AROUND the age of 50, women stop releasing eggs and can no longer have children. We consider this menopause a natural part of life, but in one respect, it is really rather unusual 鈥� there are hardly any other species in which females experience reproductive shutdown when they typically still have decades of healthy life ahead.
So what is the point of menopause? The latest findings support the idea that it is to do with our uniquely demanding offspring and the need to keep relatives who could care for them healthy and mentally sharp for as long as possible.
Classic evolutionary theory predicts that organisms should only live as long as they can pass on their genes. This mostly holds true across the animal kingdom: besides humans, only female orcas, short-finned pilot whales, belugas and narwhals tend to survive well beyond their fertile years.
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Does menopause serve a purpose?
The reason we have menopause may simply be because there are biological constraints on how long it is possible to maintain high-quality eggs, says at the University of California, Santa Barbara. The average age of the onset of menopause in humans has barely budged even as life expectancy has soared, he says. 鈥淲e definitely seem to be hitting a wall.鈥�
Men may not experience the same clear-cut end to their fertility because they can keep creating new sperm as they age, albeit with declining quantity and quality, whereas women are born with all their eggs, which accrue DNA damage over time, says Gurven.
Indeed, 鈥渢he thing to really explain is not why women have menopause, but why they live so long after menopause鈥�, he says. Over the past decade, evidence has been building to explain this unique characteristic 鈥� and the benefits it has for our evolutionary success.
Our best ideas are based around the 鈥済randmother hypothesis鈥� 鈥� the notion that once women can no longer have children of their own, they can help look after any grandchildren, who each carry 25 per cent of their genes. This may be particularly relevant for humans because we have unusually helpless young, says Gurven. 鈥淚f you look at our nearest relatives, chimpanzees, they can largely feed themselves by the time they鈥檙e 6 [years old],鈥� he says. 鈥淲ith humans, it often takes up to two decades for us to be self-sufficient.鈥�
Grandparents can come to the rescue by providing extra food for their grandchildren or taking care of them while their parents are out foraging or nursing younger children, he says.
The benefits of menopause
Recent evidence suggests we are on the right track. A study Gurven published in 2022 with , also at Santa Barbara, showed that in many hunter-gatherer groups, like the Hadza of Tanzania and the Ach茅 of Paraguay, adults over the age of 50 . This contribution ultimately allowed grandparents to pass on more of their genes. 鈥淥ur modelling suggested it was equivalent to having up to three more children,鈥� says Gurven.
Orcas and short-finned pilot whales also have highly dependent young, which may explain why females have similarly long post-reproductive lifespans. have found that post-reproductive females lead expeditions to find sustenance and increase the survival of their grand-offspring by sharing food and knowledge of where to forage. Male offspring also suffer fewer .
, San Diego, believes he and his colleagues have also found genetic evidence to bolster the grandmother hypothesis. In 2015, his group that is protective against late-life Alzheimer鈥檚 disease and is found in 20 per cent of modern humans, but not in chimpanzees or our closest extinct relatives, Neanderthals and Denisovans.
This variant may have been selected for as our period of infancy evolved to become longer, says Varki. By protecting people from Alzheimer鈥檚 disease, the new CD33 variant may have helped keep them healthy for longer so they could pitch in with providing food and care for helpless grandchildren, he says.
Since then, Varki and his colleagues have uncovered more than 10 additional genetic variants that appear to protect against cognitive decline. This suggests that women play such an important role after menopause that we even evolved genes to protect their minds, says Varki.
While these findings may not help with enduring the hot flushes and night sweats that can accompany menopause, it may be some comfort to know that these unpleasant symptoms mark the beginning of an important life chapter that probably helped shape our evolutionary success.
