Scientists believe that gonorrhea may be responsible for being one of the few species that care for their grandchildren.
The evolution of all life is driven by a single imperative, reproduction, and the biology of most animal species is optimized for that purpose at the expense of long lifespans.
Humans are one of the only species known to survive past menopause. According to the “grandmother hypothesis” this is because older women provide evolutionary important support in rearing children.
Researchers at the University of California, San Diego School of Medicine previously discovered a set of human gene mutations that contribute to this longevity by protecting older adults from cognitive decline.
And now in a new study published in the journal Molecular Biology and Evolution, they have traced the evolution of one of these mutated genes through the human genome and discovered that its rise was likely supported by selective pressure from infectious pathogens such as gonorrhea.
Key to this investigation was a comparison of the human and chimpanzee genomes, which discovered how humans have a unique version of the gene for CD33, a receptor on immune cells.
In its standard configuration, this receptor binds to a type of sugar called sialic acid that is coated on all human cells – when an immune cell senses sialic acid through CD33, it “recognizes” other cells and does not attack.
But the CD33 receptor is also expressed on immune cells in the brain called microglia, where it helps control neuroinflammation — and microglia have an important role in removing damaged brain cells and amyloid plaques associated with Alzheimer’s disease.
Abnormal CD33 receptors actually suppress this important microglial function and increase the risk of dementia by binding to sialic acid on these cells and plaques.
“This is where new gene variants come in,” the study authors said.
“Somewhere along the line of evolution, humans picked up an extra mutated form of CD33 that’s missing the sugar-binding site.
“The altered receptor no longer responds to sialic acid on damaged cells and plaques, allowing microglia to break them down.
“Indeed, high levels of this CD33 variant were independently found to be protective against late-onset Alzheimer’s,” they added.
What does it have to do with gonorrhea?
The researchers observed that gonorrhea bacteria coat themselves in the same sugars that bind to CD33 receptors.
“Like wolves in sheep’s clothing, bacteria are able to trick human immune cells into not recognizing them as outside invaders,” they say.
All in all, their paper suggests that humans initially inherited the altered form of CD33 because it protects our reproductive capacity.
However, the gene variant was later co-opted by the brain for its benefit against cognitive decline, enabling several generations of families to grow together.
“It is possible that CD33 is one of many genes selected early in life for their survival advantage against infectious pathogens, but then secondarily selected for their protective effects against dementia and other aging diseases,” the researchers said.