This year I attended the pre-SFN meeting on Evolutionary Neuroscience by the J.B. Johnston Club. I enjoyed the meeting a lot (though was somewhat baffled by their obsession with isometric lines with slope 1…) and ended up bumping into a couple of comparative papers on the amygdala (that I should have known about).
Although fairly crude, one can gain insight into brain evolution by measuring volume or counting cells across brain regions and species. This has led to much debate, for instance, regarding the PFC and its possible “enlarged status” in humans. If you do that for different amygdala nuclei, you find that “the human amygdala is evolutionarily reorganized in relation to great ape amygdala”.
This quote is also quite revealing: “Neuron numbers in the human lateral nucleus were nearly 60% greater than predicted by allometric trends, a degree of magnitude rarely seen in comparative analyses of human brain evolution (Sherwood et al., 2012). For example, the volume of the human neocortex is 24% larger than expected for a primate of our brain size (Rilling and Insel, 1999), whereas the human frontal lobe, long assumed to be enlarged, is approximately the size expected for an ape of human brain size (Semendeferi et al., 2002; Semendeferi and Damasio, 2000).”
So much for such a highly conserved structure… Interesting also that the authors discuss “evolutionary specializations” of the amygdala in terms of the social brain, not “fear processing” (as for instance described in this previous post).
Reference: Barger, N., Stefanacci, L., Schumann, C. M., Sherwood, C. C., Annese, J., Allman, J. M., … & Semendeferi, K. (2012). Neuronal populations in the basolateral nuclei of the amygdala are differentially increased in humans compared with apes: a stereological study. Journal of Comparative Neurology, 520(13), 3035-3054.
The other reference is also interesting: Barger, N., Stefanacci, L., & Semendeferi, K. (2007). A comparative volumetric analysis of the amygdaloid complex and basolateral division in the human and ape brain. American journal of physical anthropology, 134(3), 392-403.