By Tim Spector, The Conversation

“Our ancestors didn’t eat like this, so we shouldn’t.” This is the main ethos of many modern diets which advise us to exclude a number of recent additions to our plates because they were not part of our distant predecessors diet. There are many different variations on the theme – from all-encompassing “palaeolithic-style” diets to grain-free or gluten-free regimes – which are all generating a massive boom in specialised shops, products and even restaurants.

The general idea is that for most of our millions of years of evolution we were not exposed to grains, milk, yogurt or cheese, refined carbs, legumes, coffee or alcohol. As they only came into existence with farming around 10,000 years ago, our finely-tuned bodies have not been designed to deal with them efficiently.

The belief is that human evolution via survival of the fittest and natural selection is a very slow process and our genes classically take tens of thousands of years to change. This means that these “modern” foods cause various degrees of intolerance or allergic reactions, resulting not only in the modern epidemic of allergies, but also that the toxins lead to inflammation and obesity. So follow our Palaeolithic ancestors we are told, cut out these foods – and your problems are over.

This may sound imminently sensible but as it turns out, the facts on which this idea is based are rubbish. The latest research shows we are not robotic automatons fixed in time but flexible plastic beings adapting to our environments and diets much faster than anyone had realised. A study published in Nature showed clearly that major changes to our genes can occur in just a thousand years or a few hundred generations.

The researchers looked at the DNA from 101 Bronze Age skeletons across Europe from The Netherlands to Russia for key mutations. These people lived around 3,000 years ago and were busy migrating and spreading their genes. They looked in particular at one key gene (called lactase persistence) that controlled an enzyme conferring the ability to digest milk after the age of three. Around three quarters of modern Europeans have this gene allowing them to digest a glass of milk without feeling sick. Rates of the gene mutation are higher in North Europe (up to 90%) and lower in Southern Europe (around 50%).

It was previously thought this gene mutation started to dominate Europeans around 7,000 to 10,000 years ago at the onset of farming and the use of milk, so the finding that only one in 20 Bronze-age people had it 3,000 years ago was a major shock. It meant that it started later and has spread much faster than we imagined and as a consequence we have adapted to our new food source much more rapidly than the lumbering robots we are portrayed as.

Other genetic evidence of recent changes to our digestive genes comes from a worldwide study of the amylase gene which is key to breaking down starch in carbohydrates. People in areas with starch as a major part of the diet evolved to have multiple copies of the gene to help them digest it better. We found in a collaborative study using our twins that this mutation also strangely protected against obesity, and importantly we think this change only happened in the last few hundred generations.

Other genes key to how we digest food can change even more rapidly. These are the 2m or so genes in the DNA of the trillions of microbes in our gut. Although they are not human genes they are crucial to our health as they control our microbiome which digests our food and produces many of our vitamins and blood metabolites. These bacterial genes in our guts can respond rapidly to changes in our diet, and as they can produce a new generation every 30 minutes, they can evolve very fast indeed.

They also have a secret weapon called horizontal gene transfer which means they can rapidly swap genes between them to mutual advantage, without waiting for natural selection. They use this very effectively to become resistant to new antibiotics and the same process is likely for new foods.

So by all means enjoy eating at going to trendy paleo steak restaurants and decide to lose weight in the short term by going on a gluten-free diet, but don’t be fooled by the evolutionary scientific explanations which are now out of date. Your genes and your microbes are evolving faster than you realise and can cope with the new additions to our diet in the last few thousand years. The caveat is that we need to keep our gut microbes as healthy as possible. But dietary diversity, not exclusions, is the key.

Tim Spector is Professor of Genetic Epidemiology at King’s College London.