The Grandmother Hypothesis
“Long postmenopausal lifespans distinguish humans from all other primates. This pattern may have evolved with mother–child food sharing, a practice that allowed aging females to enhance their daughters’ fertility, thereby increasing selection against senescence. . . [T]his hypothesis also accounts for our late maturity, small size at weaning, and high fertility. It has implications for past human habitat choice and social organization and for ideas about the importance of extended learning and paternal provisioning in human evolution.”
—Hawkes K. : “Grandmothers, menopause and the evolution of human life histories”. Proc Nat Acad Sci USA. 3 Feb 1998; 95(3):1336–9.
Kristen Hawkes is one of the foremost anthropologists in the world. She is the Distinguished Professor of Anthropology at the University of Utah, an elected member of the National Academy of Sciences as well as the American Academy of Arts and Sciences, and serves on the executive committee of the Leakey Foundation. Her work is wide ranging and significant, focusing on the evolution of human life history. Hawkes has studied primitive tribes in Tanzania and Central America and also conducted groundbreaking studies comparing the life histories of humans and chimpanzees, our closest nonhuman primate cousins. The most noteworthy aspect of her research deals with her advocacy for, and study of, the Grandmother Hypothesis.
The Grandmother Hypothesis attempts to answer a question that has puzzled scientists for decades: Why menopause? Human ovaries tend to shut down by age fifty or perhaps younger, yet women commonly live on healthily for many years after. This contradicts evolutionary theory, which posits that losing fertility should be the end of the line; once breeding stops, evolution can no longer select for genes that promote survival.
Two evolutionary biologists laid the groundwork for the Grandmother Hypothesis. William D. Hamilton, considered by many to be the most influential evolutionary biologist of his generation, is best known for his genetic explanation of Altruism. In the 1960s, Hamilton argued that humans and other animals have a tendency to act in ways that favor the survival of their relatives, and thus perpetuate their own genetic profile.
George C. Williams’ ground-breaking work, Adaptation and Natural Selection (1966), paved the way for a post-Darwinian model of evolution that moved beyond natural selection as an explanation for every single beneficial trait. As Steven Pinker noted:
“Williams carefully dissected the concept of natural selection, delineating where it should and should not be applied. He noted that not everything that’s adaptive is an adaptation in the technical sense. If a fox’s feet tamp down a path in the snow, and that helps the fox get to the henhouse, it doesn’t mean that the feet of the fox are an adaptation to tamping down snow.”
—Steven Pinker, as quoted in The Third Culture: Beyond the Scientific Evolution by John Brockman (Simon and Schuster, 1995)
The importance of Hamilton’s and Williams’ work was noted by Hawkes in her exploration of the Grandmother Hypothesis. By carefully studying nonhuman primates and primitive cultures, such as the Hadza people of northern Tanzania, she was able to elaborate lines of evidence to show that prehistoric woman often survived long past menopause not because her reproductive cycle shut down prematurely (it is, in fact, similar to other primates in longevity), but because there was a societal advantage to her living past her reproductive capabilities. This advantage was instrumental in the survival of her family; the Grandmother was available to assist her own children in nourishing and caring for youngsters (her grandchildren), thus ensuring her own genetic survival and the flourishing of the species. Natalie Angier notes:
“When a young woman is burdened with a sucking infant and cannot fend for her family, she turns for support, not to her mate, but to a senior female relative – her mother, an aunt, an elder cousin. It is Grandma, or Grandma-proxy, who keeps the other children in baobab and berries, Grandma who keeps them alive. She is not a sentiment, she is a requirement.”
—Natalie Angier. “Theorists see evolutionary advantages in menopause”. The New York Times. 6 Sept 1997.
Chimpanzees don’t live long post-menopause, but young chimps don’t need elder females to aid in sustenance. They are capable of feeding themselves once weaned. Human children, on the other hand, are wholly dependent and fed for years after breastfeeding is completed. To aid a lactating mother with younger children, it was the Grandmother who foraged for food and aided in the older children’s nutritional status.
Hawkes’ promotion of the Grandmother Hypothesis represented a significant turning point in the study of human evolution. Not surprisingly, it challenged a patriarchal notion that big game hunting represented the primary benchmark in human evolutionary history. For years, the conventional wisdom had been that the Husband/Father provided the most vital nutritional support. The finding of stone tools and skeletal remains of large animals at two-million-year-old Homo Erectus sites of the Pleistocene Age have long lent credence to this conventional wisdom.
Hawkes presents evidence, however, to support the idea that there is good reason to be skeptical of this assumption. She notes:
“Most important is the observation that big game hunting is actually a poor way to support a family. Among the Tanzanian Hadza, for example, men armed with bows and poisoned arrows operating in a game-rich habitat acquire large animal prey only about once every thirty hunter days, not nearly enough to feed their children effectively. They could do better as provisioners by taking small game or plant foods, yet choose not to, which suggests that big game hunting serves some purpose unrelated to offspring survivorship.”
—Hawkes K.: ”Grandmothers, gathering, and the evolution of human diets.” Proc Nat Acad Sci USA. 95(3):1336–9.
It is worth noting that Hawkes does not present an alternative hypothesis of why prehistoric men chose to hunt big game rather than forage, but perhaps a clue to her thinking can be found in the title of another of her articles: Hawkes K. “Showing off: tests of an hypothesis about men’s foraging goals”. Ethol Sociobiol. 12:29–54.
