Once again about the "grandmother's hypothesis"
Gonorrhea helped ancient grandmothers overcome dementia
Olga Ivanova, Naked Science
American scientists have discovered a set of mutations of certain genes that protect the elderly from dementia and cognitive decline. It turned out that the appearance of these genes in our ancestors is associated with pressure from infectious pathogens, such as gonococcus, which causes gonorrhea. According to the researchers, such bacteria could contribute to the fact that women began to live to menopause while maintaining a sane mind. It literally helped our ancestors to raise children.
Many paleoanthropologists and other specialists in human evolution adhere to the so-called grandmother's hypothesis. It is intended to explain the origin of menopause in women (in fact, Homo sapiens is one of the few species that survives to such a biological state). If we take a wild society, it is easy to imagine that immediately after the death of a mother, for whatever reason, her young children are probably also doomed to death — from hunger, disease, attacks by predators or even relatives.
Therefore, the closer biological death is, the more advantageous it is for females from an evolutionary point of view to change the strategy — to stop reproducing their own offspring and invest in what is already available, that is, grandchildren. In this way, grandmothers help preserve their own genes in the population.
Scientists from the University of California and Princeton University (USA) decided to find out what features of human biology make such long-term health of women possible. They presented their findings in the journal Molecular Biology and Evolution (Saha et al., Evolution of Human-specific Alleles Protecting Cognitive Function of Grandmothers).
Previously, scientists have already compared the genomes of humans and chimpanzees, finding out that humans have a unique version of the CD33 gene, a receptor expressed in immune cells. The standard CD33 receptor binds to the so-called sialic acid, which covers all human cells. And when the immune cell perceives this acid through the above-mentioned receptor, it recognizes another cell as part of the body and does not attack it, preventing an autoimmune response.
The same receptor is expressed in the immune cells of the brain — microglia, where it helps to control neuroinflammation. These same microglial cells play an important role in the removal of damaged brain cells and amyloid plaques associated with Alzheimer's disease. By binding to sialic acid on these plaques, conventional CD33 receptors, in fact, suppress this important function of microglia and increase the risk of dementia.
And here a new variant of the gene enters the biological arena — the CD33 mutation, which lacks a sialic acid binding site. That is, the mutated receptor no longer reacts to this acid and allows microglia to destroy damaged cells and plaques of the brain. The higher the level of this gene, the better it protects against the onset of Alzheimer's disease.
Scientists have found out that neither Denisovans nor Neanderthals had this version of the gene. Therefore, the researchers suggested that such a mutation could actually help our ancestors better adapt to the survival of offspring — for example, the appearance of elderly women who would help raise children.
According to the authors of the work, the change in the CD33 receptor is associated with pathogenic bacteria, for example, with a microorganism that causes gonorrhea. The fact is that gonococci cover themselves with the same sialic acid that the CD33 gene binds to. And bacteria are able to deceive human immune cells so as not to be destroyed.
According to scientists, the mutated version of CD33 originated as a human adaptation to the molecular mimicry of gonorrhea bacteria and other pathogens. They confirmed that the mutation of the above-mentioned gene is able to completely neutralize the interaction between it and bacteria, and this allows immune cells to attack the latter again. Thus, having protected themselves from gonorrhea, our ancestors received an additional "bonus" — a certain protection from senile dementia.
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