MRI Species Study Offers Evidence for Gradualist Account

There are two fascinating new papers about language evolution in Nature Neuroscience (institutional access required). The first, by Rilling et al is a comparative MRI scan of human and primates, looking at the circuit level activation of the arcuate fasciculus, the neural pathway that links two major language areas of the brain, Wernicke’s Area and Broca’s area. It is this pathway that is lesioned in Aphasiacs. They found a significant difference in the levels of connectivity and the location of the terminations of the pathway across the species. In two other control pathways where no difference was predicted, they found reasonable uniformity across the species.


The significance of this study is that the Chimpanzees had an intermediate level of connectivity between the weak and the strong connectivity of the Macaques and the Humans. This indicated a gradual evolution in these pathways the further the species split from a common ancestor. So this suggests that firstly, the underlying machinery for language evolution was shared in a common ancestor in these primate species, but that also the evolution towards language capacity was gradual rather than saltational.

In a similar study, Petkov et al looked at whether Macaques have a similar voice recognition centre in the brain as humans have, seeing as both species rely heavily on vocal communication to identify conspecifics. What they found was surprising, the Macaques did indeed have a voice area sensitive to conspecifics, just like humans, except that theirs is located in an entirely different area. In humans this processing unit is located in the superior temporal sulcus, in Macaques it is in the lateral sulcus, much higher up in the brain.

What this study indicates is that, like Rilling et al’s study, there is increasing evidence that the evolution to modern human communication was gradual and based upon the adaptation of existing structures rather than a sudden saltational or exaptational leap. Interesting stuff, and adds even more fuel to the complex debate over the speed and purpose of the evolution of the language faculty.

Ghazanfar, A. A. (2008). Language evolution: neural differences that make a difference. Nat Neurosci, 11(4), 382-384. doi: 10.1038/nn0408-382.

Petkov, C. I., Kayser, C., Steudel, T., Whittingstall, K., Augath, M., & Logothetis, N. K. (2008). A voice region in the monkey brain. Nat Neurosci, 11(3), 367-374. doi: 10.1038/nn2043.

Rilling, J. K., Glasser, M. F., Preuss, T. M., Ma, X., Zhao, T., Hu, X., et al. (2008). The evolution of the arcuate fasciculus revealed with comparative DTI. Nat Neurosci, 11(4), 426-428. .

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