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September 6, 1997
Theorists See Evolutionary Advantages In Menopause
By NATALIE ANGIER
THE Hadza people of northern Tanzania are a small group of
hunter-gatherers who share a language, a culture and a distaste for
gardening. Time and again, government and church agencies have sought to
transmute them into full-time farmers, but the Hazda have always returned
to the bush, where they subsist on wild goods like fruits, honey, tubers
and game. The terrain is hard and hilly, and so is the life, but on one
incomparable resource the foragers can always rely: a pack of old ladies
with hearts like young horses.
As Dr. Kristen Hawkes of the University of Utah and her colleagues have
found in their extensive studies of the Hadza, women in their 50's, 60's,
70's and beyond are among the most industrious members of the group. They
are out in the woods for seven or eight hours a day, gathering more food
than virtually any of their comrades.
When a young woman is burdened with a suckling 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 woman's other children in baobab and
berries, Grandma who keeps them alive. She is not a sentiment, she is a
requirement. As Dr. Hawkes, Dr. James O'Connell of the University of Utah
and Dr. Nicholas Blurton Jones of the University of California at Los
Angeles report in the latest issue of Current Anthropology, a nursing
Hadza woman always has a postmenopausal helper.
There are only about 750 Hadza, and they are contemporary
hunter-gatherers, not pristine relics of prelapsarian humanity.
Nevertheless, the centrality of elder women to their group's survival has
thrown fresh kindling on the spirited debate over the origins and purpose
of human menopause.
As doctors and women thrash out the best way to ''treat'' menopause,
pitting the benefits of estrogen therapy to the heart and bones against
the risks the hormone poses to the breast and possibly the ovaries,
evolutionary scientists address the menopause mystery from a more
high-flown, though no less quarrel-prone, perspective. They ask whether
menopause is an ancient adaptation or a contemporary artifact. Is it the
well-wrought product of natural selection, or the incidental byproduct of
an unnaturally prolonged life span?
Proponents of the adaptationist camp generally see menopause as the
thriftiest solution to the problem of exorbitant offspring. By this view,
the ludicrous amount of time required for a mother to rear children to
maturity led to the need for so-called premature reproductive senescence,
an early retirement program for the ovaries. Through the mechanism of
menopause, an ancestral woman theoretically was spared the risks of
childbirth, and thus had a heightened chance of living long enough to see
her existing children out the door. Dr. Jared Diamond, a physiologist at
the University of California at Los Angeles Medical School has said that
menopause, like big brains and upright posture, is ''among the biological
traits essential for making us human.''
The artifactualists insist that prehistoric women almost never survived
past the age of 30, let alone long enough to experience the thrill of hot
flashes. By their reckoning, menopause is a modern luxury, the result of
women now outlasting an egg supply that more than sufficed for the cameo
appearances that their Stone Age foremothers called lives. ''For most of
our existence, we simply didn't live very long,'' said Dr. Alison
Galloway, an anthropologist at the University of California at Santa
Cruz. ''Menopause happens because, through technology, we've extended our
lives to the point where we run out of egg follicles. There's nothing
beneficial about it.''
Dr. Hawkes lends a new spin to the debate, combining elements of each
camp and adding a few bold spirals of her own. She agrees with the
artifactualists that menopause per se is not an adaptation -- it is not
the product of selective design. A woman's ovaries do not shut down
''prematurely,'' she says. They last 40 or 45 years, the same time as the
ovaries of our close relatives, chimpanzees and gorillas. In a sense, she
says, women do outlive their egg supply, which is fixed before birth and
cannot be added to, as men continuously generate new sperm.
On the other hand, Dr. Hawkes concurs with the adaptationists that
prehistoric women very likely often survived past menopause, and that
they were instrumental to the survival of their families. She goes
further. Only with the ascent of the grandmother, she says, were human
ancestors freed to exploit new habitats, to go where no other hominid or
primate had gone before, and to become the species we know so well.
''The Grandmother Hypothesis gives us a whole new way of understanding
why modern humans suddenly were able to go everywhere and do
everything,'' Dr. Hawkes said. ''It may explain why we took over the
In another new study that touches on the evolutionary basis of menopause
and maternal longevity, Dr. Thomas Perls of Harvard Medical School and
his colleagues describe in the current issue of Nature their finding that
women who lived to be at least 100 years of age were much likelier to
have remained fruitful well into middle age than a comparable group of
women who died at the age of 73. Looking at two sets of women born in
1896 who were equivalent in race, religion and other factors but who
differed in their life spans, the researchers found that among the 54
women who died in 1969 at the age of 73, only 5.5 percent had given birth
in their 40's. By comparison, 19.5 percent of the centenarians had
children after 40, one of them at age 53.
Because the women in Dr. Perls's analysis all had their families before
the rise of fertility-enhancing technology, the researchers see the stark
discrepancy in maternal stamina between the two groups as evidence of
innate selective drives at work. They propose that the genes allowing the
centenarians to stay fruitful for an exceptionally long time continued
working long after menopause to lengthen the women's life spans,
supporting the view that a woman's extended survival is indeed crucial to
her offsprings' prospects.
