Without doctors, mass media, and the Internet, cavemen had no choice but to listen to their bodies if they didn't feel optimal. Everyone is different. Some people have trouble with dairy, while others can't stomach wheat. One thing's certain: We can all benefit from a little self-experimentation. The paleo movement believes in learning the rhythms of your own body and eating and exercising in accordance to how you as an individual thrive best.

See also:
The Case for Primal Living
Seven Rules for Eating Paleo
Five Primal Workouts to Reboot Your Fitness