Check out this cool video explaining the history of deep learning and neural networks! People first really started to understand the brain in the late 1800s. After discovering neurons in the brain people realized their similarity to other electrical signalling equipment of the time like relays.
For years these artificial neural networks were purely theoretical. But in the 1950s the first working AI model was created (it could differentiate squares from circles). The machine had knobs between each neuron that were manually adjusted by hand to train the network.