prehistorical vs past

prehistorical is not a noun while past is a noun.

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

prehistorical (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)

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belonging to or existing in times before recorded history	earlier than the present time; no longer current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

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