So physical activity is just moving.

If you sweep the floor, make dinner, that's all physical activity.

But exercise is a kind of physical activity that's intentional, it's discretionary – undertaken for the sake of health and fitness.

And that's actually a very modern, strange behaviour, nobody ever used to do that.

You can spend your entire day sitting in chairs and pushing buttons and never have to break a sweat in order to survive.

But our ancestors, of course, had to be physically active to survive, but they also struggled to get enough calories to get through the day.

So, they were careful not to do physical activity when it wasn't either necessary or rewarding.

Common sense tells us that our bodies were programmed to be as active as our hunter-gatherer ancestors were.

But from an evolutionary perspective, we're actually hardwired not to waste precious energy in vain.

Our ancestors were physically active in order to get food or avoid being

somebody else's food. But other than that, they sensibly conserved energy.

And so if you go into a camp of hunter-gatherers, most of the time people are sitting, right, and we demonise sitting and say:

'Look, you know, sitting is the new smoking and your chair is out to kill you.'

But there's nothing strange about sitting – it's just that if that's all you do, then you run into trouble.

Only about 20 percent of Americans get the minimum level of exercise recommended by the government, which is 150 minutes a week of moderate to vigorous physical activity.

And the fact that they struggle to exercise isn't some personal failing.

People are simply doing what we evolved to do, which is to avoid physical activity when it's neither necessary nor rewarding.

Another problem is that most people exercise just because they hope to lose weight. But the truth is, our bodies have not evolved to burn more calories the more physically active we are.

We went to work with a Hadza community in northern Tanzania, they're one of the few traditional hunting and gathering communities left in the world.

And the Hadza – they don't have any domesticated animals or plants or machines or guns or electricity or plumbing or anything like that.

And every morning, men go and hunt wild animals with bows and arrows.

Women go out every day and they dig for wild tubers, root vegetables with a digging stick. They get more physical activity, walking and chopping, and digging – more activity in a day than most Americans get in a week.

And we were sure that we would find they would burn more energy every day than Americans.

In fact, Dr Pontzer found quite the opposite.

The Hadza men and women out there getting more activity every day than you get in a week, had the same energy expenditure, per day, as sedentary Americans and Europeans. Huge surprise.

Researchers were so surprised that they doubted the results, so they went out to collect more data in different hunter-gatherer communities, using different techniques.

We see the same thing again and again.

People who are more physically active don't burn more calories necessarily than people who are more sedentary.

We see this in other animals. We see this in mice in a laboratory.

Monkeys in the zoo have the same daily energy expenditure as monkeys in the wild.

Our bodies are evolved to keep energy expenditure within a kind of narrow range that we cannot afford – right?

That we can meet those demands every day.

So if you spend a lot of energy on exercise, your body will probably respond by spending less energy on other things.

But this is no excuse to be sedentary.

Please, please understand this is not an excuse to not exercise, right?

It is not a free pass. We often talk about diet and exercise as two sides of the same coin. If I don't want to exercise today, I'll just watch my diet or if I don't want to diet, I'll just exercise more.

Like you can trade them back and forth.

And these results from my work and other people's work say:

'No, no, no, no, no, it doesn't work that way.'

Diet and exercise are two separate tools with two separate jobs. You need to diet to watch your weight.

You need to exercise for everything else.

Exercise has such a big impact on, not just your muscles and your heart, it affects every part of your body, because it's affecting how those other systems spend their energy and how active they are.

And so we know that, for example, if people exercise more, things like inflammation are reduced.

But if exercising for the sake of fitness is an unnatural behaviour, how can we inspire people to be more active?

I think we should treat exercise the way we treat education.

So, education is also a very modern behaviour we never evolved to do.

Think about it, until a few hundred years ago, almost nobody on the planet was literate.

Now, you know, literacy and education are considered universal.

Dr Lieberman proposes to make exercise not only necessary, but also fun and social.

I have friends who I run with, right. And we often agree, you know, the night before, that we're going to meet at like six AM.

And then all of a sudden now it's six AM, you know, I don't want to be there, but I've already made an agreement to meet my friends.

And I can't let them down, right? And then when I meet them, they kind of make me glad that I went, after all.

You don't have to run a marathon or swim the English Channel or climb Mount Everest.

Just climbing the stairs in your building or walking to work instead of driving or walking your dog.

All of those things have enormous, enormous benefits.

The benefits are even greater as we get older.

Hunter-gatherers who survive childhood typically live about seven decades.

They become grandparents, and when they become grandparents, they don't retire and move to the south of France or Florida or wherever. They actually stay very active.

And that activity helps turn on all kinds of repair and maintenance mechanisms of the body that keep us healthy.

And so we evolved not only to live long, but also to be physically active as we age, and that physical activity actually helps us age better.

Recent studies are providing new insight into how our bodies work.

It was found, for example, that our metabolism slows down much later than previously thought.

Your metabolism is really stable from about 20 years old to 60.

That was surprising because there's this conventional idea this conventional wisdom that your metabolism slows down in middle-age, but we don't see any data for that at all.

But it's still up to us to choose to live long, healthy lives.

We have to choose now, for example, not to eat foods that are highly processed and full of sugar and fat that are really tasty and delicious, right?

We now have to choose to be physically active when we otherwise wouldn't have to do that. That's difficult, right?

Especially when it's also a matter of opportunity.

To exercise – it's become a privilege of people with wealth and means.

It used to be that only rich people could be physically inactive.

Now it's the other way around. It's only rich people who can actually afford their gym memberships and can live in places where there are, you know, access to facilities to exercise.

We need to as a society to make these resources available to absolutely everybody.