The obesity crisis gets ever larger. According to Dame Sally Davies, England's chief medical officer, being overweight is increasingly seen as the norm. Astonishingly, almost two-thirds of adults and one-third of children aged 2 to 15 in Britain are overweight or obese. As Davies points out, this represents “a profound change in the health of the nation over a relatively short period of time”. In 1980, around 7% of adults were clinically obese. Today, it is a quarter. Her report can be read here

When I heard this story, what caught my ear was not so much the – now depressingly familiar – news of obesity itself but the surrounding discussion of why it is a problem. The report itself was (wisely?) cagey about this. “The causes of obesity are complex, multi-factorial, and not completely understood”.

Commentary was less so. One fascinating discussion in particular, on BBC Radio 4’s PM programme, struck me. 

This pitted Susan Jebb, the Professor of Diet and Population Health at Oxford University, against Henry Dimbleby, the co-founder of Leon restaurants. It was a thoughtful discussion, free from caricature or finger-pointing, which circled round public policy, cultural norms, individual choice – and evolution.

Half-way through the discussion, Dimbleby offered an evolutionary explanation for our obesity ‘crisis’. From an evolutionary point of view, he reasoned, sweet and fatty foods were rare and much-needed. “We are programmed when they come in front of us to eat as much as we can because we’re going to need that to fight dinosaurs.” The dinosaurs – a nice touch – are no longer around but we still carry around the behaviour traits that were encoded into our genes during the Pleistocene.

“We evolved to have a very very sweet tooth and like very calorie dense food,” he went on to explain. “For the large part of human existence, we’ve been living in a situation when food was scarce and therefore we don’t have a mechanism that tells us to stop eating this stuff. There wasn’t an evolutionary pressure to do that.”

I have heard this explanation given many times over the years (usually whenever data on obesity is published) and it makes intuitive sense – at least until you begin to think about it.

It probably did indeed make sense to eat sweet and calorie dense food whenever we got our greasy prehistoric hands on it. It would certainly have aided our survival. But the idea that this sensible narrative, with or without dinosaurs, is the reason that we don’t have a “mechanism” that tells us to stop eating such food is – shall we say – problematic.

Firstly, who is the ‘we’ here? Did the one in three adults in the UK who are not overweight not evolve? Are they not subject to the same evolutionary urges that apparently trap so many people in obesity?

Second, was there not a surfeit of fat and sweet foods in 1980 (when there were far fewer overweight people) or indeed in 1960? My memories of the time are hazy, but I don’t recall empty shelves or a dearth of burger bars at the time.

Third, what about evolutionary counter-narratives? How about this one:

Humans evolved during the Pleistocene in an often dangerous and hostile environment. Those who were fitter, stronger and faster stood a better chance of survival than those who were slower or weaker. The evolutionary scales were tilted in favour of speed, strength and agility and even though we don’t have to run towards (or away from) our prey today, the urge for speed, strength and agility remains encoded in our genes. Just look at any primary school playground or weekend sports field if you doubt it. That’s why we have this epidemic of fitness freaks today, and why sports clubs are bursting at the seams.

Personally, I find this story about as credible as the ‘evolution makes us fat’ one.

Lastly, what exactly is a “mechanism” that tells us to stop eating food, and why does it have to be an evolutionary one? Is not the simple fact of feeling unwell or being able to do as much as you could if you ate well, a ‘mechanism’? What about the quintessentially human factor that is culture, whether in the form of encouragement from friends and loved ones or more general social opprobrium? Is not culture precisely the mechanism that can turn off, or at least turn down, evolutionary pressures?

In his defence, Dimbleby was not (I am sure) simply saying ‘evolution makes us fat’ (as the rest of the radio debate showed). But the use of this narrative, by someone who is not by profession an evolutionary biologist (people who are, I expect, particularly vulnerable to this, er, ‘meme’) does show how deeply such evolutionary Just So stories have penetrated into our culture, neatly bypassing our cognitive faculties and settling down into the comfortable positions of reserved for received wisdom.

Oscar Wilde once remarked that “everything to be true must become a religion”. Just so with evolution, as it accumulates the myths and legends that no respectable religion would be seen in public without.

Nick Spencer is Research Director at Theos

Image from wikimedia.com available in the public domain