The antonym for atavism is progression or advancement, as atavism refers to the reversion to ancestral traits or characteristics, while progression signifies moving forward or evolving.

a throwback to the past; a characteristic or trait that hasn't shown up for generations suddenly resurfacing

Synonyms: reversion, lapse, backsliding, reverting, throwback

Antonyms: progression, advance

Atavistic means relating to or characterized by reversion to ancestral traits or characteristics, often considered primitive or outdated. It suggests a throwback to an earlier evolutionary stage or form.

Like a noun. The man's white hair was a long lost atavism.