Merriam-Webster's Word of the Day for March 30, 2012 is:

atavism • \AT-uh-viz-um\  • noun
1 a : recurrence in an organism of a trait or character typical of an ancestral form and usually due to genetic recombination b : recurrence of or reversion to a past style, manner, outlook, approach, or activity 2 : one that manifests atavism : throwback

Examples:
"Examples of atavisms in animals include the appearance of reptilian teeth in a mutant chicken or vestigial hind legs in a whale." — From Kate Rheaume-Bleue's 2011 book Vitamin K2 and the Calcium Paradox: How a Little-Known Vitamin Could Save Your Life

"He was a magnificent atavism, a man so purely primitive that he was of the type that came into the world before the development of the moral nature." — From Jack London's 1904 novel The Sea-Wolf

Did you know?
"Atavism" derives via French from Latin "atavus," meaning "ancestor." "Avus" in Latin means "grandfather," and it's believed that the "at" is related to "atta," a word for "daddy." "Atavism" is a term rooted in evolutionary study, referring to instances when an organism possesses traits closer to a more remote ancestor, rather than its own parents. That sense dates to the early part of the 19th century. The word's figurative sense is a more recent development. These days one might describe a building that looks like it's from an earlier era as an atavism, or (though some people might cringe at this) apply the word to activities like reading actual paper books in the age of electronics.