RKO (courtesy of Kobal).

Humans differ from our closest relatives, the great apes, in many ways. In particular, our cognitive abilities are much more advanced than theirs, allowing us to undertake such challenging and diverse tasks as building skyscrapers, splitting the atom and composing symphonies. But what is it about our brains that gives us these advanced cognitive abilities? Although overall the human brain is larger than those of the apes, brain size can't be the main factor because animals such as elephants and whales have still larger brains.

A popular explanation for the superior intellectual abilities of humans is that our frontal cortices — which contain areas of cortex that are responsible for many cognitive tasks — are larger than those of the apes when measured as a percentage of total brain size. This oft-repeated claim has, however, been tested and found wanting by Semendeferi et al., as they describe in Nature Neuroscience. In the largest study of its type, the authors took detailed magnetic resonance imaging scans of the brains of ten humans, nineteen great apes of various species, and nine lesser apes and monkeys. Surprisingly, they found that the frontal cortices of humans, defined by reference to the central sulcus, were no larger than those of the great apes in relation to the overall size of the brain, although the lesser apes and monkeys did have relatively smaller frontal cortices.

As Semendeferi and colleagues point out, a more informative analysis would look at the relative sizes of the subdivisions of the frontal cortex, and in particular the prefrontal cortex proper. However, identifying these areas from sulcal anatomy is very difficult, if not impossible, and a detailed cytoarchitectonic study — the only way to analyse these subdivisions definitively — would be extremely costly and very difficult. As a first step, the authors compared the sizes of the frontal cortices when the precentral gyri were excluded, on the grounds that the remaining cortex contains all of the prefrontal cortex and only a few other small cortical areas. Once again, they found no significant difference between the relative sizes of this section of cortex in humans and great apes.

How can we reconcile these results with previous studies that claimed to find large differences in the relative sizes of human and ape frontal cortex? One explanation might be sample size. Previous studies used much smaller groups of subjects and looked at fewer species, whereas Semendeferi and colleagues included several examples of every extant species of great ape. Differences in how the frontal cortex is defined could also have contributed.

Of course, size isn't everything. It is likely that the human frontal cortex differs in other ways from those of apes and monkeys — for example, in being more densely interconnected (as supported by data showing that more white matter underlies the frontal cortices in humans than in apes). In addition, specific areas of the frontal cortex might have evolved to be relatively larger at the expense of other subdivisions, without altering the overall size. But one thing is certain: we will have to consider more than just size if we are to figure out what makes humans so different.