The grey matter: Human brains 'get smaller with age'

Researchers at George Washington University found that only humans have brains which progressively shrink with increasing age, and it's uniquely human as the phenomenon don't occur in chimpanzees.

Loss of neurons over time may be the result of an evolutionary trade-off -- a handicap that has to be accepted in exchange for a long lifespan and big brain, say the researchers led by Dr Chet Sherwood.

For their study, the researchers carried out magnetic resonance imaging (MRI) brain scans of 99 young and old chimpanzees aged 10 to 51. The results were compared with MRI scans of 87 humans over an equivalent age range of 22 to 88, the 'Daily Mail' reported.

The scans showed a decrease in the volume of all major brain structures over the course of human life. In contrast, ageing chimpanzees showed no significant age-related changes to their brains.

Dr Sherwood said: "Humans may be uniquely vulnerable to age-related neurodegeneration, pointing to compromises that have been struck in the evolution of an enlarged brain and an extended lifespan."

Humans are uniquely prone to brain diseases such as Alzheimer's in later life as well as a general decline in memory and "thinking" skills. In addition to having bigger brains than any other animal, humans are unusually long lived.

Physically, humans age more slowly than chimpanzees. By 30, wild chimps have worn teeth, are underweight, frail, weak, and lacking in energy. Yet female chimps remain fertile almost until the end of life.

The researchers added: "Although an enlarged brain and extended lifespan have conferred decisive fitness benefits to humans, ultimately these adaptations come at a cost. These factors combine in the later stages of life to beset many of the elderly of our species with the effects of intensified neurodegeneration."

The findings have been published in the 'Proceedings of the National Academy of Sciences' journal.