, draw a more complete picture of how religions evolved.
The rest, they say, is history. Which it is...because we have a written record. The emerging civilizations (Sumer and Egypt) bear testimony to the truth. The only question remaining is, why? Why do humans believe in immortality? Here is one possible answer.
End of Series