Mutant gene and brain size
The evolution of the human brain was determined by only one mutant gene
Daniil Sukhinov, Naked Science
The new cortex, or neocortex, is the evolutionarily youngest region of the cerebral cortex. It is isolated in all mammalian species, but it is in humans that the neocortex forms the main part of the cortex, even when compared with higher apes. It is this area of the brain that is responsible for such higher cognitive abilities as conscious thinking, speech, learning and reasoning ability.
It is believed that the increase in the volume of the cortex observed in humans is associated with a higher proliferative capacity (that is, the ability to multiply cells by division and subsequent tissue proliferation) of precursor cells of neurons in the cerebral cortex. And although the genes responsible for such increased proliferation have already, it would seem, been found in previous studies, their effect on the development of the brain of great apes has not yet been studied.
This was noticed by a group of scientists from the Max Planck Institute of Molecular Cell Biology and Genetics (Germany) and decided to find out how the activity of a gene that increases the proliferation of neuronal precursors in humans will act in the brain of chimpanzees — a characteristic representative of great apes. An article with the results of the study was published in the journal EMBO Reports (Fischer et al., Human-specific ARHGAP11B ensures human-like basal progenitor levels in hominid cerebral organoids).
However, earlier studies on great apes were not conducted for a reason, because they are banned in Europe for ethical reasons. The authors of the new work found a way out — they conducted experiments with so-called brain organoids. Organoids are model three-dimensional cellular structures with a diameter of several millimeters that can be grown from induced pluripotent stem cells (iPSCs) of chimpanzees or humans.
ISPC is obtained by reprogramming ordinary differentiated cells with an already given function so that they become stem cells again. Then almost any other cells can be obtained from them, including organoids that mimic brain tissue and even separately the cerebral cortex. Moreover, it was previously shown that studies on such organoids are identical to experiments on a real brain.
The scientists selected the previously known human-specific ARHGAP11B gene, active mainly in the precursor cells of neurons in the neocortex of the human fetus, and forced it to be expressed in the cells of the brain organoids of chimpanzees. This led to a doubling of the number of precursor neurons that play a key role in the expansion of the neocortex.
The organoid of the chimpanzee brain is about 3 millimeters in size. Ordinary stem cells are colored red, stem cells that have received the ARHGAP11B gene are colored green.
On the other hand, by blocking the expression of this gene in human brain organoid cells, the researchers reduced the number of neocortex neuron precursors to the level of chimpanzees. Moreover, blocking the similar but not identical ARHGAP11A gene, which is much more common among many animal species, does not lead to such drastic changes.
It is assumed that the ARHGAP11B gene appeared as a result of partial duplication (doubling) of the ARHGAP11A gene about 5 million years ago and further mutation of this copy. Perhaps it was these mutations and the subsequent expression of ARHGAP11B that played a key role in increasing cognitive abilities and human evolution.
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