past vs prehistoric

past is a noun but prehistoric is not a noun.

Word	Noun	Adjective	Verb	Adverb
past	Yes	Yes	No	Yes
prehistoric	No	Yes	No	No

past (noun)	prehistoric (noun)
a verb tense that expresses actions or states in the past
the time that has elapsed
a earlier period in someone's life (especially one that they have reason to keep secret)

past (adjective)	prehistoric (adjective)
earlier than the present time; no longer current	of or relating to times before written history
of a person who has held and relinquished a position or office	belonging to or existing in times before recorded history
	outdated

past (adverb)	prehistoric (adverb)
so as to pass a given point