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Why
We Die, Why We Live: A New Theory on Aging
By
NICHOLAS WADE
New York Times, July 15, 2003
As
you write yet another check to cover your children's ruinous college
bills, there is definitely a bright side to consider: if you weren't doing
this, you'd long since be dead.
This
cheerful insight comes courtesy of the evolutionary theory of aging. The
theory holds that animals generally die shortly after reproducing because
extra life would not lead to more surviving offspring, the only criteria
for success in evolution's playbook. Species
that provide parental care, however, can escape the usual curtain call for
a time because in them natural selection has a basis to favor genes that
promote post-reproductive longevity — the so-called grandmother effect. This
theory, developed by William Hamilton and others, has become the classic
explanation of the way evolution tunes the genes that shape the life cycle
of each species. But
there are various features of the human life cycle it does not explain
well: why juvenile mortality is bunched into the first years of life and
then declines, for one. Biologists
and demographers are greeting with considerable enthusiasm a new theory of
aging that extends Hamilton's idea and explains the features it doesn't
account for well. The
new theory, proposed by Dr. Ronald Lee, a demographer at the University of
California at Berkeley, was published in today's issue of The Proceedings
of the National Academy of Sciences. "I
think it's a path-breaking paper of enormous importance in the biology of
aging," said Dr. James Vaupel, director of the Max Planck Institute
for Demographic Research in Germany. The
classic theory explains aging in terms of natural selection and fertility
through the life cycle. Dr. Lee's insight is that parental care, too, is
of persistent importance. The classic theory acknowledges parental care in
the grandmother effect, but Dr. Lee gives it much greater weight, saying
it should be factored in throughout the life cycle. People start life as
receivers but gradually switch over to being givers as they have children
of their own. Dr.
Lee's theory predicts that mortality at any age through the life cycle is
caused by a combination of two factors: the classic effect of how much
reproductive life is left and the transfer effect, the economist's phrase
for parental care. In
nonsocial species in which parents do not invest in their children, there
is no transfer effect, and the classic theory applies. In social species
that have reached the optimum balance between how many children to have
and how much to invest in them, differences in fertility no longer make
much difference and the rate of aging is controlled entirely by the
transfer effect, Dr. Lee's theory states. The
evolutionary shaping of the human life cycle was presumably completed
during hunter-gatherer days, before the invention of agriculture some
10,000 years ago. Dr. Lee has tested his theory on life cycle data from
the Ache, contemporary hunter-gatherers of Paraguay, and on data from
18th-century Sweden. He finds his equation for the transfer effect gives a
better fit to these two sets of mortality data at each age than does the
classic theory of aging. Dr.
Lee's theory explains why mortality is high among infants but rapidly
drops; mutations that cause death late in childhood, when much has been
invested, are removed more quickly from a population than are mutations
that cause death in infancy. His theory can also explain the reduction of
mortality after menopause: women care for children and contribute to their
survival. Dr.
Alan R. Rogers, an evolutionary anthropologist at the University of Utah,
said in an e-mail message that Dr. Lee's paper was the "first theory
to provide a cradle-to grave account of human mortality rates." "It
is remarkable that a single theory can account both for the decline in
mortality over the first few years of life and also for the extended
postmenopausal life span of human females," Dr. Rogers stated. "This
is the most comprehensive evolutionary theory of aging that we have seen
to date," Dr. Rogers wrote in a commentary. Dr.
Lee said he developed the theory after doing basic reading in evolutionary
biology while preparing an introductory lecture in demography. "As I
started to read it didn't make sense to me," he said. Under
the classic theory, mortality should be constant throughout childhood, Dr.
Lee said, but it seemed obvious that evolution would conserve investments
made in a child and act to reduce mortality as a child grew older.
"That's how I got started," he said. Two
other problems the theory does not explain well, he said, are
postreproductive survival and the way that low fertility evolved in the
creation of menopause. "My theory shows that all three problems are
closely connected, and all can be solved through attention to parental
investment," Dr. Lee said in a e-mail message. "I don't think
anybody has realized this before." He
thought of calling his idea the "live-to-give" theory, he said,
but it sounded too New Age and Californian. The
theory does not have immediate practical consequences. But by showing the
interconnection between fertility, mortality and parental care, the theory
may point to new areas of study. "What we have right now from Ron's
theory is a framework crying out for empirical work," said Henry
Harpending, an anthropologist at the University of Utah. "The theory
tells us what to measure." Dr.
Vaupel of the Planck institute said the theory did not explain the gradual
increase in longevity that had occurred among many industrialized
populations in the last century because this improvement stems from
environmental causes, not genetic ones. But it may help explain the
evolutionary basis for "why parents are so generous about helping
their children." That's another thing to think about as you grind out those college checks. Copyright
© 2002 Global Action on Aging
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