Daniel Lieberman is a professor of human evolutionary biology at Harvard and has published numerous studies about why the human body looks and behaves the way it does. His new book is Story of the Human Body: Evolution, Health and Disease. One of his current areas of study is the advantages of barefoot running.

Your book has lots of messages in it that people might take away and use to adjust their behaviour.

That was my goal. A lot of good books have been written about the health crisis we face today but I felt that we still lack a good book that explains from an evolutionary perspective why diet and exercise really do matter. I think people often have a hard time putting together why what happened in our past is relevant to how we live today. It enriches not only our understanding of why we get sick, but also helps us figure out ways to prevent illness.

What would be the three main ideas you'd hope people would take away from your book?

First, our bodies are really a hodge-podge of adaptation that accrued over a very long and complex history, that didn't evolve only to make us healthy but evolved to make our ancestors have lots of offspring. As a result, our bodies don't do the right things in the environment we live in today.

Point number two is related: the body has evolved but also cultural evolution and natural selection is overwhelming, and the result is a mismatch, we get all kinds of new diseases that we didn't used to get.

The third one?

The final point is that our instinct when we are sick is to try to treat each other – which is right and proper. But when we have a mismatch disease caused by this poor fit between our bodies and our environments we treat the symptoms only. On the one hand people are living longer and are healthier than probably ever in human history, but also suffering in new ways that are draining the economy. The US is the worst example but the UK isn't far behind, in terms of how much you are spending in treating chronic non-infectious diseases that are preventable. We can prevent heart disease, we can prevent flat feet and myopia, but we can only do so if we consider our evolution.

Can we talk about how humans aren't adapted to the 21st century?

The big one is obesity. We evolved to put on fat wherever necessary, and that was a good thing in human history. Most people until recently had to work hard and they lived just at the margin of energy balance, and a little bit more energy stored in fat meant that you could have more babies, and your babies were more likely to survive. That was pretty powerful stuff, right? Now we're in this bizarre situation that for the first time in billions of years of evolution we have an organism that is not energy limited any more.

I'm sure there are just as many articles in the UK as they are in the US about how difficult it is for people who are overweight to lose weight. Dieting really is a disaster for everybody, it takes superhuman effort to lose weight, it can be done but it isn't easy. And that's because we're evolved not only to gain weight but to hold onto it. So if that overweight person starts dieting that's just as hard as if an underweight person starts dieting, you go into a negative energy balance and all kinds of mechanisms kick in that cause us to become less active, to reshuffle energy around our bodies to defeat that effort to lose weight. So of course obesity is our number one problem.

You talk about allergies as another mismatch disease.

Nobody really understands the epidemiology of allergies, but the fact that they are increasing so rapidly is a hallmark of a mismatch disease. There's no one thing that sweeps through our environment that gives us allergies, there's clearly something going on, and the best hypothesis out there has to do with antibiotics. Again, it's a trade-off. Not all antibiotics are evil, we shouldn't outlaw them, but we do overuse them - I don't think anybody disputes that. I'll make a bet, I don't have evidence to prove it, but if we were to cut back on how and when we use antibiotics we could decrease a lot of immune disease that range from allergies to coeliac disease or Crohn's Disease, maybe even autism, the list goes on.

So some people would argue that the conclusion from this is that we should be leading a hunter-gatherer type lifestyle.

The old paleo diet. There's some truth to it. I think the problem with the paleo diet is that there is an optimal diet that if only we were adapted to it we'd be healthier. It's based on the following argument - we're adapted to live like hunter-gatherers - if we live like hunter-gatherers we'd be healthier. But remember, hunter-gatherers were not always healthy, their bodies went through adaptations not to be healthy but to have lots of children!

What was the life expectancy of a hunter-gatherer?

Infant mortality rate was alarmingly high but the evidence is limited because there aren't many hunter-gatherers to study. Once they survive childhood they tended to live much longer, until their 70s, 80s or even 90s.

Because of the invention of sewers and toilets and antibiotics, we no longer have those infectious diseases that tend to kill hunter gatherers. So there is some benefit to understanding what we're adapted for. But some paleo diet folk won't eat dairy, or legumes, or cereal. I think it's a bit extreme because a) we can't feed the whole world on grass-fed beef, it's just not going to happen, sorry. b) there are still a lot of question marks around eating as much meat as you possibly can.

There are Mediterranean farmers and old fishermen who aren't eating a paleo diet but are living extremely long and healthy lives. Obviously you don't have to have a paleo diet to be healthy; it's impractical for most of the world and in fact only possible for very rich people.

How would a hunter gatherer get on in modern day London?

If you were to transplant a hunter-gatherer from the Kalahari to London, he or she would enjoy all the wonderful things of London, the tube, the escalators, all the great food, and they would get all the same problems that we do. It's not that people in the Kalahari go tramping about 9 to 15 kilometres a day because that's what they want to do - nobody gets up in the morning and says, hey, let's go for a jog in the Kalahari! They do it because it's their livelihood, they are coerced by their environment.

But now our environment coerces us to take the path of least resistance?

Suddenly we have all these machines and we can press all these magical buttons that move us around and allow us to live without exercise, and we're not equipped easily to make those decisions. It's very hard for me every time I walk into my building, I'm on the on the fifth floor, I struggle every single day not to take the elevator! One of the reasons I don't take the elevator is because I don't want to be called a hypocrite, it's a very powerful motivator! The fact of the matter is if we are going to effectively combat this epidemic, we have to find ways to respectfully force each other to do the right thing because we're not going to do it on our own. Everybody you know who exercises and eats a sensible diet has figured out forms of effective self-coercion.

What will humans look like in the future?

The current gaze into the crystal ball doesn't fill me with optimism. You know the movie Wall-E? We are headed that way, and unless we do something I see very little evidence that things are going to change very much. Maybe the problems will level off. So for example the US rates of childhood obesity have levelled off, so everyone's patting themselves on the back. In actual fact they've levelled off to unacceptable rates and most of those kids who are overweight or obese have a much higher possibility of suffering a wide range of diseases. China has had a tenfold increase in type two diabetes. India's a time-bomb of diabetes and obesity. The list goes on - Mexico is more obese than America. So I think the future of our planet is a lot more overweight people with a lot of very costly chronic diseases that will never be easily treated. So unless we really grapple with helping people change their diets and get them to be fit, these problems will continue to mount and cause economic woe and increased suffering. It's depressing, which is why I wrote the book!