Many's the time I've rolled around on the ground, grimacing and making animal keening noises and wondering why the hell humans evolved to experience such dramatic pain from toe-stubbing. Here is a plausible-sounding threefold answer from Chris Geiser, director of Marquette College's College of Health Sciences athletic training program. Part one is that we've just got a lot of nerves in our extremities because they're our interface to the world. But more interestingly:
Secondly and related to the first point, there is very little tissue in our toes to absorb this type of impact. Much like hitting our shin, there is no fatty tissue or muscle tissue overlying the bones in the toe to cushion the impact. Every bit of the kinetic energy created in moving our legs forward is absorbed by the skin and bone of the toe, resulting in very high compressive forces on the many nerve endings that reside there. Because the foot is at the end of the longest lever system in the body — the leg — feet tend to be moving much faster than any other part of the body when they come into contact with an unknown object. For these same reasons a pitcher can throw a baseball 90-plus miles per hour and a soccer player can strike the ball at roughly the same speed; the further away from the axis of rotation, in this case our hip, the faster the end of that segment is moving. Add the mass of our entire leg to this equation, and there's a large mass applying force to the toe at a great velocity in a small area not capable of adequately dissipating that impact. OUCH!
"The last part of this explanation comes from an evolutionary perspective. In the not so distant past, infections killed many people. Stubbing a toe can open wounds on the feet, which are constantly in contact with the bacteria-laden environment. It has been suggested that individuals who received lots of sensory information from their toes were less likely to strike them, creating an evolutionary advantage for people blessed with this type of sensory information. So there are many components to this amazingly painful question."