Evolutionary biologist Daniel Lieberman takes the long view of physical activity. His name has been connected to running and human evolution ever since his seminal Nature study Endurance running and the evolution of Homo appeared in 2004, and hes been linked to barefoot running in particular after a 2010 study, also in Nature, explored the impact of modern padded running shoes on our strides.

Liebermans research interests range wider than running, spanning physical activity across the evolutionary history of what moves humans, in the industrialized world and in traditional hunter-gatherer societies. In a new review published Monday in PNAS, Lieberman and his Harvard co-authors grapple with the active grandparent hypothesis, using biomedical research and evolutionary studies to explain how humans evolved to need physical activity, particularly in and after middle age, to increase life span and reduce the risk of disease.

Lieberman spoke with STAT from Copenhagen, where he is pursuing further research, about health span vs. life span, why stress from exercise is good for us, and which animals are couch potatoes. This interview has been condensed and edited for clarity.

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When you study physical activity, whats your starting point?

Im interested in how and why humans evolved to be physically active and how changes in our physical activity patterns affect health. My dog hangs out all day on the couch and you know, her health is not as affected by physical activity as humans. So, what is it about us? And furthermore, why?

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Studies by Ralph Paffenbarger shows that as we get older, physical activity actually becomes more, not less, important as we age. The effect is greater. That seems really interesting because humans are unusual in that we evolve to be grandparents. We evolve to live after we stop reproducing. I started thinking about how hunter-gatherers dont retire, they stay really physically active.

Do we know why physical activity keeps us healthy longer?

There are some hypotheses. The first hypothesis is that in humans, physical activity evolved to help extend health span. Prior to medicine, health span equaled life span. Today, when we get sick in our 50s or 60s from diabetes or heart disease or whatever ails us, we go to the doctor, but that didnt exist until recently.

Our general hypothesis is that we evolved all kinds of responses to physical activity that improve health span over the long term, not just when youre young, but when youre old. And that those responses are largely due to energy allocation. Until recently, energy was limited, people couldnt go to the 7-Eleven and grab 200 calories. People had to be very physically active, which takes energy.

Where does that energy go?

One previous idea is that physical activity prevents us from spending extra energy on things that may be good for reproductive success but arent good for our health. And thats fat and hormones. When youre physically inactive, you increase your reproductive output by increasing hormones like estrogen, for example, and progesterone, which increases your risk of cancer. Testosterone as well. Also, you store fat. Fats babies. Until recently it was all about storing energy to improve your reproductive success. Now we live in this weird world where people consume more than enough.

What does exercise do, beyond burning calories?

The other hypothesis is that physical activity is also important for health because its stressful. If I were to go for a run right now, my mitochondria would start pumping out reactive oxygen species, Id be putting a little micro cracks in my bone, Id be glycating proteins. But of course, physical activity isnt bad for us. Its good for us. And the reason its good for us is that our bodies mount a whole series of responses to those stresses that are beneficial.

The analogy I sometimes like to use is, imagine you spill a cup of coffee on the floor. And then you clean up the floor, but you actually end up cleaning the floor a little bit more than it was before. Its like an overshoot.

We know from all kinds of data that physical activity turns on all these repair and maintenance mechanisms. Crucially, we never evolved to turn them on as effectively in the absence of physical activity because we never were physically inactive, right? Nobody ever had machines to do their work for them. So weve never evolved to keep our bodies ticking along and preventing senescence in the absence of physical activity.

How does that play out in diseases more common as we get older?

Therere so many examples: cancer, cardiovascular disease, Alzheimers. In cardiovascular disease, people who are not physically active dont generate stress in their peripheral circulatory system that causes arteries to stay elastic. As a result, people who arent physically active tend to become hypertensive as they get older, and hypertension is arguably the major cause of illness in the world today. People who stay physically active dont become hypertensive. People in subsistence populations who stay physically active dont become hypertensive. Im not discounting the effect of diet, but physical activity plays a very important role in keeping our hearts strong and our cardiovascular systems from becoming hypertensive.

Cancer is another one. Physical activity upregulates all kinds of cells, like natural killer cells, a white blood cell type that actually seek out and eliminate cells that are cancerous. Physical activity decreases blood sugar levels and cancer cells tend to have a sugar addiction.

One of the most important things about physical activity is that it lowers systemic inflammation. It turns out that the major organ that regulates inflammation in your body is muscle.

People today are living longer than our hunter-gatherer ancestors but were living longer with chronic disease.

What differences do you see between life span and health span?

If youre a cynic, you can say people today are living longer than our hunter-gatherer ancestors. Thats true, but were living longer with chronic disease. Looking at life span is a very, I would say, impoverished way of looking at health. When you die is important, but its not the only thing. How long youre healthy and free from disease is also very important.

What can we learn from hunter-gatherers today, like the Hadza people you study?

Exercise is not a magic bullet, but they dont get sick from the kinds of diseases we get. They dont get diabetes. As far as we can tell, cancer rates are much lower. Heart disease doesnt exist or its very, very rare. Remember, cancer and heart disease kill 2 out of 3 Americans.

And for most populations, calories are limited. They have to optimally allocate energy across the life span to grow up, to take care of their bodies, and reproduce. So we evolved to take it easy when possible, but there wasnt that much opportunity to take it easy.

How much activity do we need now to have a good health span?

We have lots of epidemiological evidence that just a little bit of activity, like 10 minutes a day or an hour a week, can lower your relative risk of mortality considerably. You dont need to swim the English Channel or run a marathon. With the commercialization and commodification of exercise, we make people feel like they have to do an Ironman or CrossFit, but you dont need that to get the benefits of physical activity.

You mentioned your sleeping dog. What about other animals?

Apes are very inactive. Theyre couch potatoes. People today are more active than your average wild chimpanzee. That tells us weve been selected in our evolutionary history to be more active than our close relatives. Thats important to our health. It looks like there might be something different about humans, and we dont have conclusive evidence. So in the paper, we called for more data. More studies need to be done.

And does that something different help some of us become grandparents?

Were selected to live after we stop reproducing in order to increase our reproductive success. And we do so by helping our children and our grandchildren. Thats the secret of human longevity. Plenty of other anthropologists have written about this. Were sort of just adding physical activity to the fact that humans evolved to be grandparents.

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New research explores the 'active grandparent hypothesis' and evolution - STAT