This article was taken from the September issue of The Critic. To get the full magazine why not subscribe? Right now we’re offering three issue for just £5.

Our ancestors have spent a few hundred years in cities at most. Before that, they spent a million years or more on what was essentially a perpetual camping trip, most of it in Africa. Little wonder then that people are more easily scared of snakes than cars, of deep water than speed, of spiders than guns. We are, to a significant extent, adapted to the environment we evolved in, rather than the one most of us now inhabit.

This mismatch explains quite a lot about our modern problems, and Adam Hart, an entomologist and broadcaster, has set out to see just how far, and how convincingly, mismatch can explain things like allergies, obesity, and our addiction to drugs, social media and even fake news. His book is especially valuable because it does not fall for simplistic “just-so” stories without checking the actual evidence first.

In the case of obesity, for example, the mismatch theory goes that our ancestors would have gorged on sugary fruit and fatty grubs whenever we got the chance, because a famine was just around the corner. Now there’s no famine but perpetual abundance, this tendency makes us fat and diabetic. This is known as the thrifty gene hypothesis and it makes superficial sense.

But Hart finds it wanting. With the exception of Samoans, who put on weight especially easily but do not become diabetic easily probably because of very long, very hungry canoe journeys in which the fat survived and the lean did not, most of the evidence points away from thrifty genes once examined in detail. For instance, only nine of 115 genes associated with obesity show evidence of selection and five of those have been selected to promote lean bodies.

Rather, Hart argues, what makes us put on fat today is that we no longer get chased by sabre-toothed tigers and the like. When we ceased to become regular prey, which was quite a while ago, the disadvantage of being tubby disappeared. So our genes “drifted” into allowing us to put on too much weight once confronted with easily available and affordable carb-rich diets.

The steady increase in auto-immune disease, allergy and dietary intolerance may be best explained by an impoverishment in the range and abundance of bacteria that inhabit our guts. We have become so relentlessly hygienic that we no longer get the bugs in our system that the body expects, and this causes an immune over-reaction.

Here I feel Hart sells his own thesis short. This “hygiene hypothesis” never fully made sense for me until I read An Epidemic of Absence, by Moises Velasquez-Manoff. That book solved my problem, which was puzzlement as to why the immune system should overreact if underused. It’s not as if it gets bored. Velasquez-Manoff’s sources pointed out that a parasite quickly evolves the ability to dampen the immune reaction of its host. That starts an evolutionary arms race, in which the host evolves an overreactive immune system. Take away the parasite and the thing overreacts against grass pollen.

Here is another intriguing mismatch. Why do we like alcohol so much that we get addicted to it?

Sure enough, there is abundant evidence that removing worms from (for example) Ethiopian villages is followed by the appearance of hay fever, asthma and the like for the first time. Experiments with mice have found that infecting them with worms reduces allergies, inflammatory bowel disease and even diabetes and multiple-sclerosis-like conditions.  Treating people with worms has also been shown to work against these diseases, though it’s hardly a good bargain since worm infections are no fun. Hart mentions worms only in passing, focusing mainly on bacteria, but there is little doubt that many of our modern conditions derive from the extinction of both parasites and friendly creatures within us.

In an ingenious chapter, Hart argues that our problem with social media stems from the fact that we are designed to have a social network of about 150 people, not thousands. This number, named after the evolutionary biologist Robin Dunbar, derives from a neat correlation between brain size and group size in primates and fits surprisingly neatly with the number of people in an old-fashioned address book, or the size of a company before it becomes too unwieldy to manage. Today we have real and virtual social networks that far exceed this number, and we are not well placed to cope.

Here is another intriguing mismatch. Why do we like alcohol so much that we get addicted to it? Neatly, there are genetic clues. A mutant version of a gene called ADH4 arose around 10 million years ago in the ancestors of African great apes, making us 40 times as good at digesting ethanol. Getting tipsy was a small price to pay for the calories. It probably gave significant advantage to foraging on fallen, rotting fruit.

Rosling pointed out that a chimpanzee would pick up the right answer six times as well as humans

Along with other drugs which stimulate the brain system that is triggered by orgasm, argues Hart, “the modern environment is one of great temptation and a wealth of new and potent opportunities to highjack a brain that evolved to enjoy, and reward, the pleasures of sex and ripe berries.”

Perhaps the most original of Hart’s arguments is the one where he explains fake news. We have a deeply-evolved tendency to take things on trust. “There is a very thin line between the trust necessary for everyday life and the gullibility or credulity that can lead us towards believing fake news.” We put a halo on our heroes, either as a neurological side effect of the tendency to fall in love with high-status mates, or as a by-product of having followed leaders into battle against enemy tribes for millions of years. Us-and-them tribal loyalty, a deeply-ingrained human tendency, then does the rest, leaving us polarised into groups with unshakeable beliefs in “facts” that are sometimes unshakeably wrong.

My favourite example of this tendency is the experiment that Hans Rosling, the now sadly dead author of Factfulness, did with more than 1,000 people. He simply asked them, “Has the percentage of the world population that lives in extreme poverty (a) halved, (b) doubled, or (c) stayed the same in the past 20 years?” Only 5 per cent got the right answer that it had halved; 65 per cent thought it had doubled. Rosling pointed out that if he wrote the three answers on three bananas and threw them to a chimpanzee, it would pick up the right answer 33 per cent of the time, doing six times as well as human beings at answering a question about human society.

It is this mismatch, between our tendency to believe persuasive people and our exposure to charlatans with global megaphones, that explains the worst aspect of the modern world: the headlong rush into cults, whether of Trump, Muhammad, Marx, Corbyn, Ayn Rand or Foucault. (That’s my list, not Hart’s, by the way.) This book is therefore a good place to start to understand almost everything about the twenty-first century, from obesity to Donald Trump, by recognising that we are still apes.