Study Compares Aging of Human, Chimp Brains


August 1, 2011

Chet Sherwood in lab coat and gloves sits at lab table with arms crossed in front of him

Chet Sherwood

By Menachem Wecker

With the imminent release of the film Rise of the Planet of the Apes, the question of how well human brains compare to those of other primates looms large. A study led by Chet Sherwood, associate professor of anthropology, which provides an unprecedented look at the changes in chimpanzee brains over their lifespan, shows that while human brains degenerate with age, chimpanzee brains do not.

Dr. Sherwood, who is a member of GW’s Center for the Advanced Study of Hominid Paleobiology and Institute for Neuroscience, is the lead author on the study, “Aging of the Cerebral Cortex Differs Between Humans and Chimpanzees.” The study appeared July 25 in the journal Proceedings of the National Academy of Sciences.

According to research by Dr. Sherwood and colleagues at six other universities, human brains – particularly the regions called the hippocampus and the frontal lobe – decline in volume with age, while chimpanzees, which are humans’ closest living relatives, experience no such changes.

Humans do not lose a significant amount of neurons during the normal aging process, Dr. Sherwood explained in an interview with GW Today. Shrinkage in size comes instead from a loss of neurons’ branches and axons, myelin sheaths surrounding the axons and synaptic connections.

“There are also reductions in the levels of genes expressed in the brain and other molecular modifications, but these probably do not contribute to overall size shrinkage,” Dr. Sherwood said. “In contrast, in Alzheimer’s disease and other pathological illnesses of aging, there can be significant loss of neurons in particular regions of the brain.”

Dr. Sherwood and his colleagues used MRI – magnetic resonance imaging – to measure the size of 87 brains of humans between the ages of 22 and 88, and 99 brains of chimps between the ages of 10 and 51.

“Although other animals experience some cognitive impairment and brain atrophy as they age, it appears that human aging is marked by more dramatic degeneration,” said Dr. Sherwood.

The changes Dr. Sherwood and his colleagues observed in human brains all occurred beyond the average maximum lifespan of chimps in the wild, whose longevity is significantly shorter than humans.

“What’s really unusual for humans is the combination of an extremely long life and a large brain,” said Dr. Sherwood. “While there are certainly benefits to both of these adaptations, it seems that more intense decline in brain volume in the elderly of our species is a cost.”

Dr. Sherwood said the jury is still out on the question of how, if at all, biological factors that protect chimpanzee brains from degeneration might inform approaches to trying to prevent, or at least mitigate, human brain degeneration.

“It is interesting to note that studies of changes in gene expression in the cerebral cortex during aging of humans, chimpanzees, macaque monkeys and mice all show declines in the number of genes that are ‘turned on’ in old age for all species,” he said. “However – and here’s the interesting part – the actual genes that are down-regulated differ from species to species. This suggests that there are different species-specific mechanisms of neuro-degeneration.”