The effect the modern western diet has had on our health and wellbeing has been a rumbling issue for decades; it has become more controversial recently thanks to the influential evolutionary perspective on the subject. When the organic food movement began, the premise was that traditional farming practices were the norm, from which we have departed by adopting intensive techniques to produce highly processed food. This could be dubbed the Prince Charles Line. But seen in the light of evolution, as Daniel Lieberman considers it, farming itself was the Great Unnatural Practice. At the extreme, the evolutionary perspective has given rise to the Paleo lifestyle, an attempt to return to the way we lived before the invention of farming 10,000 years ago: to live "as evolution intended".

Lieberman is far from endorsing such crass simplifications. Evolution, as he stresses, intends nothing. What actually evolves is due to many competing tendencies, and the result is always a work in progress with no destination. Notable figures including David Attenborough and the geneticist Steve Jones have remarked that human evolution has now ceased. Lieberman says this cannot be true: human beings continue to show a great degree of heritable variation, and reproductive success is also variable; so evolution of some kind must still be active. He goes further by regretting that for most of his career as a professor of human evolutionary biology he taught the old line that human biology has hardly changed since the Ice Age.

The question is: if human evolution is ongoing, what factors are influencing it now, as opposed to in the distant past? Genomic studies show that more than 100 genes indicate positive selection since farming began 10,000 years ago. Beyond the best-known example – the lactose tolerance gene, which has gone from almost zero to nearly 100% in northern Europe – most of them have not yet been characterised, but many almost certainly relate to the infectious diseases that accompanied hugely expanded human populations living hugger-mugger with domestic animals.

Lieberman is good at explaining why simplistic nostrums aren't appropriate for considering creatures such as human beings. We are not designed specifically to do one thing, but to be adaptable to many. Thus dark skin protects against UV radiation in equatorial regions but is gradually lost in northern climates probably because the weaker sunlight found there would generate inadequate levels of vitamin D in heavily pigmented skin. The body is designed to be good enough to do many things, but not to be especially good at any one of them. It doesn't have a large margin of error, unlike engineered structures such as bridges and aeroplanes, designed to withstand several times the stresses they will actually experience. Thus specialised animals out-compete us in speed, strength and visual or aural acuity, which was a problem when humans lived more or less in a state of nature.

Essentially, Lieberman's message is that modern life has created mismatches between our abilities and adaptations and the stresses we place on the body by often living thousands of miles from our family's origins, sitting down all day long rather than actively gaining a livelihood by physical work, and snatching at quick, high-energy foods.

The evolutionary approach produces some counterintuitive surprises. Fresh fruit juices are as much junk food as a cola drink – they produce a sugar rush, so it's better to eat fresh fruit with its additional fibre; chewing gum as a child is a healthy pastime, if the gum is sugar-free (teeth and jaws need the exercise that modern diets fail to provide). Without going the whole Paleo wild boar, Lieberman suggests we can learn something from the hunter-gatherer lifestyle. He is a celebrated advocate of barefoot running, for example.

Since his book is all about trade-offs between incompatible attributes, it is only right to point out that, while his message is sound, it could have been made at half the length. In discussing the human plague of obesity he seems unable to resist the textual bloat that is equally a trait of our times. But given the vast outpouring in recent years of crank diets and earnest attempts to correct western lifestyle ills by some kind of Rousseauist regression, The Story of the Human Body is a reliable guide to a problem that is going to get worse before it gets better.

• Peter Forbes is the author of Dazzled and Deceived: Mimicry and Camouflage.