If I had a buck for every time I was asked this question, I'd be a rich man. It's a great question, and helps frame a conversation about what it means to be fit.
My response to the question is to pose another: How often do you eat? I don't ask this to suggest that if you're taking in calories you better burn them or they will be stored as fat, although it's true. I ask because it reminds me of our early life as a species and why Homo erectus didn't need a gym membership. He was moving all the time. And the main motivation to move was to eat. As a hunter-gatherer, eating was inseparable from moving. We grazed and chased. And moving is the most important element of exercise.
To be sedentary is to be sick. Two million years of evolution designed us as exquisite movers, unlike any other mammal.
Human evolution, from ape to Homo erectus, is all about our ability to move and what that allowed us to eat. Australopithecus was a pea-brained brute brain one-third the size of ours, with a huge jaw to chew tough fruits, vegetables and tubers. He stood a diminutive four feet tall, but he stood! This was progress and freed up the hands for future tool use. Enter Homo erectus, big-brained and biped, long-legged and -armed, with sharp teeth to tear meat. It was the addition of meat to the diet that provided the dramatic increase in calories, fat and protein to fuel the development and maintenance of a big brain.,,
Now you might ask, how exactly did he get the meat on his plate? A very reasonable question that created significant trouble for anthropologists for a long time. Remember, there were no fast-food joints, and the first spearhead dates back to around 20,000 years ago. Yet Homo erectus was enjoying his carnage 2 million years ago.
To make a long and fascinating story less long, H. erectus practiced what is known as "persistence hunting," whereby he chased down his prey until it collapsed from hyperthermia. Not bad for a biped. The ability not to die in a similar fashion as the prey derives from our cooling system. Unlike the animals that were being chased and had to pant to cool, we sweated. Panting is perfect for sprints and that is what all mammals with coats need to survive, short bursts of extreme speed that require intense oxygen delivery to the muscle. The way this is achieved in these four legged creatures is by yoking the stride to breathing.
So for instance, when a lion is galloping along, when its front feet strike the ground, its guts slog forward, compressing the lungs and forcing a full exhalation; and when it extends its legs, the guts slog back, pulling the diaphragm back and sucking in a turbocharged breath of air. This is great in the short term, but not sustainable. The beauty of sweating is that it allows for a disconnection of breathing, cooling and running. This is why, given sufficient distance, a human can outrun a horse, and in fact does, most Octobers in Prescott, Ariz. in the 50-mile Man Against Horse Race.
Now this may have been a long journey, but the bottom line is that we were made to move every day. That is the first and most important message. Your body doesn't care how you do it, but it will reward you if you do. You don't stop moving because you're old. You're old because you stop moving.
[1.] Leonard W. et al. "Effects of Brain Evolution on Human Nutrition and Metabolism," Annual Review of Nutrition 2007 Aug Vol. 27:311-327
[2.] Wray G. et al. "A potential role for glucose transporters in the evolution of human brain size." Brain Behavior and Evolution 2011 78(4):315-326
[3.] Wray G. et al. "Genomic signatures of diet-related shifts in primate evolution." Proceedings of the Royal Society. 2011 B278:961-969
[4.] D.M. Bramble and D.E. Lieberman, "Endurance running and the evolution of Homo", Nature, 432: 345-353, November 18, 2004