Person dependent (like above comments on barefoot killing peoples shins, etc.) and cultural, like the Mexican-Indian (native) folks who drink, and run for 100's of miles (Born to Run book inspiration).

http://en.wikipedia.org/wiki/Tarahumara_people The word for themselves, Rarámuri, means "runners on foot" or "those who run fast" in their native tongue according to some early ethnographers like Norwegian Carl Lumholtz, though this interpretation has not been fully agreed upon. With widely dispersed settlements, these people developed a tradition of long-distance running up to 120 miles (190 km) in one session, over a period of two days through their homeland of rough canyon country, for intervillage communication and transportation as well as to hunt.

They also have the tendency to eat when hungry, sleep when sleepy (as in, anytime / anywhere) which has threads to other "genius" discussions (lets call being able to run 100's of miles a type of "physical genius" for a thought experiment). I know when I do barefoot walking on a frequent basis my chronic lower back pain starts to dissipate.

Side note: We also aren't naturally meant to sit all day in an office chair, yet...