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Human brains shrink as people grow old, unlike even our closest animal relative, says a new study in the Proceedings of the National Academy of Sciences that highlights what researchers call the unique character of human aging.
The human brain normally can shrink up to 15% as it ages, a change linked to dementia, poor memory and depression. Until now, researchers had assumed this gradual brain loss in later years was universal among primates and other animals.
But in the first direct comparison of humans to chimpanzees, a brain-scanning team led by George Washington University anthropologist Chet Sherwood found that chimpanzees don't experience such brain loss. From that, researchers concluded that only people are afflicted by this oddity of longevity.
'We are very weird animals,' said Emory University anthropologist Todd Preuss at the Yerkes National Primate Research Center in Atlanta, who wasn't involved in the study. 'Among neuroscientists, the assumption has been that species are all the same, but this shows there is something really unusual about the late-life biology of the human species.'
In their study, the scientists used magnetic resonance imaging scans to measure changes in five key brain structures involved in memory, reasoning and other mental processing, as well as overall brain volume. They compared measurements from 87 adult humans ranging in age from 22 to 88 years old with the brain volumes of 99 adult chimps ranging from 10 to 51 years old. They measured the brain's gray matter of neurons and the white matter of its connecting neural fibers. On average, human brains weigh about three pounds. Chimp brains only weigh a third as much.
No matter where the researchers looked, they found the human brains lost significant volume over time, while the chimpanzees didn't. 'We found no age-related changes in chimpanzees,' said Dr. Sherwood.
Until recently, the study of brain differences between humans and other species was a scholarly pursuit with little impact outside the field of anthropology. But the rapid growth of the aging population world-wide is turning evolutionary biology into a pressing topic of public health. By 2030, about one in every five people in the U.S. will be over 65 years old, more than twice the number of elderly a decade ago, according to the U.S. Administration on Aging.
Unlike chimpanzees and other primates, elderly humans are prey to a host of neurodegenerative diseases, such as Alzheimer's disease. Researchers hope that by understanding the basic biology of the brain, they can devise ways to treat or postpone the harmful mental effects of age.
To clean out cobwebs in the attics of the brain, aging baby boomers have turned to brain fitness programs, diet nostrums and other enhancers. Specialized websites, software programs and videogames promise to improve fading memories and weakened cognitive abilities through mental exercises, with varying degrees of success.
Stress can affect brain size. So can depression, research shows. Diet can be a factor, too. More broadly, though, humanity's unusual shrinking brain just may be the price our species pays for living so much longer than other primates, experts said. Barring serious illness or injury, a human being can expect to live up to 80 years or more -- almost twice the normal lifespan of a chimpanzee in the wild.
During those extra decades of life, natural cell-repair mechanisms may wear out and neural circuits wither, the researchers said. As the brain normally ages, it acquires the neural equivalent of sore knees and stiff fingers. Natural grooves in the brain widen. Healthy swellings subside. And tangles of damaged neurons become dense thickets of dysfunctional synapses.
'It seems that this is the cost,' said Dr. Sherwood. 'We experience more extensive atrophy in the brain that results in this obvious shrinkage, of a kind that is not seen even in our closest relatives, chimpanzees.' 2011-08-08
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