- The unique human brain
Unlike human brains, chimpanzee brains don’t get smaller as they age, suggesting that pronounced neurological decline is a uniquely human byproduct of our oversized brains and extreme longevity.The brains of chimpanzees don’t undergo a general reduction in volume as the animals age, a sign of general cognitive decline that humans experience. The finding, published today (July 25) in Proceedings of the National Academy of Sciences, suggests that extreme neurological degeneration is the price humans pay for having evolved big brains and long life spans.
Human beings and chimpanzees follow a similar lifespan in absolute years until you get late in life, and then humans have somehow managed to extend our lifespan,” said anthropologist and neuroscientist Todd Preuss of the Yerkes National Primate Research Center in Atlanta, who was not involved in the study. This study shows that during those extra years, we experience a sharper downturn in brain function, he added.
Many species experience subtle brain decline as they age, but in humans the process of neuronal degeneration is so extreme that it leads to a 10 to 15 percent decline in brain volume over the lifetime, said study author Chet Sherwood, an anthropologist at The George Washington University in Washington, D.C.
To see whether this brain shrinkage occurred in other animals, Sherwood and his colleagues looked to our closest living relatives, chimpanzees. The team used MRI to measure the volume of 87 human brains, ranging in age from 22 to 88 years, and 99 10- to 51-year-old chimpanzee brains.
Unlike the human organs, which started to shrink when the subjects were in their mid-40s and decrease in size more dramatically after 70, chimpanzee brains didn’t shrink at all with age. This suggests that the shrinkage humans experience is primarily attributable to the extra 40 years of life that humans enjoy beyond the average ape lifespan of 45, Sherwood said.
Furthermore, because human brains are 3.5 times bigger than chimpanzee’s, human brains use much more energy and generate a greater amount of damaging oxidizing chemicals as a result. The production of oxidizing chemicals and the longer timespan during which they can accumulate can explain the damage that causes the extreme cognitive decline in our old age, Sherwood said.
Elephants and whales can live as long as humans and also have much bigger brains than their close relatives, so it would be really interesting—if impractical—to see if they undergo similar brain shrinkage as they age, he added.
But the cognitive decline associated with living longer may have been balanced by the benefits of increased lifespan, such as providing a greater opportunity to transfer knowledge to the next generation or allowing grandparents to help rear their children’s offspring, Preuss said.
C. Sherwood, et. al, “Aging of the cerebral cortex differs between humans and chimpanzees,” PNAS, doi:10.1073/pnas.1016709108, 2011.