Chances are we’re all going to live into our eighties. Probably even into our nineties. Despite the risks of cancer, heart disease, diabetes, stroke, and all the other ailments of our time—even despite car accidents, earthquakes, terrorist attacks, avian flu—it is a statistical probability that as Americans we will survive to a ripe old age. Many of us will indeed suffer from chronic diseases and acute injuries, and most of us will who do will survive them. Modern medicine is fantastic at keeping us alive, and it’s getting better every day.

However, it’s debatable whether modern medicine is good at keeping us healthy. So if we’re likely to live into our eighties and nineties it’s up to each of us to make sure we can afford to live that long, not just financially, but physically.

That’s where exercise comes in. Not government recommended, twenty minutes a day, three times a week exercise; we’re talking intense, rigorous exercise for an hour a day, six days a week. Sixty minutes, six days a week, minimum.

Minimum!? Is it humanly possible to exercise more? It is, and we’re all built for it. Our bodies actually expect us to exercise that much, and if we don’t meet those expectations our bodies will slowly decay. Most people think of this as aging, but it’s not. Aging and decay are two separate things. Aging is inevitable, while decay is entirely preventable. Why do we decay? We can blame evolution.

Specifically we can blame our genes for decay. Sometimes what’s good for us as a species is not so great for us as individuals. In the days when famine was a real concern, which was not so long ago on an evolutionary scale, it made sense for the older generation to decline and die off rather quickly so that our limited resources would be available to the younger generation, ensuring the kids would live to reproduce and so carry our genes down the timeline. While the parents and adolescents would go out every day to hunt and gather, engaging in intense exercise and staying fit and healthy in the process, the grandparents would stay home and take care of the children. No doubt a useful occupation, but a relatively sedentary one. Once strong muscles atrophy, bones become brittle, and joints become stiff. Once the children become parents, and parents become grandparents, there’s not really much use for great-grandparents, so our genes tell our bodies to wither and die rather than consume precious calories that can be used to ensure our genes survive to the next generation.

Depressing? Hell yes it is. Our genes are not our friends; essentially they use us as self-propelled Xerox copiers to replicate themselves, then replace us for newer models. They give us a few golden years to impart some wisdom to our grandchildren while their parents are out bringing home the mammoth, and then we’re done. Decay is the default mode our bodies go into, but it is not inevitable. An hour of exercise a day will override the signals for decay, and stimulate growth.

The biological process behind this phenomenon lies in two groups of hormones: cytokine-6 and cytokine-10 (c-6 and c-10). We can think of these as decay and growth signals for the body. We have a constant trickle of c-6 (decay) running through our bodies, telling our white blood cells to demolish aging and damaged cells. This is a good thing—every cell in our bodies has a planned life span. Cells that are past their prime run the risk of turning cancerous, so we want to replace them regularly. Ideally we want to stimulate growth by replacing those cells with stronger versions. For this to happen we must flood our bodies daily with c-10, and this only happens with vigorous exercise.

Why exercise? Exercise tells our genes that we are still useful, still relevant, that we’re still contributing to the tribe. Exercise tells our genes that we’re still hunting and gathering and collecting more calories for the group than we’re consuming individually. And so long as our genes believe that to be true they’ll let us stick around. More importantly for us, given that modern medicine will probably keep us alive for a long time, exercise will ensure that our bodies continue growing, and we will keep our physical ability to function as we age. And by function I don’t mean shuffle around with a walker; I mean run, jump, climb, hike, ski, or whatever it is you enjoy doing into your eighties and nineties.

But six days a week? Can’t I get away with four? Think of it this way. The cells in your body are a yearly budget allotted to you by your genes. If you don’t spend your entire budget, if you don’t use your muscles, bones, sinews, tendons and capillaries to their full potential, your genes will take that as evidence you don’t need them, and your budget for the next year will be smaller. This is not aging. This is decay. Just be thankful our genes are idiots. We don’t actually have to run down a gnu and drag it back to the cave. We can fool our genes by running in the park, lifting some weights, taking a spin class—as long as our hearts pound and our muscles burn we can fool our genes into thinking we’re still relevant.

So go ahead and be selfish. Be an individual. To hell with the grandkids, they can fend for themselves. An hour a day, six days a week isn’t a burden—it’s giving the next generation the finger.