Over the years, the brain shrinks. The tangle of neurons becomes looser, more distant, like a ball of wool that has been pulled too far. While not everyone ages in the same way—as some still-spirited centenarians attest—there are still important similarities within our species: age, that vicious executioner, very often strikes in the same place, in specific areas of the brain, according to the same pattern.
To understand why, and where these repeated similarities from individual to individual come from, researchers from the Heinrich Hein University in Düsseldorf compared our aging to that of certain monkeys. If other primates do not suffer the same fate, then perhaps they could give us the key to doing the same, these scientists imagined, hoping to find there something to slow our decline.
189 chimpanzees, all put to sleep for the occasion because they can’t sit still, and 480 humans took part in the experiment. There was no rejuvenation cure in this work: resisting age-related decline, curing aging, apparently requires much more than analyzing a few MRI scans. But the study, which appeared in Science Advances Wednesday, August 28, nevertheless brought to light some observations which shed new light on what scientists already knew about the evolution of our species, and its weaknesses with regard to the effects of time.
The price of evolution
6 million years ago, man and ape separated. A slow split, which extended over thousands of generations, caused by different living conditions, and therefore by natural selection. It was at this time that Man, who was not yet quite a man, acquired, among other things, a larger brain, with a more developed prefrontal cortex.
The result of genetic divergences, the appearance of this area of the brain located at the front of the encephalon allowed for better mobility and better reasoning. But, and this is what the German scientists detail in their work, these acquisitions were not “free”. They were obtained at the cost of significant counterparts, including that of being much more sensitive to aging.
The researchers thus showed a correlation between the speed of aging and the date on which the different areas of the brain appeared. “This correlation is specific to humans and is not visible in the other monkeys studied, chimpanzees, but also olive baboons and rhesus macaques, their closest cousins,” explains Felix Hoffstaedter, neuropsychology researcher and first author of the study. In other words, what is most primary in our head is also what is most durable. And what is most new, most “powerful”, is also what is most fragile.
Ways to age better
This “compromise” of evolution, as the researchers call it, would explain in particular why human brain aging is more significant and faster than that of monkeys, reduced to an equal lifespan. And allows us to better understand why monkeys seem less affected by diseases linked to degeneration, such as Alzheimer’s. If this phenomenon was relatively well-known to scientists, the comparison with monkeys has allowed us to clarify a little more the role of evolution in this mechanism.
The story is similar to the pacts that humans sometimes make in movies with the devil to acquire additional faculties. Here, obviously, no contract was made with evolution. These faculties appeared by chance, then persisted, because they allow individuals to survive longer and have more children. However, for the moment, scientists are still unaware of the precise reasons that could explain such cerebral fragility.
“Compromises” like this are not uncommon. Our species seems to have acquired its intelligence at the cost, also, of a more energy-consuming brain and a particularly long development time. Most baby monkeys become adults more quickly. For Felix Hoffstaedter, the fact that the elements that separate us from primates are “recent” in the history of evolution, makes them less “complete”. “As if evolution had not finished its work”, the researcher says.
It will probably take scientists years to fully understand the mechanisms that led to what we are. But the advent of supercomputers and AI, used in the German work in particular, has given new life to these research projects. They could eventually allow us to better detail, in addition to morphological changes, how the skills and behaviors that result from them appeared. And also advance medicine, which still has difficulty understanding the pathologies of aging.