The price we humans pay for our long lives may be a shrinking brain, a new study suggests.
George Washington University researchers used magnetic resonance imaging to measure the brain volume of 99 chimpanzees ranging from 10 to 51 years of age and compared it to brain structure volumes measured in 87 humans ranging from 22 to 88 years of age. Their conclusion: Human brains shrink over time, the result of an extended lifespan, while chimp brains don't.
Moreover, the hippocampus, the area of the brain responsible for encoding new memories and maintaining spatial navigation, as well as the region of the brain most prominently affected by Alzheimer's disease, also shrank in humans, but not in our most closely related primate relatives. The researchers speculate that the humans' "unique vulnerability" to develop Alzheimer's may be due, in part, our more pronounced brain atrophy, even in normal, healthy aging.
In an interview with CBS News, lead researcher Chet Sherwood speculated that shrinkage may be the evolutionary price we have to pay for having large brains in the first place. He noted that most female mammals die when they can no longer reproduce; but human women can live 40 years past menopause. He said that longer lifespans may have evolved so that grandparents could help with rearing children over the many years it takes for their brains to develop fully.
However, that raises a conundrum: If our brains shrink as we grow older, predisposing us to dementia, we become more vulnerable and dependent as we age. So AgingCare.com asked Dr. Sherwood: How would evolution favor our getting older, since dependent elders with dementia cannot help raise children and require help themselves? Dr. Sherwood declined to answer that question, but pointed to earlier studies that suggest that long lifespans, large brains, and investment by grandparents in the welfare of their grandchildren probably evolved very early in human history.