A study led by Kristen Hawkes with mathematical biologist Peter Kim found that in a computer simulation of a population of chimpanzees the contribution of grandmothers in caring for infants and children increased the average lifespan by almost 25 years. Their findings ultimately support the ‘grandmother hypothesis’, which attempts to explain why humans survive long after their childbearing years have passed, a quality which is somewhat unique to humans.
The postmenopausal survival of humans is anomalous because at first glance it appears to be at odds with the dogma of evolution. Ultimately our evolutionary prerogative is to pass on our genes, and so more children means a greater contribution to the gene pool. Once an organism is past its reproductive years, the reasoning goes, they become an evolutionary dead end. Sustaining older organisms without the ability to reproduce is also an added draw on the resources available to the rest of the community. Humans, therefore, are unusual in that they survive for decades post-menopause, long after they have lost the ability to reproduce.
One explanation for this phenomenon is the grandmother hypothesis, which posits that post-menopausal grandmothers help care for their grandchildren, thereby conferring an evolutionary advantage. In doing so they are simultaneously increasing their grandchildren’s probability of survival and allowing their daughters to bear more children, as they are partially relieving them of their parental responsibility of childrearing. In this way they are ensuring that their genes will be passed on through their offspring and subsequently on to their grandchildren.
The grandmother hypothesis was first proposed by evolutionary biologist William Hamilton in 1966, but only gained popularity after a field study of Tanzanian hunter-gatherers by anthropologist Kristen Hawkes. Hawkes found that when grandmothers helped with foraging, their grandchildren were often heavier and healthier and weaned at a younger age. Earlier weaning also decreased the interval between births, as once children were fully weaned their mothers were free to give birth again at little cost to their other children.
In the years following researchers began to put the grandmother hypothesis to the test. A 2010 mathematical simulation began with a population of 1,000 people and tracked changes in lifespan in populations with the aid of grandmothers and without. The researchers who conducted the study found that the average lifespan was not affected in either population, casting doubt on the validity of the grandmother hypothesis. Hawkes countered by pointing out several flaws in the simulation, and the authors ultimately conceded that their simulation did need several adjustments.
A new study led by Kristen Hawkes which was published today in the Proceedings of the Royal Society employed a new simulation that began with animals with chimpanzee lifespans. Female chimpanzees do not live long past their childbearing years, whereas humans survive for decades even after they have lost the ability to reproduce. When the simulation began each individual only lived an average of 25 years after reaching adulthood, making their lifespan closer to that of a chimpanzee. After 24,000 to 26,000 years of evolution, however, the length of time that the individuals survived as adults doubled. Researchers found that the simulated creatures survived an average of 49 years after reaching adulthood, a lifespan which is more similar to humans.
Living Longer? Thank Grandma. Credit: Shutterstock
The simulation began with only 1% of females reaching grandmother age, and a 5% possibility of newborns acquiring a gene mutation which would decrease or lengthen their lifespan. By the end of the simulation the number of females which reached grandmother age climbed to 43%. To avoid artificially inflating the grandmother’s contribution the researchers included several parameters which limited the contribution of grandmothers: women were only considered grandmothers between 45 and 75 years of age, and they could only care for one child after the age of 2 at any given time. Even with these constraints, and only a minimal amount of 'grandmothering', the contribution of grandmothers still had a significant effect on the lifespan of the simulated creatures.
These findings come on the heels of a study published in Science in September in which the authors reported that ‘grandmother’ killer whales increase the survival of offspring. Their contributions were most pronounced in aiding the survival of grown male killer whales, but less so among their daughters. These results were widely touted as additional support for the grandmother hypothesis, and suggest it may also be relevant for the few other animals with a similar lifespan.
Another explanation for the increased longevity of humans contends that it is just an artifact of decreased predation. Before modern humans mastered the art of hunting they were more susceptible to attack by other animals, but as of now humans have virtually no natural predators. Over the past few years, however, the grandmother hypothesis has been gaining support as it has been accruing evidence in its favor. The grandmother hypothesis, however, is not an explanation for the steadily rising lifespan of humans, especially in developed countries: it is only supposedly working in conjunction with other variables such as increased hygiene, medical intervention, etc. So the next time Grandma complains about looking after the kids, remind her that it is her evolutionary duty to do so. Or maybe not- chances are that won't go over well, and you're probably better off staying on Grandma's good side.