Humans may get Alzheimer's precisely because our species evolved a highly developed brain—the regions responsible for self-awareness run hot.
Apes do not get Alzheimer's the way humans do. Scientists comparing different species have found an uncomfortable answer: we may lose our sense of self precisely because our species evolved to have one.
A genome running overclocked
Swedish geneticist Svante Pääbo compared liver, blood and brain tissue across humans and other apes. Liver and blood gene activity was similar, but activity in the human brain had shot up.
Later work kept piling on. Scientists at Leipzig University found nearly 300 genes with altered activity in Alzheimer's brains—and those genes had changed fastest during human evolution.
In researcher Thomas Arendt's words, Alzheimer's is based on 'typically human' genome changes. Teams at Duke University and in China and the UK found further links between evolution and disease risk.
A Ferrari engine inside the skull
Evolution grew the human parietal lobes, and the precuneus tucked inside them. Those regions stitch sight, sound and touch into a model of the world, and handle self-awareness, autobiographical memory and the sense of being the same person over time.
The trouble is, this area burns enormous energy, throws off heat, and sits in the brain's thinnest blood supply. Its myelin insulation is fragile, too.
Neuroscientist Emiliano Bruner's analogy: it is putting a Ferrari engine into a Fiat 500—it goes fast, runs hot, and the brakes are a problem.
Yale's Amy Arnsten calls that braking 'calcium regulation'. Neurons that hold an idea or plan in mind fire persistently, relying on calcium. As the system falters with age, calcium rises to harmful levels and touches off tau clumping. And the closer a species is to humans, the more of this vulnerable apparatus it has.
Evolution has long handed us defects: our upright spine invites herniated discs, a narrow pelvis makes childbirth hard. Accepting that trade-off is harder when it hits the organ of thought.
Why it matters
If Alzheimer's is rooted in the fast-changing human genome, prevention and treatment cannot focus only on amyloid and tau—the weak points evolution left behind, from energy metabolism to blood supply and calcium regulation, may be where the disease starts.
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