If it feels like a slog to get to the gym and hit the weights, the reason may be much deeper than laziness — it could be evolution asking you to stay home.Our guest, paleoanthropologist Daniel Lieberman, says regular exercise — working out, on purpose, to get healthier — was never part of the plan for natural selection. (Hey, as long as the species keeps breeding, right?) But we’re also not supposed to sit around all day and do nothing. We did evolve to use our bodies and even to work them pretty hard — when necessary. With medical professionals lamenting our sedentary lifestyles, the key, Daniel Lieberman told us, is to exercise the right way. He'll join us Friday at 11 a.m. to share his research. (Rebroadcast)