How come our brains are bigger than those of other mammals?
It all began about 3 million years ago. From that period until the emergence of Homo sapiens, some 200.000 years ago, our brains almost quadrupled: from 350 cc to 1300 cc. Of the three main parts of the brain, the neocortex (cerebral cortex), the cerebellum, and the brain stem (including the limbic system where emotions are generated), the growth manifested itself primarily in the neocortex. In general terms, the neocortex has the function to control and refine our emotions, bolstered by a huge memory and by sensory and motor fields, which process the incoming information and control conscious locomotion. Apart from that, the neocortex contains areas where voice and language are formed. For the more abstract issues such as association, analytical thinking, creativity and intuition no specific areas are known, although the prefrontal cortex seems to play a role in this.
Up until now scientific research has come up with different but overlapping explanations for this extraordinary phenomenon. First, in 2011, Oliver Fedrigo at de Duke University in North Carolina published an article about the energy supply of the brain, in which he compared humans with chimps. His genetic research showed that the interaction of several genes causes the ability of the brain to reach such a big volume. Our brains make up 2% of our body weight and 20% of our energy consumption. In other primates the latter is 7%. The most prominent energy supplier is glucose. So-called glucose transporters determine the amount of glucose that enters the brain (as well as other energy-hungry body parts, like muscles). He found several genes that code for these carriers, different for brain and muscles: one that encodes for the activation of the glucose transport in the brain, another for the same in muscle tissues. These two genes were compared in chimpanzees and humans. It turns out the human versions of these genes are a mutated version of that of the chimpanzee. The human DNA produces glucose transporters for the brain three times more than in the studied apes. Conversely, chimpanzees transfer much more glucose to their muscle tissue.
In her laboratory at the Institute of Biomedical Sciences at the Federal University of Rio de Janeiro, Suzana Herculano-Houzel routinely dissolves brains into a soup of nuclei — cells’ genetic control rooms. Each neuron has one nucleus. By tagging the nuclei with fluorescent molecules and measuring the glow, she can get a precise tally of individual brain cells. Using this method on a wide variety of mammalian brains, she has shown that, contrary to long-standing assumptions, it is not so much the brain volume, but foremost the number of neurons and their distribution that defines the ‘intelligence’ of a brain. Although the human brain is smaller than that of elephants and whales, the human neocortex contains 16.3 million neurons, while the elephant’s similar brain part has only 5.6 million neurons. So it is all about the density of neurons1.
In a pair of papers publishing in Cell (a journal that publicizes new cell biological research results)2, two teams of researchers identified a gene family (NOTCH2NL), that appears to play an essential role in human-specific cortex development and may have been a driving force in the evolution of our large brains. These genes delay the differentiation of cortical stem cells into neurons, resulting in the production of more neurons across the course of development. The genes are found exclusively in humans, are heavily expressed in neural stem cells of the human cerebral cortex, and are located on the part of the genome implicated in neurodevelopmental disorders.
Plainly said, in (proto-) humans a gene family that regulated the differentiation of neural cells in the cortex underwent mutations that slowed down this process and made neurons keep on duplicating much further than in other mammals. This gene family is located in a part of our genome that is also responsible for (other?) disorders as macrocephaly3, microcephaly (lately also linked to the Zika-virus) and is associated with a range of neurodevelopmental ailments, including ADHD, autism spectrum disorder, and intellectual disability. From this, one may boldly conclude that big brains are rather a disorder which happened to be beneficial for survival.