Mice fed a diet of McDonald's burgers and fries not only put on weight but start to develop human-like genetic traits, scientists have found.
The same effect is not seen when mice are given a typical chimpanzee diet of raw fruit and vegetables.
Most of the changes seen in the mice occurred in the liver, where a number of genes linked to metabolism became more active.
According to researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig, the findings support the theory that food choices helped to make us human.
Although humans and chimps evolved from the same ancestor, they have very different diets.
Chimps are almost completely vegetarian, while humans consume large amounts of animal protein and fat, and also cook their food.
One of the Max Planck scientists, PhD student Mehmet Somel, said: "There have been numerous hypotheses proposing a role for dietary change in human evolution.
"One of the most popular is that a switch to a high quality diet allowed our ancestors to fuel larger brains (the "expensive tissue hypothesis)".
"This suggestion sounds very reasonable. But we had yet no direct evidence that human and chimp diets differed in their molecular effects.
"This we find in our study - with respect to liver - and also that diet-induced differences in mice overlap with differences between humans and chimps.