One of the great puzzles in evolutionary biology is why humans live so long after they stop having children. Most animals reproduce until late in life, and their biological usefulness fades when they can no longer reproduce. But human women typically live decades after menopause. Why?
Enter the grandmother hypothesis. It’s a scientific idea that suggests grandmothers might have played a key role in the success of our species.
The idea was first seriously proposed in the 1990s by anthropologist Kristen Hawkes and her colleagues, who studied the Hadza people of Tanzania, one of the last remaining hunter-gatherer societies. They noticed that older women, especially grandmothers, spent a lot of time foraging for food, not for themselves, but to feed their grandchildren.
This behavior might not just be helpful — it might have changed the course of human evolution.
The grandmother hypothesis suggests that women’s long post-reproductive life span evolved because grandmothers who helped care for their grandchildren increased the chances that those grandchildren would survive and grow up to reproduce. By assisting their daughters in raising offspring, grandmothers indirectly passed on more of their genes.
This shift likely allowed humans to raise children more quickly and with better survival rates. Human babies are especially helpless and take years to grow up. If mothers had to raise one child entirely on their own before having another, population growth would be slow. But if grandmothers pitched in, mothers could wean their children earlier, have more babies and still ensure their existing children survived.
This could explain another evolutionary mystery: why humans live in multigenerational groups. Our ability to cooperate across generations, something that most animals don’t do, might have its roots in grandmothering. With older females providing knowledge, food and care, humans became better able to adapt and survive in changing environments.
Evidence for the grandmother hypothesis shows up in studies of modern societies. Research has found that children with involved grandmothers are more likely to survive and thrive, especially in environments where resources are scarce or life is challenging.
But the hypothesis doesn’t just apply to grandmothers. Some researchers have extended the idea to older males as well, though the case is weaker. Unlike women, men can theoretically reproduce their whole lives. Yet the contributions of elder males as protectors, teachers or providers could also help younger generations succeed, even if they aren’t directly parenting.
The grandmother hypothesis also opens the door to deeper questions. If our extended life spans evolved to support child-rearing, could it explain why humans developed complex language, teaching and culture? Some scientists think yes —because older people could pass on not just genes, but wisdom.
In the end, what may have started as a practical need — to feed and care for hungry grandchildren — might have helped shape the entire structure of human society.
So the next time your grandmother hands you a snack or offers advice, remember: She might just be carrying out a role that helped humans become human.
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
