Answer: It is speculated that in the Neolithic era, being adopting more advancements than that of their ancestors of the Paleolithic era, would bury the bodies of the fallen as a sign of respect and even show evidence of the collective belief of an afterlife.
Definitions:
Though prehistoric may indicate a number of periods, for the sake of this answer we can define it as a sort of 'primordial' way of human living.
Though this may fit into the earlier category known as the Paleolithic Era, the hunter-gatherer lifestyle these ancient peoples adopted makes it harder to truly speculate their motives for their actions.
Differences:
The key differences between these two time periods is that the one involved constant moving and shifting to wherever there pray had went (Paleolithic) and the latter involved agriculture, the sort of self-sustainability of one's food supply through the growth of crops and tending of cattle (livestock such as cows, goats, etc).
Essential Explanation:
As a result of the neolithic peoples having the ability to 'settle down' and focus more on creating arts and textiles, they were able develop one of the first known recognitions of a possible life after death. Their burial of loved ones with tools they may have used during their lifetime can be shown as a sign of respect but mainly it can be believed that being buried with these tools allows them to be used with them in the afterlife.
This idea adopts a present continuity too; we can see that especially the Pharaohs of Ancient Egypt were buried with their belongings (including animals) in the hopes of arriving with them in the afterlife.
For reference:
- Paleolithic Period: ~2.5 million years ago
- Neolithic Period: ~12,000 Years ago