A comparative neuroimaging study performed by researchers from Emory University in Atlanta, Georgia, in collaboration with colleagues from the University of Oxford, provides clues to how human language evolved.
In the past, it was believed that the increase in brain size during human evolution occured mainly to accomodate our complex linguistic abilities. But the findings of this new study suggest that the emergence of language also required major modifications in how the brain is wired.
Using diffusion tensor imaging (DTI, a type of functional magnetic resonance imaging which I described in this post about synaesthesia), Rilling et al compared the structure of arcuate fasciculus, a large white matter tract, in humans, chimpanzees and macaques.
It is believed that the organization and terminations of the arcuate fasciculus of humans and macaques differ somewhat. However, the fibre tract in each of the species has until now been examined using different techniques (DTI in humans and axonal tracing in macaques), and so a precise comparison has not been possible. In the current study, Rilling et al have, for the first time, compared the arcuate fasciculus of humans, chimpanzees and macaques, using the same method.
In humans, this bundle of nerve fibres connects two regions of the cerebral cortex that are vital for language. From diffuse regions of the frontal lobe, including Broca’s area, which is involved in speech production, the arcuate fasciculus projects backwards, arching around the Sylvian fissure, the prominent cleft which separates the frontal and temporal lobes. It then descends deep into the temporal lobe, where it branches and terminates in several different areas, including Wernicke’s area, which is known to process the meaning of words.
By contrast, the connections between frontal and temproal lobes in the chimp and macaque are much weaker, as illustrated in the schematic diagram above. In chimps, the arcuate fasciculus branches less extensively in the temporal lobe, and makes far fewer connections there. And in macaques, the fronto-temporal connections are weaker still; the fibre tract barely reaches the temporal lobe, and does not form any branches.
This study therefore provides some evidence that the organization of the arcuate fasciculus was strongly modified in the human lineage, and that this was one of the brain specializations that led to the emergence of language.
In the macaque, the brain regions analogous to Wernicke’s area are involved in higher order processing of visual information. It seems, then, that when humans diverged from other primates during the course of evolution, another specialization necessary for language was the addition of new cortical fields such as Wernicke’s area.
This would in turn have necesitated a reorganization and enlargement of the arcuate fasciculus (leading to a disproportionate increase in the white matter volume of the frontal and temporal lobes), and would have displaced the visual cortical areas towards the back of the brain.
Rilling, J. K., et al. (2008). The evolution of the arcuate fasciculus revealed with comparative DTI. Nat. Neurosci. doi: 10.1038/nn2072. [Abstract]