prehistoric vs past

prehistoric is not a noun while past is a noun.

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

prehistoric (noun)	past (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)

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of or relating to times before written history	earlier than the present time; no longer current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outdated

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