Study suggests we have this STI to thank for changing grandmothers

The arms race between the human immune system and gonorrhea might have had the helpful side effect of promoting healthy brain tissue later in life.

That little boost to cognitive health in our twilight years may have played a small part in ensuring that grandmothers were sharp-minded enough for evolution to keep them.

While it’s fiendishly difficult – and perhaps impossible – to determine which evolutionary factors are responsible for life beyond the ages when we no longer reproduce, researchers at the University of California, San Diego are getting closer to some possible explanations.

In 2015, a team of researchers led by molecular medicine professor Ajit Varki discovered that humans have a unique type of immune receptor that protects against Alzheimer’s disease and sets us apart from other primates.

In a paper published this month, the team found that the spread of this immune receptor variant in our species was not entirely random, but rather the result of intense selection pressure over a relatively short period of time.

Research has shown that some of our closest relatives – Neanderthals and Denisovans – did not have this version of immune receptors encoded in their genomes. Something drove humans to develop this special immune receptor early in our history as a species, the researchers said.

Likely culprits are human-specific infectious pathogens like Neisseria gonorrhoeae which try to disguise themselves by dressing in the same sugar coating as human cells, which tricks patrolling immune cells into thinking the bacteria are harmless.

Gonorrhea became very good at tricking the human immune system into thinking it was just another human cell. But the human immune system has found a way to fight back.

The researchers showed that the newly developed immune receptor could see through the disguise and kill the invading bacteria, while the old immune receptor variation could not.

Getting rid of gonorrhea is useful for the survival of the species because this disease can disrupt 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 investigation by evolutionary scientists for some time.

Once evolved, this immune receptor was likely co-opted by immune cells in the brain, called microglia, for a different purpose: protection against aging, the researchers suggest.

The human immune system does not usually attack itself voluntarily, but it has to when cells begin to break down.

The huCD33 receptor, which appears to have evolved in response to sneaky bacteria, had the added benefit of being able to recognize decaying brain tissue and thus protect cognitive function in the elderly.

Microglia use the huCD33 receptor to eliminate damaged brain cells and amyloid plaques associated with Alzheimer’s disease. Whether this could have played a role in paving the way for evolution to add a few more precious years to our lives in an effort to help raise families is open to debate.

Grandparents bring benefits to the human species as they help care for children and pass on important cultural knowledge. And gonorrhea can be to thank for that.

This article was published in Molecular biology and evolution.

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