Unlike the brain of our closest living relatives, the chimpanzees, particular parts of our human brain shrink in volume as we age, probably as an evolutionary consequence of our longer lifespan, suggest US researchers who report how they used MRI scans of chimps' brains to arrive at their findings in an early online issue of the Proceedings of the National Academy of Sciences published on 25 July 2011.

First author Dr Chet C. Sherwood, associate professor of anthropology at George Washington University's Columbian College of Arts and Sciences, told the press that:

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

To investigate how this might relate to brain size, they set out, in the first study of its kind in this field, to compare total and regional brain volume of chimps at different ages with that of humans.

But as there appears to be a gap in data available on chimps' brains, the researchers used magnetic resonance imaging (MRI) to measure brain volumes in a cross-sectional sample of 99 adult chimpanzee brains aged from 10 to 51 years.

The researchers measured total brain volume and the volume of certain regions, including: total neocortical gray matter, total neocortical white matter, frontal lobe gray matter, frontal lobe white matter, and the hippocampus.

They then compared these measurements with brain structure volumes measured in 87 adult humans aged from 22 to 88 years.

They found that while the volume of brain structures in chimpanzees did not change much as they aged, there was a decline in the sizes of all the brain structures measured in humans.

Further analysis ("using an iterative age-range reduction procedure") showed that the strongest effects in humans came from those individuals who were older than the maximum longevity of chimpanzees (in the wild, few chimps live past their 45th birthday).

"Thus, we conclude that the increased magnitude of brain structure shrinkage in human aging is evolutionarily novel and the result of an extended lifespan," write the researchers.

The team were particulary interested in the hippocampus, because this is particularly vulnerable to age-related degeneration in humans. Among other things, the hippocampus encodes new memories and helps with spatial navigation. It is an area of the human brain that suffers the most damage in Alzheimer's disease (AD).

Affecting mostly older people, Alzheimer's is the result of gradual loss of the structure and function, and also death, of brain cells or neurons. Chimps don't get the disease.

Perhaps humans are vulnerable to Alzheimer's disease because we have more pronounced brain atrophy than other species, including our close relatives, even when we age normally and healthily, said the researchers.

Humans have evolved to possess a large brain and a very long lifespan:

"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," commented Sherwood.

"Aging of the cerebral cortex differs between humans and chimpanzees."
Chet C. Sherwood, Adam D. Gordon, John S. Allen, Kimberley A. Phillips, Joseph M. Erwin, Patrick R. Hof, and William D. Hopkins.
PNAS published ahead of print July 25, 2011, doi:10.1073/pnas.1016709108
Link to Abstract.

Additional sources: George Washington University.

Written by: Catharine Paddock, PhD