Human frontal lobes are not relatively large

Abstract

One of the most pervasive assumptions about human brain evolution is that it involved relative enlargement of the frontal lobes. We show that this assumption is without foundation. Analysis of five independent data sets using correctly scaled measures and phylogenetic methods reveals that the size of human frontal lobes, and of specific frontal regions, is as expected relative to the size of other brain structures. Recent claims for relative enlargement of human frontal white matter volume, and for relative enlargement shared by all great apes, seem to be mistaken. Furthermore, using a recently developed method for detecting shifts in evolutionary rates, we find that the rate of change in relative frontal cortex volume along the phylogenetic branch leading to humans was unremarkable and that other branches showed significantly faster rates of change. Although absolute and proportional frontal region size increased rapidly in humans, this change was tightly correlated with corresponding size increases in other areas and whole brain size, and with decreases in frontal neuron densities. The search for the neural basis of human cognitive uniqueness should therefore focus less on the frontal lobes in isolation and more on distributed neural networks.

How the flow and processing of information shapes the cerebrum

“…However, as the sum of two different powers (e.g., x 1 + x 1.5 or x + 1) is never a power relation, at least some of the above relations do not truly reflect a 50 power law . Similar relations with little variability from the trend have been reported for the number of neurons in the cerebrum, the cerebellum and the rest of the brain (Barton and Venditti, 2013.…”

How the flow and processing of information shapes the cerebrum

“…Similar relations with little variability from the trend have been reported for the number of neurons in the cerebrum, the cerebellum and the rest of the brain (Barton and Venditti, 2013.…”

The Human Frontal Lobes: Not Relatively Large but Still Disproportionately Important? A Commentary on Barton and Venditti

“…An often-cited candidate is the expansion of the frontal regions of the neocortex which, it has been argued, became exceptionally large during human evolution [Rilling, 2006]. Recent work, however, suggests that this was not the case [Barton and Venditti, 2013].…”

“…The whole neocortex becomes disproportionately large as brain size increases because white matter, which contains nerve fibres connecting different structures, expands much more rapidly than grey matter or neuron number. Although the ballooning of white matter probably reflects size-related constraints on maintaining connectivity, it can produce the appearance that the cortex -or a specific cortical region -is selectively enlarged when viewed as a proportion of total brain size [Barton and Venditti, 2013].…”

“…Shared ancestry leads to the expectation that more closely related species should resemble each other more than more distant relatives, and renders comparative data non-independent. Using comparative methods that correct for this non-independence, Barton and Venditti [2013] re-visit several datasets to address the issue of allometry and resolve the debate over the size of the human frontal lobes. To do so they examine the relationship between the size of frontal and nonfrontal structures across non-human primates and assess whether humans fall withIt is widely accepted that there was something exceptional about human cognitive evolution.…”