Oct. 23, 2012 – Computer simulations provide new
mathematical support for the "grandmother hypothesis" – a famous theory
that humans evolved longer adult lifespans than apes because
grandmothers helped feed their grandchildren.
"Grandmothering was the initial step toward making
us who we are," says Kristen Hawkes, a distinguished professor of
anthropology at the University of Utah and senior author of the new
study published Oct. 24 by the British journal Proceedings of the
Royal Society B.
The simulations indicate that with only a little
bit of grandmothering – and without any assumptions about human brain
size – animals with chimpanzee life spans evolve in less than 60,000
years so they have a human lifespan. Female chimps rarely live past
child-bearing years, usually into their 30s and sometimes their 40s.
Human females often live decades past their child-bearing years.
The findings showed that from the time adulthood is
reached, the simulated creatures lived another 25 years like chimps, yet
after 24,000 to 60,000 years of grandmothers caring for grandchildren,
the creatures who reached adulthood lived another 49 years – as do human
The grandmother hypothesis says that when
grandmothers help feed their grandchildren after weaning, their
daughters can produce more children at shorter intervals; the children
become younger at weaning but older when they first can feed themselves
and when they reach adulthood; and women end up with postmenopausal
lifespans just like ours.
By allowing their daughters to have more children,
a few ancestral females who lived long enough to become grandmothers
passed their longevity genes to more descendants, who had longer adult
lifespans as a result.
Hawkes conducted the new study with first author
and mathematical biologist Peter Kim, a former University of Utah
postdoctoral researcher now on the University of Sydney faculty, and
James Coxworth, a University of Utah doctoral student in anthropology.
The study was funded by the National Science Foundation and the
Australian Research Council.
How Grandmothering Came to Be
Hawkes, University of Utah anthropologist James
O'Connell and UCLA anthropologist Nicholas Blurton Jones formally
proposed the grandmother hypothesis in 1997, and it has been debated
ever since. Once major criticism was that it lacked a mathematical
underpinning – something the new study sought to provide.
The hypothesis stemmed from observations by Hawkes
and O'Connell in the 1980s when they lived with Tanzania's Hazda
hunter-gatherer people and watched older women spend their days
collecting tubers and other foods for their grandchildren. Except for
humans, all other primates and mammals collect their own food after
But as human ancestors evolved in Africa during the
past 2 million years, the environment changed, growing drier with more
open grasslands and fewer forests – forests where newly weaned infants
could collect and eat fleshy fruits on their own.
"So moms had two choices," Hawkes says. "They could
either follow the retreating forests, where foods were available that
weaned infants could collect, or continue to feed the kids after the
kids are weaned. That is a problem for mothers because it means you
can't have the next kid while you are occupied with this one."
That opened a window for the few females whose
childbearing years were ending – grandmothers – to step in and help,
digging up potato-like tubers and cracking hard-shelled nuts in the
increasingly arid environment. Those are tasks newly weaned apes and
human ancestors couldn't handle as infants.
The primates who stayed near food sources that
newly weaned offspring could collect "are our great ape cousins," says
Hawkes. "The ones that began to exploit resources little kids couldn't
handle, opened this window for grandmothering and eventually evolved
Evidence that grandmothering increases
grandchildren's survival is seen in 19th and 20th century Europeans and
Canadians, and in Hazda and some other African people.
But it is possible that the benefits grandmothers
provide to their grandchildren might be the result of long
postmenopausal lifespans that evolved for other reasons, so the new
study set out to determine if grandmothering alone could result in the
evolution of ape-like life histories into long postmenopausal lifespans
seen in humans.
Simulating the Evolution of Adult Lifespan
The new study isn't the first to attempt to model
or simulate the grandmother effect. A 1998 study by Hawkes and
colleagues took a simpler approach, showing that grandmothering accounts
for differences between humans and modern apes in life-history events
such as age at weaning, age at adulthood and longevity.
A recent simulation by other researchers said there
were too few females living past their fertile years for grandmothering
to affect lifespan in human ancestors. The new study grew from Hawkes'
skepticism about that finding.
Unlike Hawkes' 1998 study, the new study simulated
evolution over time, asking, "If you start with a life history like the
one we see in great apes – and then you add grandmothering, what
happens?" Hawkes says.
The simulations measured the change in adult
longevity – the average lifespan from the time adulthood begins. Chimps
that reach adulthood (age 13) live an average of another 15 or 16 years.
People in developed nations who reach adulthood (at about age 19) live
an average of another 60 years or so – to the late 70s or low 80s.
The extension of adult lifespan in the new study
involves evolution in prehistoric time; increasing lifespans in recent
centuries have been attributed largely to clean water, sewer systems and
other public health measures.
The researchers were conservative, making the
grandmother effect "weak" by assuming that a woman couldn't be a
grandmother until age 45 or after age 75, that she couldn't care for a
child until age 2, and that she could care only for one child and that
it could be any child, not just her daughter's child.
Based on earlier research, the simulation assumed
that any newborn had a 5 percent chance of a gene mutation that could
lead to either a shorter or a longer lifespan.
The simulation begins with only 1 percent of women
living to grandmother age and able to care for grandchildren, but by the
end of the 24,000 to 60,000 simulated years, the results are similar to
those seen in human hunter-gatherer populations: about 43 percent of
adult women are grandmothers.
The new study found that from adulthood, additional
years of life doubled from 25 years to 49 years over the simulated
24,000 to 60,000 years.
The difference in how fast the doubling occurred
depends on different assumptions about how much a longer lifespan costs
males: Living longer means males must put more energy and metabolism
into maintaining their bodies longer, so they put less vigor into
competing with other males over females during young adulthood. The
simulation tested three different degrees to which males are competitive
What Came First: Bigger Brains or Grandmothering?
The competing "hunting hypothesis" holds that as
resources dried up for human ancestors in Africa, hunting became better
than foraging for finding food, and that led to natural selection for
bigger brains capable of learning better hunting methods and clever use
of hunting weapons. Women formed "pair bonds" with men who brought home
Many anthropologists argue that increasing brain
size in our ape-like ancestors was the major factor in humans developing
lifespans different from apes. But the new computer simulation ignored
brain size, hunting and pair bonding, and showed that even a weak
grandmother effect can make the simulated creatures evolve from
chimp-like longevity to human longevity.
So Hawkes believes the shift to longer adult
lifespan caused by grandmothering "is what underlies subsequent
important changes in human evolution, including increasing brain size."
"If you are a chimpanzee, gorilla or orangutan
baby, your mom is thinking about nothing but you," she says. "But if you
are a human baby, your mom has other kids she is worrying about, and
that means now there is selection on you – which was not on any other
apes – to much more actively engage her: 'Mom! Pay attention to me!'"
"Grandmothering gave us the kind of upbringing that
made us more dependent on each other socially and prone to engage each
other's attention," she adds.
That, says Hawkes, gave rise to "a whole array of
social capacities that are then the foundation for the evolution of
other distinctly human traits, including pair bonding, bigger brains,
learning new skills and our tendency for cooperation."
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