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Why We Fall Apart
Biologists Use Engineering Technique to Learn More
About Human Aging
Aug. 27, 2004 - Methods used for studying the aging
of complex equipment can be used to better understand the aging of
humans, according to biologists, who say our bodies function best at
about 10 years old and if we could hold that rate we may live to be
5,000.
The quest to understand and control aging has led
University of Chicago biologists Leonid Gavrilov and Natalia Gavrilova
to draw inspiration from what might seem an unlikely source: reliability
engineering.
The reliability-engineering approach to
understanding aging is based on ideas, methods, and models borrowed from
reliability theory. Developed in the late 1950s to describe the failure
and aging of complex electrical and electronic equipment, reliability
theory has been greatly improved over the last several decades. It
allows researchers to predict how a system with a specified architecture
and level of reliability of the constituent parts will fail over time.
But the theory is so general in scope that it can
be applied to understanding aging in living organisms as well. In the
ways that we age and die, Gavrilov and Gavrilova find, we are not so
different from the machines we build. "The difference is minimized if we
think of ourselves in this unflattering way: we are like machines made
up of redundant components, many of which are defective right from the
start," the two write in the September issue of IEEE Spectrum.
In reliability theory, aging is defined through the
increased risk of failure. More precisely, something ages if it is more
likely to fall apart, or die, tomorrow than today. If the risk of
failure does not increase as time passes, then there is no aging.
By looking closely at human aging data, the
University of Chicago researchers noted striking similarities between
how living organisms and technical devices age and fail. In both cases,
the failure rate follows a curve shaped roughly like a bathtub. The
curve consists of three stages, called infant mortality, normal working,
and aging. Death rates are rather high during infant mortality, but then
drop to a low constant rate during the normal-working period. In humans,
"this period is all too short, just 10 to 15 years, starting at about
age 5," write Gavrilov and Gavrilova, a husband and wife team. "If only
we could maintain our body functions as they are at age 10, we could
expect to live about 5000 years on average."
Machines and humans even share these strange
characteristics at very old age. As humans approach the age of 100, the
risk of death stops increasing exponentially and instead begins to
plateau. "If you live to be 110, your chances of seeing your next
birthday are not very good, but, paradoxically, they are not much worse
than they were when you were 102," the authors write. "There have been a
number of attempts to explain the biology behind this in terms of
reproduction and evolution, but since the same phenomenon is found not
only in humans, but also in such man-made stuff as steel, industrial
relays, and the thermal insulation of motors, reliability theory may
offer a better way."
An immediate consequence of the last observation is
that there is no fixed upper limit to human longevity--there is no
special number that separates possible from impossible values of a life
span. This conclusion flies in the face of the common belief in the
existence of a fixed maximal human life span and a biological limit to
longevity. The University of Chicago researchers were able to adapt
reliability theory to biological aging by thinking of humans as a
collection of redundant parts that do not, in themselves, age.
But to get the model to work exactly, Gavrilov and
Gavrilova had to make a radical assumption. Instead of humans starting
life in pristine condition, the reliability equations suggest that we
actually start with a great many defective parts. "If we accept the idea
that we are born with a large amount of damage, it follows that even
small improvements to the processes of early human development--ones
that increase the numbers of initially functional elements--could result
in a remarkable fall in mortality and a significant extension of human
life," write Gavrilov and Gavrilova.
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