It would seem – what is the connection? It turned out that the most direct. 500-200 thousand years ago, in the course of natural selection, changes were fixed in six genes responsible for brain development. In particular, the number of connections between neurons has increased, so we have become smarter. However, the same genes were found to be responsible for the development of Alzheimer’s disease.
This disease is the fate of man, it does not occur even in our closest relatives – primates. Kun Tang, a population geneticist at the Shanghai Institute of Biological Sciences who led the study, suggests that Alzheimer’s occurs when the aging brain can’t keep up with the high metabolic rate required for highly developed human intelligence. Alzheimer’s disease is a neurodegenerative disease that is responsible for 50-75% of dementia cases. The disease begins gradually, steadily progresses, effective treatment for it has not yet been found. The main symptom is loss of short-term memory and weakening of thinking. In the later stages, the brain is so severely affected that the patient becomes almost helpless.
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Chinese scientists decided to study the DNA of modern humans in order to find traces of ancient evolutionary changes in the genes responsible for the increase in the number of neural connections. To do this, they examined the genome of 90 people of African, Asian and European origin.
Genome analysis was complicated by the fact that changes caused by changes in population size could easily be confused with those that occurred naturally. To isolate the latter, scientists tried to assess the historical dynamics of the population and then find DNA fragments that do not agree with it, which means that they most likely appeared as a result of natural selection.
Thus, the researchers managed to look back 500 thousand years into the evolutionary past of man (although he became a man in the modern sense, most likely, only 200 thousand years ago). Previous methods of analyzing genetic change did not allow studying evolutionary history beyond 30 years ago.
Подробнее см. H. Zhou et al. «A Chronological Atlas of Natural Selection in the Human Genome during the Past Half-million Years», BioRxiv, May 26, 2015.