According to TOS's "All Our Yesterdays," Spock's ancestors a mere 5,000 years ago were savages. That would be some awfully fast history, with starflight, a possible dark age and everything else thrown in. (Remember that Vulcan logic is a philosophy, not a behavioral trait. Yet Spock's trip through the atavichron is enough to strip that away, as though it were genetic.)
It had been previously established (I think) that Spock could feel or sense the death of all the Vulcans on the Intrepid
in "Immunity Syndrome." I understood Spock's lack of logical behavior the result of a telepathic link to the savage Vulcans of 5,000 years in the past.
MCCOY: Are you trying to kill me, Spock? Is that what you really want? Think. What are you feeling? Rage? Jealousy? Have you ever had those feelings before?
SPOCK: This is impossible. Impossible. I am a Vulcan.
MCCOY: The Vulcan you knew won't exist for another five thousand years. Think, man. What's happening on your planet right now, this very moment?
SPOCK: My ancestors are barbarians. Warlike barbarians.
MCCOY: Who nearly killed themselves off with their own passions. Spock, you're reverting into your ancestors five thousand years before you were born!
SPOCK: I've lost myself. I do not know who I am. Can we go back?