Why We Die, Why We Live: A New Theory on Aging

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Why We Die, Why We Live: A New Theory on Aging

By NICHOLAS WADE

 

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.