Usually assumed hypothesis suggests early humans were able to redirect energy to their brains because of a reduced digestive tract.
But Zurich primatologists have now disproved this theory, demonstrating that mammals with relatively large brains actually tend to have a somewhat bigger digestive tract.
Ana Navarrete, PhD student and the first author on the study, has studied hundreds of carcasses from zoos and museums to come to the conclusion.
"The data set contains a hundred species, from the stag to the shrew," explained Navarrete.
The scientists involved in the study then compared the size of the brain with the fat-free body mass.
Senior author Karin Isler stresses that, "it is extremely important to take an animal's adipose deposits into consideration as, in some species, these constitute up to half of the body mass in autumn."
But even compared with fat-free body mass, the size of the brain does not correlate negatively with the mass of other organs.
Nevertheless, the storage of fat plays a key role in brain size evolution.
The researchers discovered another rather surprising correlation: the more fat an animal species can store, the smaller its brain.
Although adipose tissue itself does not use much energy, fat animals need a lot of energy to carry extra weight, especially when climbing or running. This energy is then lacking for potential brain expansion.
"It seems that large adipose deposits often come at the expense of mental flexibility," said Isler.
"We humans are an exception, along with whales and seals - probably because, like swimming, our bipedalism doesn't require much more energy even when we are a bit heavier," she added.
The study has just been published in Nature.