The ritual treatment of the dead among early humans is a hot topic for anthropologists, so you may find more people better suited to answer at /r/askanthropology.

This is more an anthropology question than a history question, so try /r/AskAnthropology. However, if you are looking for a book that covers this topic, you could do worse than checking out "A Social History of Dying by Allan Kellehear". It deals more with dying than treatment of the dead, but the two are of course very closely related.