What I understood from the PopSci that passes HN every now and then is that the &#x27;Alzheimer protein&#x27; is just some normal protein that the brain makes during normal operation every day. In your sleep the brain contracts and &#x27;flushes it out&#x27; somehow, but over the years the protein accrues and you suffer dementia. Obviously that&#x27;s super oversimplified and probably wrong, my source being skimmed over PopSci and all.<p>Since dementia is a disease that happens at a late age, there doesn&#x27;t need to be a reason why it is there. It happens a long time after reproduction, so its effect on evolution is minimal, worse it could even be evolved as a mechanism to curb older specimens in the species.