Awork of art is considered prehistoric if it is:

Prehistoric is before recorded history, So anything made before recorded history is prehistoric.

Prehistoric, meaning preserved for a long period of time. The art component, insinuates it's unique and has a story behind it.

Generally spontaneous human combustion is something that still bewilders some, human combustion happens very rarely and doesn't affect the surrounding areas, so unless you want to set your self on fire, human combustion will remain spontaneous and very rare.