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Abstract
Magnetic resonance images of the brain were obtained from 2 gorillas (Gorilla gorilla
gorilla), 4 orangutans (Pongo pygmaeus), 14 chimpanzees (Pan troglodytes), and 4 bonobos
(Pan paniscus). The region on the motor cortex of humans identified as responsible
for motor skill of the hand (the "knob") was identified in this sample on consecutive
1-mm axial scans. The shape of the knob area was traced on each scan from both hemispheres,
and the area from all scans was summed to calculate the knob volume. The width of
the knob was also measured and correlated highly with knob volume. A significant population-level
leftward asymmetry in the volume and width of the knob was revealed (p < .05). Species
differences in knob asymmetry and overall volume were not significant, but the variability
in overall volume between species was substantial. Selection for the evolution of
a neuroanatomical representation of the hand in primates and an evolutionary trend
toward population-level right handedness are discussed.
Scenarios regarding the evolution of cognitive function in hominids depend largely on our understanding of the organization of the frontal lobes in extant humans and apes. The frontal lobe is involved in functions such as creative thinking, planning of future actions, decision making, artistic expression, aspects of emotional behavior, as well as working memory, language and motor control. It is often claimed that the frontal lobe is disproportionately larger in humans than in other species, but conflicting reports exist on this issue. The brain of the apes in particular remains largely unknown. In this report we measure the volume of the frontal lobe as a whole and of its main sectors (including cortex and immediately underlying white matter) in living humans, and in post-mortem brains of the chimpanzee, gorilla, orang-utan, gibbon and the macaque using three-dimensional reconstructions of magnetic resonance (MR) scans of the brain. On the basis of these data we suggest that although the absolute volume of the brain and the frontal lobe is largest in humans, the relative size of the frontal lobe is similar across hominoids, and that humans do not have a larger frontal lobe than expected from a primate brain of the human size. We also report that the relative size of the sectors of the frontal lobe (dorsal, mesial, orbital) is similar across the primate species studied. Our conclusions are preliminary, because the size of our sample, although larger than in previous studies, still remains small. With this caveat we conclude that the overall volume of the frontal lobe in hominids enlarged in absolute size along with the rest of the brain, but did not become relatively larger after the split of the human line from the ancestral African hominoid stock. Aspects other than relative volume of the frontal lobe have to be responsible for the cognitive specializations of the hominids.
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