A large brain is arguably the greatest defining characteristic that sets humans apart from other animals and, specifically, from other apes. Only cetaceans, such as dolphins and other toothed whales, have similarly large brains. Though they also have a large neocortex, the prefrontal cortex of dolphins is much more underdeveloped than in humans, an area that accounts for decision-making, problem-solving, and acting with long-term goals in mind. A major question is how the human brain developed so quickly in its relatively short period of time.
One of the first obstacles to overcome allowing for a large brain is the amount of energy required to support it. Even though the human brain only accounts for 2% of the mass of the body, it consumes 20% of the body's energy. In order for a large brain to develop, a species would need a consistent food supply in order to support such a power hungry organ. This could also explain the slowed adolescent development that we see in humans compared to other hominids. The high energy needs can be spread out over a longer period of time and the energy needs would not be as immediate. Also, the fact that the prefrontal cortex is one of the last regions to fully develop and isn't completed until the late teens to mid- twenties. This would explain many of the poor decisions made by teens and (at times) their inability to grasp the consequences of their actions.
But I digress. The incredibly fast evolution of the human brain may be controlled, in part, by noncoding regions of DNA. Specifically, the ZNF588 gene is a transcription regulating protein that is active in human, but not in chimpanzee forebrains. Errors in this gene will typically lead to mitochondrial diseases, which makes me wonder if the activation of this gene allows more energy to be accessed at critical times of brain development resulting in a larger cerebral cortex.
(Photo credit: National Institute of Mental Health)
Gene Mutations
Like everything in evolution, the raw material for natural selection is mutations. As discussed above, a mutation that allowed for the expression of a transcription protein turned on the ZNF588 gene. Two other genes are related to the brain development of mammals and possess mutations that coincide with the large brain development of humans. The microcephalin (MCPH1) and the abnormal spindle-like microcephaly associated protein (ASPM) have mutated faster than normal in human ancestors and are prime candidates for increasing the size of the cerebral cortex. The MCPH1 gene evolved quickly in the lineages leading up to Homo sapiens, whereas ASPM saw its fastest evolution occur in the time after human ancestors split from chimpanzees. Other genes, such as ARHGAP11B, also have been shown to influence the development of the brain after the separation from chimpanzees.
Selection for Big Brains
The real question is, "How could humans develop such big brains in such a short period of time?" I believe that the answer can still be seen within the context of natural selection, but the discrepancy can be explained because brain development is such an unusual characteristic to quantify the rate of natural selection. The rate of selection is proportional to the level of advantage that the mutation provides. In this video about Rock Pocket mice, the mutation of their coat color spread relatively quickly because the advantage was so great.
It's hard to understate the advantage of having a brain that can do so much. Even in regards to the large amount of energy it requires, this enhanced organ allows for problem-solving and predicting outcomes. The ability to communicate with others and express complex ideas could help them survive. The social aspect of pooling resources and developing complex hunting strategies to take down much more physically impressive game far outweigh the drawbacks of increased energy consumption and prolonged development. I think that the addition of mutations and the extreme advantage that they provide can explain the fast rate of encephalization that we see in humans.
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