The older a woman is at last birth, the more years she must stick around
to keep her family afloat. (The researchers also believe that menopause,
the complete cessation of fertility, evolved to prevent aging women from
dying in childbirth while they still had dependent young, but their
analysis of the centenarians offers no direct proof of that.)
The notion that menopause has adaptive value for women and their
offspring is itself getting a bit long in tooth. Dr. George C. Williams,
a renowned evolutionary biologist, first proposed it in 1957 to explain
the seemingly anomalous nature of human menopause. Other female primates,
and even species like fin whales and elephants that live into their 80's,
can continue bearing young to the bitter end, he pointed out.
Why are humans different? And why does menopause occur universally among
women, and during a short window of time -- at the half-century mark,
give or take four years -- while other depradations of age, like gray
hair or presbyopia, occur gradually and randomly? Dr. Williams saw in
menopause the thumbprint of natural selection, and suggested that the
early cessation of reproduction paradoxically enhanced a woman's
reproductive ''fitness,'' by assuring that she lived long enough to see
her children bear children themselves -- that she became a grandmother.
Others have picked up and expanded on the rudimentary Grandmother
Hypothesis. Dr. Diamond has proposed that aging women, and men, were
repositories of essential information in preliterate times, living
libraries for their clans, able to distinguish edible from poisonous
plants and to recall events of long ago that remained pertinent to
In traditional societies, clan members are often related, and so by
aiding the tribe the elders help themselves. Only women need protection
from the hazards of advanced maternity, Dr. Diamond says, and so only
women need to undergo premature reproductive senescence to keep them
around for the benefit of their kin.
Yet efforts to demonstrate the adaptive value of menopause have proved
elusive. Dr. Kim Hill and Dr. Magdalena Hurtado, anthropologists at the
University of New Mexico, spent years studying the Ache, a group of
hunter-gatherers living in eastern Paraguay. They tried to estimate the
impact of postmenopausal women on the welfare of their children and
grandchildren, to see if the presence of a grandmother had a measurable
effect, for example by reducing the mortality of grandchildren.
The anthropologists concluded that the Ache grandmothers did not make
enough of a cumulative difference to their families to justify, in
Darwinian terms, the loss through menopause of their own reproductive
Using mathematical models, Dr. Alan Rogers of the University of Utah
estimated that a postmenopausal woman would have to double the number of
children her children bore, and eliminate infant mortality among those
grandchildren, to make menopause look like a sound strategy for
propagating one's genes. That is not a grandmother -- that is Neutron
''Adaptive menopause is an interesting idea, and I'm trying to keep an
open mind,'' said Dr. Steven N. Austad, a professor of zoology at the
University of Idaho in Moscow and author of ''Why We Age'' (John Wiley &
Sons, 1997). ''But I just don't see evidence to support it.''
Before anybody consigns the Grandmother Hypothesis to a conceptual
nursing home, however, Dr. Hawkes and her colleagues offer evidence in
the Hadza study that elder women can make an enormous, and quantifiable,
difference to their kin. The researchers found that whenever the mother
of young children gave birth to a new baby and was absorbed by the rigors
of breast-feeding, it was only through the intervention of a senior
female relative that the older children's weight stayed up. And the
harder the elders foraged, the higher the numbers on the researchers'
Importantly, the elder Hadza women were flexible in how they apportioned
their assistance. If a woman could help her nursing daughter, she did. If
she had no daughter, she helped a niece, or a cousin once removed.
Dr. Hawkes pointed out that in the Ache study showing little benefit from
grandma, the researchers had focused on the relationships between
mothers, their children and their children's children. Dr. Hawkes and her
colleagues were more inclusive in their analysis. ''If we restricted
ourselves to counting the reproductive success only of women whose moms
were still alive, we'd underestimate the effect of help from senior women
by a huge amount,'' Dr. Hawkes said. ''And you expect, with strategic
critters like ourselves, that natural selection would favor adjusting
help to where it was needed most.''
Dr. Hawkes proposes that what distinguishes a human female from her
chimpanzee or gorilla cousins is not that the woman goes through
menopause and the chimpanzee does not, but rather that the chimpanzee, at
45 and with ovaries failing, is globally decrepit and close to death,
while a woman can live decades after follicular fadeout.
But young chimpanzees do not need elder females to help them. Once they
are weaned, they feed themselves, and so there are no selective pressures
to keep Aunt Chimsky alive. Only human children are fed for years after
leaving the breast, and a senior female can serve up nuts and berries as
well as can a mother, and better when mother is lactating.
Dr. Hawkes and her co-workers suggest that the extension of life past
menopause was a watershed event in human prehistory. With a labor force
of elder females available to help provision the young, adults were then
free to colonize new territories unavailable to those primates that did
not provision their weaned young, and that were thus restricted to
feeding grounds where the pickings were easy enough for juvenile fingers.
Suddenly humans could migrate to places where it required full adult
strength and cunning to extract food. They could go wherever they
pleased. After all, there were older women around to help.
Supporting this proposition are the foraging patterns of the Hadza. Very
young children find much of their food themselves, but they depend on
adults for half their calories, and those calories come from foods, like
deeply buried tubers, that only an adult can obtain. Growing old, then,
may be nothing new, and the postmenopausal years worthy of celebration
and gratitude. Grandma is great, she is strong, and she baby-sits for
Copyright 2001 The New York Times Company
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