The Paleo diet has won thousands of fans with its seemingly straightforward logic that what was good for our metabolism when we lived in caves is likely to be good for us today.
High in meat, fish and vegetables, it largely excludes dairy and cereal or anything else that emerged in the agricultural revolution, arguing that it was a protein-rich regime that fuelled massive brain growth for early humans.
But a study from University College London appears to cast doubt on that claim. It seems that early man did not steer clear of carbohydrates at all. In fact, early communities must have been eating starchy plants for such rapid brain growth.
Professor Mark Thomas said carbohydrates should be "put back" into the Paleo diet because it is likely that they, alongside meat, allowed humans to become the most dominant species.
"The global increase in the incidence of obesity and diet-related metabolic diseases has intensified interest in ancestral or 'Palaeolithic' diets," he said. "Surprisingly, however, there is little clear agreement on what quantitatively constitutes a healthy diet, or indeed a Palaeolithic diet, with much conflicting information.
"Eating food suited to the way our metabolisms evolved is a fantastic idea, but if you buy a book on the Paleo diet it's probably rubbish. We know so little about what the Palaeolithic diet was."
The Paleo diet, which appeared in the 1960s and uses the American spelling, is based on the idea that after the discovery of toolmaking, early humans were able to kill more animals and move away from a fibrous, plant-based diet to begin eating more meat. In recent years it has been championed by celebrities including actors Megan Fox and Uma Thurman.
By around 1.8 million years ago the human digestive tract had shrunk, which archaeologists suggest demonstrates that humans had started eating cooked meat which did not need as much time to digest. At the same time human brains began to increase in size, speeding up rapidly from around 800,000 years ago, in the Mid-Pleistocene.
A diet rich in meat was thought to be behind the changes but Professor Thomas said that it was a combination of the invention of cooking and the emergence of starch-digesting enzymes in the saliva and pancreas that allowed the rapid growth.
"Plant carbohydrates and meat were both necessary and complementary dietary components in human evolution," he said.
"Concentrated starch from plant foods was essential to meet the substantially increased and enlarged brain."
The Paleo diet has been criticised for neglecting important food groups, with the British Dietetic Association branding it a "dangerous fad" which risked triggering bowel cancer from eating too much meat.
Naveed Sattar, professor of metabolic medicine at the University of Glasgow, said the diet could help people lose weight through calorie restriction but eating a more balanced diet was sensible.
"The Paleo diet is in some form like the Atkins Diet in that it encourages greater protein intake and less carbohydrate intake, although perhaps it's better that more fibre is included.
"If one wishes to lose weight it does work, but note this is short-term."
Paleo diet: The rules
Meat from grass-fed livestock
Fish and other seafood
Fresh fruit and vegetables
Oils such as olive, coconut and flaxseed
Salt and sugar
Refined vegetable oils
Legumes such as beans, peas and lentils.