This small boost to cognitive health in our twilight years may have played a small role in ensuring that grandmothers were sharp-minded enough to evolve to sustain them.
While it is extremely difficult—and may even be impossible—to know the evolutionary factors responsible for living past ages where we are no longer reproducing, researchers at the University of California, San Diego, are getting closer to some potential explanations.
In 2015, a team of researchers led by Professor of Molecular Medicine Ajit Varkey discovered that humans have a unique type of immune receptor that protects against Alzheimer’s disease and sets us apart from other primates.
In research published this month, the team found that the prevalence of this altered immune receptor in our species was not entirely random, but rather the result of intense selection pressure over a relatively short period.
The research showed that some of our closest relatives – Neanderthals and Denisovans – did not have this version of the immune receptor encoded in their genomes. Something prompted humans to develop this special immune receptor early in our history as a species, the researchers said.
Possible culprits are human infectious pathogens such as Neisseria gonorrhoeae who try to disguise themselves by wearing the same sugar cap as human cells, tricking the patrolling immune cells into thinking the bacteria are harmless.
Gonorrhea has successfully tricked the human immune system into thinking it was just another human cell. But the human immune system has found a way to respond.
The researchers showed that the newly developed immune receptor can see through disguise and kill invading bacteria, while the older form of the immune receptor cannot.
Getting rid of gonorrhea is beneficial to the survival of the species because the disease can spoil human reproduction.
The new version of the immune receptor is called huCD33. Thanks to the way this version is modified into two subtly different structures within our bodies, it has been the subject of investigations by evolutionary scientists for some time.
The researchers suggest that once this immune receptor evolved, it may have teamed up with the brain’s immune cells, called microglia, for a different purpose: to protect against aging.
Usually the human immune system does not attack itself on purpose, but it does need to when cells begin to break down.
The huCD33 receptor, which appears to have evolved in response to infiltrating bacteria, had the additional benefit of being able to recognize degenerating brain tissue and thus protect cognitive function in old age.
Microglia use the huCD33 receptor to clear damaged brain cells and amyloid plaques associated with Alzheimer’s disease. Whether this played a role in paving the way for evolution to add a few precious years to our lives in order to help raise families is an open topic for debate.
Grandparents provide benefits to the human race as they help nurture children and pass on important cultural knowledge. The reason for this may be gonorrhea.
This paper was published in Molecular Biology and Evolution.