It's owed in larger part than you'd think to one specimen:
When a surgeon cut into Henry Molaison's skull to treat him for epilepsy, he inadvertently created the most important brain-research subject of our time a man who could no longer remember, who taught us everything we know about memory. Six decades later, another daring researcher is cutting into Henry's brain. Another revolution in brain science is about to begin.
It's a long read that's worth your while.