We tend to separate animation into two time periods: before computer animation and after. But 2-D animation, which animated features used exclusively before computers, went through its own changes in technology as well. One could argue that the biggest of these was the invention of the multiplane camera, which allowed animators to add depth and movement to backgrounds in ways they weren't able to previously.
The first multiplane camera was actually invented in 1933 by a former Disney employee named Ub Iwerks. Disney developed their own in 1937 thanks to William Garrity, and it was used to animate classics like Snow White and the Seven Dwarfs, Pinnochio and Peter Pan. Next time you watch your favorite old-school Disney film, take a moment to appreciate how much effort went into making the backgrounds look as realistic as possible.