Anatomically modern humans emerged around 200,000 years ago. Over the next hundred thousand years or so, natural selection drove changes in half a dozen genes for brain development—increasing the connectivity of neurons and making our species “smarter” than our early ancestors. But according to new findings posted online at bioRxiv, that intellectual capacity came with a cost: Alzheimer’s may have evolved alongside human intelligence, Nature reports. Those same genes are implicated in the origins of the age-related brain disease that destroys memory. 

Humans are the only species we know of that develops Alzheimer's disease. It’s not found in even our close chimpanzee relatives. To find evidence for the evolution of the disease, a team led by Kun Tang of the CAS-MPG Partner Institute for Computational Biology searched through the genomes of 90 people of African, Asian, or European descent. 

They were looking for variation that’s driven by changes in natural selection, but in order to isolate selection’s signatures, the team had to separate out the effects of changes in population size. So after they estimated how population sizes changed over time, Nature explains, they identified genome segments that didn’t match up with the population history. This revealed DNA stretches that were shaped by selection. And this allowed the researchers to look back at various selection events that occurred as far back as half a million years ago, revealing the evolutionary forces that helped shape the origin of our species.

Around 50,000 to 200,000 years ago, they found, changes in six genes involved in brain development offered us a new intellectual capacity. The team thinks that Alzheimer’s disease developed as aging brains struggled with the new metabolic needs that came with increasing intelligence. 

[Via Nature]