atavism

The reappearance of an ancestral characteristic that had been absent for several generations; a reversion to something ancient or primitive.

• atavistic/ˌætəˈvɪstɪk/(adj)返祖的
• atavist/ˈætəvɪst/(n)返祖者

• atavistic impulse
• atavistic fear
• cultural atavism
• atavistic urge

1. 1.The mob's behavior revealed an atavistic savagery that no amount of civilization had erased. (reversion to primitive instincts)
2. 2.Occasionally a whale is born with tiny hind limbs — an atavism from their land-dwelling ancestors. (ancestral trait reappearing)

• throwback/ˈθroʊbæk/— reversion to earlier type, informal
• regression/rɪˈɡrɛʃən/— return to earlier state
• reversion/rɪˈvɜːrʒən/— returning to a former condition

from Latin "atavus" (ancestor, great-great-great-grandfather) — literally "the old father" — the return of the old father's traits