Encyclopedia of Evolution.pdf - Online Reading Center

nerve cells—the very specialization that stem, germ, and cancer
cells do not have. Second, processes that promote cell immortality
can easily slip into a destructive mode. Telomerase may stimulate
cancer. The more active form of superoxide dismutase causes ALS
(amyotrophic lateral sclerosis, or Lou Gehrig’s disease) in humans
when it functions in nerve cells. As the years roll by, telomeres
shorten, oxidative damage occurs, and the molecular armamentarium
of the cells appears unable to stop these processes from
eventually killing an individual.
2. Inevitability of cancer. Among the many genes in a eukaryotic
cell are tumor-suppressor genes. These genes become active
when a cell experiences a potentially carcinogenic mutation. These
TSGs cause the mutated cell to commit biochemical suicide, thereby
preventing the cell from causing cancer. Benzopyrene in cigarette
smoke mutates the p53 TSG in lung cells, making these cells vulnerable
to cancer. This is the principal reason that smoking causes lung
cancer. Mutations in the p53 gene are also responsible for cancers
in response to ultraviolet light and to certain fungal toxins. There are
other TSGs, which protect cells from other kinds of cancer.
Because humans are diploid organisms (see mendelian genetics),
each cell has two copies of each TSG. If a mutation occurs in
one of these copies in a cell, the other can serve as a backup in
that cell. Eventually both will mutate, as the body is bombarded by
radiation and exposed to carcinogenic chemicals (including many
that are natural components of food or the environment). Cancer
therefore appears to be inevitable after a long enough period of
time. People who inherit mutated TSGs from their parents have an
increased risk of cancer. If a person inherits one copy of a mutated
TSG, cancer can result quickly in a cell because there is no functional
backup TSG. This is what happens in women who inherit one
mutated copy of the BRCA1 gene, a TSG that protects them from
breast cancer. These women are at greatly increased risk of breast
cancer. As the years roll by, cells lose their tumor-suppressor genes
and cancer results.
3. Inevitability of mutations. Many other mutations accumulate
in chromosomes, besides those in the tumor-suppressor genes.
In time, the accumulation of mutations would ruin the genes. Mutations
cannot be prevented.
One of the major causes of mutations is the very air that animals
breathe! Among the results of oxidative stress, described
above, is DNA mutation. Eventually, oxygen gas would create so
many mutations that the genes would stop working. At the same
time, oxygen is essential for most cells to release energy from food
and put it to use. Oxygen gas is essential to all higher life-forms
(anaerobic life-forms are all simple and slow), yet it would eventually
kill all of these life-forms.
Another major cause of mutations is radiation, especially
ultraviolet radiation from the sun. The ozone layer shields the world
from much, but not all, ultraviolet radiation. Sunlight is necessary
for life, but this same sunlight would eventually kill all cells and
cell lineages by creating mutations and ruining their genes. As the
years roll by, mutations accumulate.
Therefore, in a world of oxygen and ultraviolet radiation, it
appears that no individual organism could live forever. Natural
selection does get rid of many mutations that would otherwise build
up through generations of exposure to oxygen and radiation. Mutations
do accumulate in germline cells—and they are the source of
life history, evolution of
the genetic variation (see DNA [raw material of evolution]; mutations;
population genetics) upon which natural selection acts.
Aging and Death Are in the Body
From ages 30 to 75, the average male human loses 44 percent of
his brain weight, 64 percent of his taste buds, and 44 percent of his
lung capacity. The older man’s heart output is 30 percent lower, and
his nerves 10 percent slower. His brain receives 20 percent less
blood. During exercise, the older man absorbs 60 percent less oxygen
into his blood. Just as cells accumulate molecular damage, so
the organs accumulate damage due to wear and tear. Repair mechanisms
exist but are imperfect.
Aging Is the Product of Natural Selection
The longer an organism lives, the more likely it is to meet its demise
from factors unrelated to its own health. In the wild, it would be
practically inconceivable for an animal to live a thousand years
without experiencing an accident or disaster or cancer. Some
plants live a long time but experience much damage during that
time. Giant sequoia trees, many of them more than two millennia
old, almost all show signs of fire damage. Even in an imaginary
population of physically immortal animals, there would be very few
extremely old ones. (The immortals of mythology avoided this problem
by not being physical.) Natural selection would favor the ability
of their bodies to repair themselves for at most a few decades. Any
genes that especially promoted the repair of cells in very old individuals
would confer little benefit, since there would be so few of
these individuals. Besides, repair mechanisms have costs and risks
associated with them. Examples include:
• The cells that have the greatest ability to regenerate are the
very cells that have the greatest risk of cancer—which is, after
all, simply cell division that has gone out of control. Examples
include skin and intestinal cells.
• The same physiological processes that protect younger individuals
can accumulate and cause problems in older individuals.
Urate molecules in the blood act as antioxidants, but they also
accumulate in joints, causing gout.
Early in the 20th century, evolutionary biologist J. B. S. Haldane
(see haldane, J. b. s.) pointed out that natural selection cannot
eliminate mutations that are deleterious to individuals who have
passed their reproductive life span. Biologists Peter Medawar and
George Williams expanded this concept. Pleiotropic genes have
more than one effect. If the effect of a gene on young individuals is
good but the effect on older individuals is bad, natural selection will
actually favor the gene, despite its deleterious effects on older individuals.
Natural selection has favored a long, but not vast, period of
post-reproductive life in humans (see life history, evolution of).
Natural selection favors characteristics that promote successful
reproduction, even at the expense of individual survival. This
is why evolutionary fitness is defined as successful reproduction.
Organisms that devote the most effort and resources to reproduction
when they are young, even if they “wear themselves out” in doing
so, can produce more offspring than can organisms that reproduce
fewer offspring over a longer period of time. This has been confirmed
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