In 1981, science essayist Jeremy Bernstein wrote a piece for The New Yorker that touched upon a historic backgammon game two years earlier, in which reigning champ Luigi Villa lost to a computer. It was the first time an artificial-intelligence program had defeated a world champion at a board or card game. In the essay, Bernstein wrote: "What does this mean for us, for our sense of uniqueness and worth -- especially as machines evolve whose output we can less and less distinguish from our own?" He might have asked that decades ago, but the question is now more relevant than ever. Google's AlphaGo recently won four out of five matches against Go master Lee Sedol. And that's just the latest example: In the intervening years since Villa's loss, humans have challenged numerous robots and programs. Let's take a look back at the most memorable of those competitions.