rial humans. Thus, the study captured more than the full range of life spans and brain aging that you would expect to see in the evolutionary past of chimps and humans.
Focusing on the frontal lobe and hippocampus, the most shrink-prone regions of the human brain, the researchers found the expected loss of brain size with age in the human volunteers. But the chimps' brains showed no size change at all.
Long lives, shrinking brains
That's not to say that chimps' brains don't change with age — other studies show cellular changes and subtle structural changes in animals as they get older — but the direct comparison makes it clear that "whatever is going on is certainly more extreme in humans," Sherwood said.
Sherwood suspects that as natural selection began to favor the large human brain, humans had to deal with the associated costs, including relatively helpless infants and an extended childhood as that brain develops. In most mammals, females die once they can't reproduce anymore. But human women can expect to live 40 years past menopause. This longevity may have evolved so that grandparents could help their own children with childrearing, thus ensuring that their offspring's offspring would survive to spread their genes.
The cost of having grandma and grandpa around as nannies is that the brain's repair mechanisms may not have evolved to pick up the slack.
"If you're going to take on another 40 years of life span and these neurons have to function, that ultimately seems like it's going to be hard to keep up with," Sherwood said.
In fact, humans outlive their reproductive usefulness for so long that it's amazing their brains don't degrade faster, said Todd Preuss, a neuroscientist at Yerkes National Primate Research Center who was not involved in the current study. Preuss and his colleagues are studying the subtler differences between human and chimp brains, trying to understand how molecular and cellular variations separate us from our nearest living relatives.
"There is the opportunity here for much more microscopic and biochemical work," Preuss told LiveScience. "The particular vulnerability of the human brain to Alzheimer's disease suggests that there's something unusual about our brain chemistry."
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