Frontal Lobe Size
Certainty styling is being phased out topic by topic.
Hover over keys for definitions:The frontal lobe is defined as the portion of the brain anterior to the central sulcus. Absolutely, the size of the human frontal lobe is approximately 3-4 times that of great apes; however, information to date suggests that evolutionary increase in the relative size of the entire frontal lobe does not distinguish humans from apes. The frontal lobe does not show disproportionate volumetric increase in humans relative to great apes (Semendeferi and Damasio, 2000). Proportinately, the human frontal lobe occupies approximately 35-38.5% of the cerebral hemispheres, which does not fall discretely outside of the ranges found in all great ape species. Nor are human frontal lobe volumes greater than would be predicted based on regressions drawn through nonhuman primate data (Semendeferi and Damasio, 2000).
The frontal lobe is the largest lobe of the human cerebral cortex and have long been theorized to be the seat of human cognitive and intellectual advancement (Semendeferi, et al, 1997). In his influential anatomical study, Brodmann (1912) reported that the frontal lobe of a chimpanzee occupied a smaller percentage of the cerebral hemisphere than in a human. Some subsequent classical analyses were able to replicate these results, while other analyses found that, as von Bonin (1948) stated, "man has precisely the frontal lobe which he deserves by virtue of the overall size of his brain''.
With the advent of magnetic resonance imaging technology, it has become possible to more reliably assess human frontal lobe evolution by employing larger comparative samples including all great ape species, especially given the relative ease of distinguishing this lobe based on sulcal morphology (Semendeferi and Damasio, 2000; Semendeferi, et al., 1997). These more recent analyses have predominantly supported von Bonin's conclusions. Contemporary analysis of larger post-mortem samples echoes this support (Smaers, et al., 2011).
However, the frontal lobe is functionally heterogenous and comprised of more than a dozen discrete cytoarchitectonic territories. It has been suggested that subdivisions of the frontal cortex that are more associated with working memory and executive function, like the frontal pole (Semendeferi, et al., 2001), may be especially expanded in human evolution despite relatively conservative changes in overall frontal lobe size.
The absolute size of the frontal lobe is larger in humans than in apes, but the human frontal lobe does not appear disproportionately expanded when controlling for overall increases in brain size.
Universal
To date, modern evidence suggests that the large size of the human frontal lobe may be accounted for by the large size of the human brain coupled with a tendency for frontal lobe size to increase disproportionately as brain size increases in primates, as a taxon. Across primates, frontal cortex volume, measured from magnetic resonance imaging, shares a clear positive allometric relationship with brain volume (exp=1.18), occupying an increasingly larger percentage of the brain as brain size increases (Bush and Allman, 2004). Using post-mortem cases, human data also fall in line with predictions from non-human primates when changes across the antero-posterior axis of the lobe are considered, e.g., humans do not appear to show disproportionate increase in more anterior portions of the frontal lobe (n.b., discrete cytoarchitectonic areas in the frontal lobe were not individually assessed) (Smaers, et al., 2011).
To date, evidence suggests that the large size of the human frontal lobe may be accounted for by the large size of the human brain coupled with a tendency for frontal lobe size to increase disproportionately as brain size increases in primates, as a taxon. Across primates, frontal cortex volume, measured from magnetic resonance imaging, shares a clear positive allometric relationship with brain volume (exp=1.18), occupying an increasingly larger percentage of the brain as brain size increases (Bush and Allman, 2004). Using post-mortem cases, human data also fall in line with predictions from non-human primates when changes across the antero-posterior axis of the lobe are considered, e.g., humans do not appear to show disproportionate increase in more anterior portions of the frontal lobe (n.b., discrete cytoarchitectonic areas in the frontal lobe were not individually assessed) (Smaers, et al., 2011).
Despite the absence of evidence for an evolutionarily derived expansion of the human frontal lobes, it has been argued that overall increases in frontal lobe size can easily account for cognitive differences related to planning and executive function observed between humans and other apes (e.g., Gibson, 2001). At the anatomical level, comparative data on human frontal lobe volume largely evidence evolutionary continuity with great apes and other primates. In the search for human uniqueness in the frontal lobe, many research efforts have turned to investigating possible differences below the level of gross volume, e.g., local and long-range connectivity (e.g., Schenker, et al., 2005; Smaers, et al., 2011; Semendeferi, et al., 2010), differences in the number of cells in particular classes (e.g., glial cells: Sherwood, et al., 2006; cholinergic cells: Rhaganti, et al., 2008; interneurons: Sherwood, et al., 2010), and variation in discrete cytoarchitectonic territories (e.g., Semendeferi, et al., 2001; Schenker, et al., 2010).
Dramatic (hyperallometric) increase in frontal lobe volume relative to whole brain volume has been observed in the primate lineage, but not in carnivores (Bush and Allman, 2004).
References
-
Primate prefrontal cortex evolution: human brains are the extreme of a lateralized ape trend., , Brain Behav Evol, 2011, Volume 77, Issue 2, p.67-78, (2011)
-
Spatial organization of neurons in the frontal pole sets humans apart from great apes., , Cereb Cortex, 2011 Jul, Volume 21, Issue 7, p.1485-97, (2011)
-
Broca's area homologue in chimpanzees (Pan troglodytes): probabilistic mapping, asymmetry, and comparison to humans., , Cereb Cortex, 03/2010, Volume 20, Issue 3, p.730-42, (2010)
-
Inhibitory interneurons of the human prefrontal cortex display conserved evolution of the phenotype and related genes., , Proc Biol Sci, 04/2010, Volume 277, Issue 1684, p.1011-20, (2010)
-
Differences in cortical serotonergic innervation among humans, chimpanzees, and macaque monkeys: a comparative study., , Cereb Cortex, 03/2008, Volume 18, Issue 3, p.584-97, (2008)
-
Evolution of increased glia-neuron ratios in the human frontal cortex, , Proc Natl Acad Sci U S A, Sep 12, Volume 103, Number 37, p.13606-11, (2006)
-
Neural connectivity and cortical substrates of cognition in hominoids., , J Hum Evol, 2005 Nov, Volume 49, Issue 5, p.547-69, (2005)
-
Bigger is better: primate brain size in relationship to cognition. , , Evolutionary anatomy of the primate cerebral cortex, Cambridge , p.79-97, (2001)
-
Prefrontal cortex in humans and apes: a comparative study of area 10., , Am J Phys Anthropol, 2001 Mar, Volume 114, Issue 3, p.224-41, (2001)
-
The brain and its main anatomical subdivisions in living hominoids using magnetic resonance imaging., , J Hum Evol, 2000 Feb, Volume 38, Issue 2, p.317-32, (2000)
-
The evolution of the frontal lobes: a volumetric analysis based on three-dimensional reconstructions of magnetic resonance scans of human and ape brains., , J Hum Evol, 04/1997, Volume 32, Issue 4, p.375-88, (1997)
-
The frontal lobe of primates; cytoarchitectural studies., , Res Publ Assoc Res Nerv Ment Dis, 1948, Volume 27 (1 vol.), p.67-83, (1948)
-
