South Korean professional Go player Lee Sedol, an 18-time international title winner, conceded defeat in a match broadcast live, with one YouTube stream watched by tens of thousands of people worldwide, while domestic cable provided frequent updates.
AlphaGo, built by Google subsidiary DeepMind, made history in October, by becoming the first program to beat a human professional player at the ancient Asian board game.
But Lee was considered a much bigger hurdle for a machine to overcome in what many experts consider to be the most complex board game in existence.
Go, most popular in countries such as China, South Korea and Japan, involves two players moving black and white stones on a square grid with the aim of seizing the most territory.Experts did not expect an AI program to beat a human professional for at least a decade, until AlphaGo's victory over Fan Hui last year.
AlphaGo's ability to learn on its own in a human-like manner, key to its success, also underscores the advance of AI.
In recent years, scientists have made strides in getting computers to think and learn in ways more similar to people, with the eventual goal that AI will one day assist humans in advanced fields, such as healthcare and scientific research.
Sedol will face AlphaGo another four times in the tournament: this coming Thursday, Saturday, Sunday and next Tuesday. The games start at 1pm in South Korea, therefore 6am South African time. The matches will be live-streamed on YouTube.
There is a $1 million prize for the winner. If AlphaGo wins, the prize money will be donated to UNICEF, STEM and Go societies.
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