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The Genetic Origins of Human Intelligence

The human neocortex is roughly three times as great as would be expected for a primate.

It has been know for a while that the ARHGAP11B gene is unique to humans. The ARHGAP11B gene protein is located in mitochondria. This gene causes the stem cells in the brain to multiply, in a manner similar to rapidly-dividing tumour cells (basal progenitor amplification). The result is an explosion of stem cells in the brain and a larger brain. That could be the single reason why the human neocortex is about three times bigger than that of our closest relatives, the chimpanzees. The mechanism was explained in 2020 by Wieland Huttner's team at the Max Planck Institute for Molecular Cell Biology and Genetics (paper). A few months later the same team, in collaboration with Hideyuki Okano of Keio University in Japan (a pioneer of the technology to generate transgenic non-human primates) and the Central Institute for Experimental Animals in Japan, transported the gene into monkeys: they generated baby transgenic marmosets equipped with the gene ARHGAP11B. After 101 days, the team measured their neocortex and found that it was both enlarged and folded, just like the human brain is (paper). This seems to prove that this gene alone causes primate brains to develop a larger neocortex.

Huttner's team had already shown (in 2015) that this was the case for mice and ferrets but this is the first time that the fact has been proven for a primate (paper).

A hypothesis is that this gene evolved by partial duplication of ARHGAP11A, which is ubiquitous in animals. The ARHGAP11B protein contains a sequence of 47 amino acids that is not found in the ARHGAP11A protein, and this sequence could well be responsible for our bigger brain. In 2016 Huttner's team showed that these 47 amino acids are the result of a simple mistake, a single C-to-G base substitution in the gene, that probably occurred between 1.5 million and 500,000 years ago, which causes the loss of 55 nucleotides from the messenger RNA, which causes a shift in the reading frame, which causes the 47 amino acid sequence (paper).

One can therefore hypothesize that a tiny genetic mutation made human brains bigger than the brains of all other mammals. Specifically, it's the neocortex that is unusually big in human brains.

The neocortex is the most recently evolved part of the cortex. It only exists in mammals (although it is similar to the dorsal cortex of reptiles). It first appeared in the early small mammals that emerged from their reptilian ancestors during the transition of the Triassic/Jurassic periods.

It is hard to understate the importance of the neocortex. There are many features of the human neocortex that are unique. The human brain contains about 86 billion neurons. The cerebellum contains about 80% of the brain's neurons, but it constitutes only about 10% of brain mass/ volume. The cerebral cortex contains only about 20% of all neurons (about 16 billion), but it constitutes about 80% of brain mass because of larger neurons and many more dendrites, axons, synapses, etc. In most mammals the olfactory system dominates the senses (and so Lucia Jacobs for example thinks that the brain evolved from a "nose"). Humans are an exception: our sense of smell is not particularly good, but other cognitive skills are more sophisticated.

There are several features of cortical development that differ from the development of other organs of the body and even from the rest of the brain. For example, none of its constituent neurons is generated within the cortex itself. There is an orderly migration of neurons from their birthplace towards the outer layers of the neocortex: each new neuron moves from inside to outside. In 2004 Todd Anthony at the Rockefeller University showed that radial glial progenitors in the ventricular zone produce almost all excitatory neurons in the neocortex (paper). Anthony Rossi and Vilaiwan Fernandes at New York University showed that a mechanism of temporal patterning presides over this sequential production of distinct types of neocortical neurons (paper). The neocortex has six layers and neurons in each layer have different functions. Progenitors sequentially generate distinct types of neurons that migrate outwards and settle in a specific layer, from deep to superficial. The end result is that the identity and location of a cortical neuron is tightly related to the neuron's birthdate. At the beginning of neurogenesis the first-born neurons move to the deepest layer of the neocortex, while late-born neurons will migrate through these early neurons to settle in the superficial layers of the neocortex.

The neocortex consists of glutamatergic excitatory neurons and GABAergic inhibitory interneurons. Glutamatergic neurons generate the main output of neural circuits. GABAergic interneurons provide a rich repertory of inhibition that regulates circuits. Neocortical interneurons are incredibly diverse in their morphology, chemical expression, electrical properties, and synaptic connectivity. Most attention is usually reserved to excitatory neurons, the neurons that triggers electrochemical activity in neighboring neurons. Inhibitory neurons are not as numerous but their function, which is the opposite, seems important to properly modulate brainwaves. A single inhibitory neuron can coordinate the work of thousands of excitatory neurons, switching them on and off precisely to produce the correct rhythms for memory and other functions. The result of this coordination is brainwaves that we can measure, and this brainwaves reflect the flow of information within the brain. When inhibitory neurons don't work well, it's like a feedback device that stops working well in a motor: neurons fall out of synch and the brain machine doesn't work as well.

The neocortex is therefore the theater of a complex interplay of signals. A hypothesis is that this complex system is what creates the symbolic mind of humans. That symbolic mind is responsible for human civilization in all its power and oddities. Humans covered the planet with roads, buildings and plastic. Humans waste time and resources to bury the dead (think of the pyramids), waste time and resources to worship invisible deities (and even fight wars over them), waste time and resources in all sorts of rituals (think of traditional weddings, or of clothes like suits & ties, tuxedos, gowns). The evolution of human behavior doesn't make a lot of sense. Even the domestication of animals makes no sense (100 goats per person in 10,000 BC!) In fact, nomadic barbarians (Germans, Mongols, Turks) came to dominate the world From the point of view of an ant there is nothing interesting about human artifacts: what is the use of a road or a skyscraper for an ant? Each species makes things that are useful to itself. Human artifacts are useful to humans, but not to most of other animals. What is truly exceptional about humans is the sheer amount of things that we can make (how much we can change the planet).

A random mutation in the brain of Homo Sapiens caused that brain to grow a larger neocortex which made humans to think in complicated ways, i.e. to create a symbolic system, which caused domestication, agriculture, cult of the dead, religion, city-states, etc. After the fact, we (the abovesaid mutated brains) convinced ourselves that the new brain (our brain) was smarter than the old one, and that the various innovations make a lot of practical sense.

Not only a genetic mutation may be responsible for our "intelligence", but tiny mutations may also be responsible for most of what humans have done on this planet. See "A New History of Prehistory Part 3 - The Missing Mutation: Are We Really Smarter than our Ancestors?"

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