The extension of human life post-menopause may have been the watershed event in human prehistory. With Grandmother able and willing to assist in provisioning the youngsters, adults were free to explore new lands unavailable to other primates who could not roam far from natural food sources available to the just-weaned young.
According to Hawkes:
“‘The Grandmother Hypothesis gives us a whole new way of understanding why modern humans were suddenly able to go everywhere and do everything. It may explain why we took over the planet.’”—Natalie Angier. “Theorists see evolutionary advantages in menopause.” The New York Times. 6 Sept 1997.
Menstruation As A Defense
“When I was seven I learned I was to undergo this monthly bleeding. I was disgusted, not because of the blood, but by the design – that our bodies were so inefficient they couldn’t do anything better with the blood, like absorb it. I never bought the explanation.”—Margie Profet, interviewed by Shari Rudavsky in Omni, May 1994
One would be hard pressed to find a scientist with a background as unique as that of evolutionary biologist, Margie Profet. Largely self-taught, without a PhD (a degree, she says, “is not a magic ticket”) nor formal training in biology, she nonetheless was the recipient of a MacArthur Genius Fellowship.
Profet’s educational background includes a degree in Political Philosophy from Harvard and a second Bachelor’s degree in Physics from the University of California, Berkeley. She also studied Mathematics at the University of Washington in Seattle. Her work on human menstruation then is something of a surprise. Not nearly as surprising, however, as the genesis of her groundbreaking paper, “Menstruation as a Defense Against Pathogens Transported by Sperm,” which was published in the September 1993 edition of The Quarterly Review of Biology. Her own explanation of how her study came about is so unique – it was inspired by a dream when she was awakened by her cat – that it is worth quoting in length, from the Omni interview cited above:
“Gelato was a whiny, very smart cat. I loved this animal for some dumb reason. He’d always meow in the middle of the night to go out and hunt. He was so persistent; he always won. One night he awoke me at 3:00 a.m. Earlier, I’d had a conversation with my sister about variability in menstrual flow. Who knows why – you know, sisters talking. And I had a vision in dream of a cartoon from grade school. The girls watched menstruation films and the boys sports films. . . The films’ little images showed ovaries, the uterus: ‘During the month, the uterus builds up this nice lining. But if it doesn’t get a fertilized egg, then it doesn’t need that lining, and it just comes out as blood.’
“I saw the pale yellow ovaries and the red lining of the uterus, and the red was flowing out of the cervix. But there were all these tiny black triangles with pointy tips embedded in the uterus and they were coming out with the flow. As soon as Gelato woke me up, I knew the black triangles were pathogens. And I said, ‘Oh, so that’s why,’ and I went back to sleep. The next morning, puttering around the house, I thought, Didn’t I have some weird dream last night? Then I thought, How would pathogens get up there; the only thing that gets up there is sperm. Maybe pathogens ride on, hitchhike on sperm. . . That’s why I gave Gelato acknowledgement.”
Like Kristen Hawkes, overturning the established paradigm of male-centric hunter-gatherer societies, Profet’s work challenged long-standing notions. Science, historically being a male-dominated field, gave little thought to the purpose of menstruation, accepting for two thousand years Aristotle’s idea that menstrual blood contains a substance, the materia prima, that is the source of new life. In Aristotle’s time, menstruation was and, in some circles, still is a source of myths, taboos and humor. As John Travis notes:
“Menstruation has also been used to cast women in a threatening light. Parts of the Bible contend that menstruating women are polluted and dangerous to men. The Roman historian Pliny wrote that menstruating women cause wine to sour, vines to wither, grass to die, and fruit to fall. As recently as 1974, Lancet published a letter speculating why flowers wilt if held by menstruating women.”
—Travis J.: “Why do women menstruate: scientists seek a reason for this feminine phenomenon”. Science News. 12 April 1997.
When scientists have studied menstruation, they have attempted to answer the question posed by evolutionary biology: If it is not advantageous, then why does it exist? Profet’s was among the first serious attempts to explain this in a way that gained widespread scientific and public recognition. Her theory was based on the idea that menstruation serves as a two-pronged assault on invasive pathogens: It sheds the outer lining of the uterus, where pathogens are liable to be lurking, and it swathes the area in blood, which brings immune cells into the battle against microbial infection.
Profet went on to write two very controversial books with her MacArthur Fellowship, proving that she was a highly original thinker. Protecting Your Baby-to-Be: Preventing Birth Defects in the First Trimester (1995) and its follow-up, Pregnancy Sickness: Using Your Body’s Natural Defenses to Protect Your Baby-to-Be (1997), argued that morning sickness is likely to be a natural response to everyday toxins found in foods.
Perhaps the oddest aspect of Profet’s life to date is that she went missing for seven years, only resurfacing in May of this year. Journalist Mike Martin wrote a haunting study of her disappearance in Psychology Today, quoting friends of hers who had last seen her in 2005, at which point she had already ended contact with her mother, with whom she had been very close. The article alerted her to the notion that people were looking for her, and she resumed contact with her mother, explaining only that she had been in “severe physical pain” and didn’t want to trouble anyone.
On a Nature news blog from May 31, 2012, Martin quoted Margie’s mother, Karen:
“Margie is finally home now, recovering from her long ordeal and hoping to find work in the near future. She is very happy to be reunited with her family, and we are overjoyed to have her back.”