The Evolution of Human Brain Set It Up for Failure

Apes don’t develop Alzheimer’s—at least, not like we do.

Although we share a hefty amount of our DNA with our closest primate relatives, humans are uniquely at risk of developing the disease that can erode our sense of who we are.

When I make the diagnosis in my neurology clinic, I tell patients and family members that in most cases, doctors can’t point to a single, definitive cause of the illness.

Age, genetics, health, lifestyle factors, and environmental triggers all play a role.

But a growing body of research offers another dimension: We can lose our sense of self precisely because our species evolved to have one.

In recent years, scientists have been comparing the brains of humans with those of other, closely related species, and teasing out how the genetics, structure, and molecular mechanisms differ.

What they’re finding is that the same characteristics that support our unparalleled cognitive abilities make us susceptible to Alzheimer’s disease.

Certainly, the biological hallmarks of Alzheimer’s look different inside the human brain than in the brains of other species.

Melissa Edler, a comparative neurobiologist at Kent State University, and her team have identified distinct patterns in how humans and chimpanzees accumulate amyloid plaques and tau tangles, the toxic proteins that build up in brains with Alzheimer’s.

She believes that looking at multiple species is going to be crucial to understanding how this disease develops.

If nonhuman primates aren’t developing Alzheimer’s, “why aren’t they?” she asked, when we spoke about her work.

“What’s different about their brains?” One answer dates back to the early 2000s, when the Swedish geneticist Svante Pääbo (who went on to win the Nobel Prize for sequencing the Neanderthal genome) wondered what might account for the profound behavioral differences between humans and other apes—after all, we build civilizations while nonhuman primates cannot, without assistance, start a fire.

Pääbo and his team of international researchers collected brain, blood, and liver tissue from humans, chimpanzees, orangutans, and macaques.

But instead of comparing the genes themselves, they measured the genes’ activity—effectively, how hard each gene was working to make messenger RNA, which carries instructions from DNA on how to make proteins.