Different selection pressures on different brain regions in primate evolution? An evolutionary attempt to understand what makes humans (somewhat) unique.
By Karan Ahuja and Tanushree Dargar
There is an expansion in overall brain size in primates but different parts of the brain show a distinct evolutionary pattern along with the addition of some parts. The struggle for existence provides the selective advantage for the enlargement of some parts of the brain with the loss of function of some parts in order to enhance the sensory-motor function for the adaptation in the environment. Some studies stated the role of complexity in the social brain as one of the causes of unusual brain size increase in primates when compared with other vertebrates. Its main claim was due to unusual complexity in life, primates required comparably larger brain size. 1
The driving force for the mosaic evolution of various parts of the brain is linked to the function they perform.
Neocortex: The introduction of neocortex as a highly multi-laminated cortex is known to be the highly important event during the evolution of the mammalian telencephalon. This cortical region is known to be a most important nervous element as it is directly related to the emergence of those capabilities which differentiates humans from other mammals. Adult human neocortex contains various distinct functional areas like visual area in the occipital lobe, auditory area in the temporal lobe and the Broca’s language area in the left frontal lobe. The transcriptome of developing human neocortex is heterogeneous across distinct cortical areas. But the transcriptome of an adult human, as well as non-primate brain, reflect more similar gene expression patterns in the neocortex region. 2,3
Cerebellum: Cerebellum shows expansion in size but at a pace slower than rest of the brain, with an almost 2.9 times larger than expected in primates of similar body size which explains its involvement in language, neuro-cognitive functions. 4,5
Frontal lobe: The frontal lobe is as large as expected to be. It is more biased in white matter development, i.e. 4.7 times larger than their pongid sample. While the gray matter is only 3.6 times larger. The size of the primary motor area varies with the body size during human evolution; i.e. shows expansion with brain size. Premotor area also increases with the brain size. Large-brained primates and humans are shown to have disproportionate large frontal lobes and it is believed that the major cause of this is the role of frontal lobes for integration across sensory and association units and for those cognitive processes generally referred to as ‘executive functions’. 6
Prefrontal area: Prefrontal area is larger, both as a percentage of total brain volume, as well as allometrically. It too shows bias towards more white matter.
Visual cortex: The relative size of the Visual cortex is smaller than expected for the primate of the same size while it is ~1.5 times larger in absolute terms than it is in the chimpanzee. This explains the quality vision of humans. Humans show an increase in the size of the temporal lobe and hence elaboration in its functions as well. Fine movement of eyeballs led to the development of foveal vision in primates. Several theories suggested the role of visual mechanism in large brains evolution among primates. The relationship among visual mechanism and encephalization helps to explain large brains in frugivores. They suggested the coevolution of color vision and frugivory in primates as dichromatic vision (seen in lemurs and many New World monkeys) enables them to distinguish between fruits of similar color and trichromatic vision aids in the perception of fruits against green leaves. Thus, it was stated as parvocellular mediated colour vision may be the cause of larger brains in frugivores. Opioid receptor peptide agonist is expressed in the primary visual cortex in primates whereas, in mice, it is expressed in GABAergic interneurons.
Olfactory bulb: The size of the olfactory bulb is reduced comparatively which is consistent with the finding that olfaction in humans is relatively poor, with the enhancement in the function of other senses. 7
Temporal lobe: It shows an overall increase in volume with relatively more increment in white matter volumes. The enlargement in the temporal lobe is accustomed to the improved auditory functioning, memory, emotions, conceptual understanding.
After studying the evolutionary pattern of some of the major brain regions, we can say that evolution took place in accordance with the usage of the brain region.
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2. Bae, B.-I., Jayaraman, D. & Walsh, C. A. Genetic changes shaping the human brain. Dev. Cell 32, 423–34 (2015).
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