For brain surgeons, Penfield’s map was a practical boon, helping them plan out their surgeries. But for scientists interested in more basic questions about the brain, it was downright fascinating. It revealed that the brain organized the sensory information coming from the skin into a body-like form.
There were differences between the homunculus and the human body, of course. It was as if the face had been removed from the head and moved just out of reach. The area that each body part took up in the brain wasn’t proportional to its actual size. The lips and index finger were gigantic, for instance, while the forearm took up less space than the tongue.
That difference in our brains is reflected in our nerve endings. Our fingertips are far more sensitive than our backs. That’s because we don’t need to make fine discriminations with our backs, while we use our hands for all sorts of things–like picking up objects or using tools–that demand sensory power.
The shape of our sensory map reflects our evolution, as bipedal tool-users. When scientists have turned to other species, they’ve found homunculi of different shapes, the results of their different evolutionary paths.