A shrinking brain may be the price that has to be paid for being human, a study has found.
Only humans have brains that progressively get smaller with increasing age, the research has shown.
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.
Age-related shrinkage of large-scale brain structures such as the hippocampus and frontal lobes has only ever been observed in humans.
Now scientists have confirmed that they do not even occur in our closest relatives, the chimpanzee. They appear to be uniquely human.
US researchers led by Chet Sherwood, from George Washington University in Washington, carried out magnetic resonance imaging 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 scans showed a decrease in the volume of all major brain structures over the course of human life. In contrast, aging chimpanzees showed no significant age-related changes to their brains.
Dr Sherwood’s team wrote in the journal Proceedings of the National Academy of Sciences: “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 the age of 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.
Humans stay physically fit for longer than their chimpanzee cousins, but women reach the end of their reproductive life in middle-age. The menopause roughly matches the age at which chimps and other great apes naturally die.
Living longer is said to have benefits that contribute to the development of bigger brains, including the nurturing help provided by grandmothers and other elderly helpers. But this may also result in a “unique setting in which humans undergo more progressive neurological ageing”, said the scientists.
They 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.”