This is very much an anthropology question, not a question for historians. The answer to this goes back to early hominid (not even homo sapiens) behavior, millions of years. Burial of the dead is one of the very early indicators of advanced intelligence, in that it indicates a conceptual understanding of death and consciousness, an attempt to recognize the death and mark it out socially in some way, and also the realization that a decomposing corpse is harmful for others healthwise and also an unpleasant sight for those wishing to recall the deceased fondly. The question of when hominids began to bury dead is unanswered, with anthropologists uncovering earlier and earlier evidence. Recently, the remains found in the Rising Star cave in South Africa have challenged our notions on this topic: if the bodies there were deposited purposefully by Homo naledi, as a form of burial, it would be (if I'm not mistaken) the oldest example of this behavior in hominids discovered to date. See here.