The Science of Aging
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Content:
- Understanding the science behind aging.
- Lifespan Versus Longevity
- Aging & Mortality
- Wear & Tear Theories of Aging
- Aging As a Preprogrammed Process
- Metabolic Theories of Aging: “The Brighter the Candle, the Quicker It Burns”
- Things to note;
- Why Would Evolution Select for Limited Lifespans?
Understanding the science behind aging
The science of aging is the study of how living organisms change over time.
It aims to understand the biological processes that lead to age-related changes in the body and mind.
Consider the various stages in the lifespan of Homo sapiens.
Infancy and childhood are characterized by continual growth in height and body mass.
Basic motor and intellectual skills develop: walking, language, etc.
Infancy and childhood also represent a period of vulnerability wherein a youngster is dependent on adults for water, food, shelter, protection, and instruction.
Adolescence witnesses a final burst of growth in the body’s skeletal framework.
More importantly, a series of dramatic developmental changes occur an accumulation of muscle mass, maturation of the gonads and brain, and the emergence of secondary sex characteristics that transform a child into an independent and reproductively capable adult.
Adulthood, the longest stage, is a period devoid of dramatic physical growth or developmental change.
With the notable exception of pregnancy in females, it is not unusual for adults to maintain the same body weight, overall appearance, and general level of activity for two or three decades or more.
Understanding the underlying causes and instigating triggers of aging and the changes that accompany it is of great biomedical importance.
Lifespan Versus Longevity
From Palaeolithic to Medieval times the average life expectancy of a newborn baby oscillated over a range of 25 to 35 years.
Beginning with the Renaissance, however, this number gradually increased such that, by the beginning of the 20th century, the average life expectancy of persons born in developing countries reached the mid-40s.
Today, 100 years later, the current world average is 67 years, and that for developed nations is approaching 80.
These dramatic increases have led to speculation about how long this trend might be expected to continue.
Can future generations expect to routinely live past the century mark?
Is it possible that human beings possess the potential, with proper care and maintenance, to live indefinitely?
Aging & Mortality
Are aging and death non-determinant or stochastic processes wherein living creatures inevitably reach a tipping point after a lifetime’s accumulation of damage from disease, injury, and simple wear and tear? Alternatively, are aging and death genetically programmed processes analogous to puberty that have evolved through a process of natural selection?
In all likelihood, aging and death are multifactorial processes to which both stochastic and programmed factors contribute.
Wear & Tear Theories of Aging
Some researchers theorize that aging and mortality are the inevitable outcomes of a lifetime’s accumulation of damage from injury, disease, and exposure to deleterious environmental constituents such as ultraviolet light. These theories note that while repair and turnover mechanisms exist to restore or replace many types of damaged molecules, they are less than perfect.
Hence, some damage inevitably leaks through damage that will accumulate over time, particularly among those cell populations that undergo little, if any, turnover.
Ironically, many prominent sources of molecular and cellular damage also are essential for terrestrial life: water, oxygen, and sunlight.
Aging As a Preprogrammed Process
While molecular wear and tear undoubtedly contributes to aging, several observations suggest a prominent role for programmed, deterministic mechanisms.
Female menopause, for example, provides an example of an age-associated physiologic change that is genetically programmed and hormonally controlled.
The following paragraphs describe several current theories regarding deterministic, programmed mechanisms for controlling aging and death.
Metabolic Theories of Aging:
“The Brighter the Candle, the Quicker It Burns”
(One of the many variants of this famous quote attributed to the ancient Chinese philosopher Lao Tzu summarizes the salient features of metabolic theories of aging).
Its origins can be traced to the observation that the larger members of the animal kingdom tend to live longer than the smaller ones.
Reasoning that the causal basis for this correlation derived from something connected with size, rather than size itself, scientists focused their attention on the organ most frequently associated with life and vitality the heart.
In general, the resting heart rate of small animals such as hummingbirds, 250 beats per minute, tends to be higher than that of large animals such as whales, 10 to 30 beats per minute.
Estimates of the total number of times each vertebrate animal’s heartbeat throughout a lifetime exhibited an amazing convergence on 1 × 109.
Is the vertebrate heart physically or genetically limited to 1 billion beats?
A more nuanced variation of this heartbeat hypothesis was put forward by Raymond Pearl in the 1920s.
Pearl’s metabolic or rate of living hypothesis posited that an individual’s lifespan was reciprocally linked to their basal metabolic rate.
It was calculated that every vertebrate animal expends a similar amount of total metabolic energy per unit body mass, 7 × 105 J/g, over their lifetime.
While intuitively appealing, the identification of a mechanistic link between lifespan and energy expenditure or metabolic rate has proven elusive.
Adherents of the mitochondrial theory of aging suggest that what is being “counted” is not heartbeats or energy, but the ROS that is the by-product of respiration.
Over time, the continued generation of energy and the related consumption of O2 leads to the accumulation of ROS-induced damage to DNA, proteins, and lipids until, eventually, a universally conserved tipping point is reached.
Cells experiencing caloric deficits adjust (reprogram) their metabolic pathways to utilize available resources in a more efficient manner that concomitantly decreases the yield of collateral ROS.
Things to note;
- Aging and longevity are controlled via the complex interplay between random and deterministic factors that include genetic programming, environmental stresses, lifestyle, cellular countdown clocks, and molecular repair processes.
- Wear and tear theories hypothesize that aging results from the accumulation of biomolecular damage over time.
- Water, oxygen, and light are essential for life, but possess an intrinsic capacity to damage biological macromolecules.
- ROS are continually generated as a by-product of aerobic metabolism, particularly by the electron transport chain.
- The deleterious effects of Reactive Oxygen Species (ROS) are often amplified by free radical chain reactions.
- The reactivity of their aromatic ring systems and ability to absorb UV light render the nucleotide bases of DNA particularly vulnerable to UV or chemical damage.
- Missing or damaged nucleotide bases provide a potential source of deleterious gene mutations.
- Their critical roles in energy production and apoptosis, along with their endogenous genome, render mitochondria a central player in many theories of aging and death.
- Because the telomere caps at the ends of our chromosomes progressively shorten with each division of a somatic cell, they are hypothesized to serve as a molecular countdown clock.
- Evolutionary selection of a limited lifespan may optimize the vitality of the population rather than that of its members.
Why Would Evolution Select for Limited Lifespans?
The idea that animals would have evolved mechanisms designed specifically to limit their lifespan would appear, at first glance, to be counterintuitive.
If the driving force behind evolution is the selection of traits that enhance fitness and survival, should not this translate into an ever-increasing life expectancy?
However, while extending lifespan may represent a desirable trait from the point of view of the individual, it does not necessarily follow that this applies to a population or species as a whole.
A genetically programmed limit on lifespan could benefit the group by eliminating the drain on available resources imposed by members no longer actively involved in the production, development, and training of offspring.
Indeed, the current three-generation lifespan can be rationalized as providing time:
- for newborns to develop into reproductively active young adults,
- for these young adults to produce and nurture their offspring,
- and for older adults to serve as a source of guidance and assistance to young adults facing the challenges of childbirth and childrearing.
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