Brains shrink in humans, potentially causing a number of health problems and mental illnesses as people age, but do they shrink to the same extent in the closest living relatives to humans – the chimpanzees? New research says no, making brain shrinkage in aging humans unique. The research appears in the July 25, 2011 online issue of Proceedings of the National Academy of Sciences.
Chet Sherwood, an anthropologist at George Washington University in Washington, D.C., and a team of scientists from seven other U.S. universities put forward the question to see if comparable data on the effects of aging could be found in chimpanzees. Such data on regional brain volumes in chimpanzees was not available until now.
The researchers – anthropologists, neuroscientists, psychologists, biologists, and veterinary professionals – used magnetic resonance imaging (MRI) to measure the space occupied by various brain structures in adult humans and chimpanzees, including the frontal lobe and the hippocampus, an area of the brain associated with short-term and long-term memory.
They found that chimpanzees do not display significant loss, or atrophy, in the size of their brains and other internal structures as they age.
Instead, Sherwood and colleagues suggest that as humans evolved the ability to live longer, the result was a “high degree of brain degeneration” as people get older. Sherwood said:
We were most surprised that chimpanzees, who are separated from humans by only 6-8 million years of independent evolution, did not more closely resemble the human pattern of brain aging. It was already known that macaque monkeys, separated from humans by about 30 million years, do not show humanlike, widespread brain atrophy in aging.
Because humans and chimpanzees grow, develop and age on different schedules, the study compared humans from age 22 to 88 and chimpanzees from age 10 to 51. For both species, this encompassed the whole adult lifespan under natural conditions. Humans have a longer lifespan than chimpanzees. In the wild, the lifespan of chimpanzees is about 45, at the oldest. With medical care in captivity, they can live into their 60s. On the other hand, humans without access to modern medical care and who live in traditional hunter-gatherer societies can live to their mid-80s.
The researchers used MRI to measure the volume of the whole brain, total neocortical gray matter, total neocortical white matter, frontal lobe gray matter, frontal lobe white matter and the hippocampus in a cross-sectional sample of 99 chimpanzees and 87 adult humans.
The authors wrote:
Traits that distinguish humans from other primates include enlargement of the brain and increased longevity.
Consequently, they say, humans are unique among animals in being susceptible to certain neuropathologies, such as Alzheimer’s disease, in the later stages of life. Even in the absence of disease, however, healthy aging in humans is marked by variable degrees of neural deterioration and cognitive impairment.
This research points to the uniqueness of how severe brain aging is in humans. While there are certainly many similarities between humans and other animals in the degenerative processes that occur in the brain, our research indicates that even healthy, normal aging in humans involves more pronounced brain deterioration than in other species.
Taken together with particular environmental and genetic risk factors, this might help to explain the fact that only humans are vulnerable to developing dementing illnesses like Alzheimer’s disease in old age.
Sherwood and colleagues conclude that evolution led to both a large brain and a long lifespan in humans. They point out that the benefits of these traits are much debated, but they surmise it might be related to an increased reliance on social learning of skills. Sherwood explained:
As a result, we suggest that the high energy cost of a large brain in humans leads to more wear and tear that cannot be easily repaired because most neurons are not renewed. As a consequence, human brains become more vulnerable to degeneration towards the later stages of life.
Bottom line: Chet Sherwood, an anthropologist at George Washington University, and his team of researchers published research in the July 25, 2011 online issue of Proceedings of the National Academy of Sciences, suggesting that the brains of chimpanzees do not decrease in size with aging, as do the brains of humans.
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