There was an important step prior to alpha go. At the time the combinatorics were in favor of Go. But someone had the bright idea to do a probabalistic search of the space. The key idea was to play a ton of random games and rate each position based what percentage that spot was included in winning games. This blew away all other go ai at the time. Sadly this was about the time I stopped having time to follow the space, so I’m not sure how this idea was further incorporated in Go AI. But it was truly a revolutionary idea at the time