On this day back in 1928 Walt Disney released Steamboat Willie. It was the first Disney cartoon with synchronized sound:
Disney understood from early on that synchronized sound was the future of film. Steamboat Willie was the first cartoon to feature a fully post-produced soundtrack which distinguished it from earlier sound cartoons such as Inkwell Studios' Song Car-Tunes (1924–1927) and Van Beuren Studios' Dinner Time (1928). Steamboat Willie would become the most popular cartoon of its day.
It put Disney and his studio on fast-forward. Here's a link to Steven J. Gould's "A Biological Homage to Mickey Mouse", in which he discusses Mickey's evolution in biological terms.
H/t Jerry Coyne.