Why was AlphaGo's victory over Lee Sedol considered a decade ahead of schedule?

The answer

In March 2016, DeepMind's AlphaGo defeated 18-time world champion Lee Sedol 4 games to 1 in Seoul, South Korea, in a match watched by over 200 million people worldwide. Go has roughly 10^170 possible board positions - vastly more than chess - making brute-force search impossible. AlphaGo combined deep neural networks trained on 30 million moves from expert games with Monte Carlo tree search and reinforcement learning through self-play. In Game 2, AlphaGo played Move 37, a creative placement that no human expert would have made, which was later called one of the most beautiful moves in Go history.