Encyclopedia of Evolution.pdf - Online Reading Center

life history, evolution of
Why Do Humans Die?
The traditional religious and modern creationist explanation (see creationism)
about why humans are mortal is that God cursed the human
race in response to Adam’s sin. In the biblical book of Genesis, Adam
was the first man, and God created Eve from one of his ribs. Adam
ate the fruit that God had forbidden him to eat (many traditionalists
blame Eve for giving him the fruit), and he was transformed from an
immortal into a mortal. The entire human race is descended from
Adam and has inherited his mortality. Aside from the many difficulties
of this attempted explanation, it is not even an explanation. It leaves
the question unanswered: What actually happened to introduce the
physiological processes of aging and death into the human body?
The evolutionary explanation is that mortality is an inescapable
part of being an organism. In addition, natural selection has
actually favored the evolution of mortality. That is, a limited life span
confers greater fitness than would an unlimited life span, if the latter
were even possible.
Aging is the result of senescence. Senescence is not death
but is a programmed, gradual breakdown of biological processes,
which leads to death. The sequence of events in senescence is in
the cells, in animal bodies, and the product of natural selection.
Aging and Death Are in the Cells
1. Aging of cells. As cells, and lineages of cells, become older, the
chromosomes begin to lose the DNA caps at their ends (known as
telomeres). A telomere is a nucleotide sequence repeated about
2,000 times. The sequence is TTAGGG in most animals and fungi;
plant telomeres have an extra T; and protist ciliates have TTTT-
GGGG or TTGGGG telomeres. Each time a cell duplicates, its chromosomes
duplicate, and each time this happens, portions of the
telomere caps are not replicated. This occurs because the polymerase
enzyme, which copies the chromosome, cannot start right
at the end. An 80-year-old person has telomeres about five-eighths
as long as those of a newborn. An enzyme called telomerase
restores telomeres, but this enzyme is not active in most cells.
Most cells are capable of only a limited number of cell divisions,
whether in an animal body or in a test tube tissue culture.
Skin cells, for example, begin losing their smoothness as they go
through more divisions, which explains why old people have wrinkled
skin, and why the skin of people who spend a lot of time in the
sun is wrinkled: Ultraviolet light damages skin cells, which undergo
divisions to repair the damage, more than is the case with a person
who does not spend much time in the sun.
There appears to be a correlation between telomere loss and
aging. Evidence includes:
• Genetically engineered skin cells that produce telomerase live
longer in culture.
• Genetically engineered mice that could not produce telomerase
aged rapidly.
• Werner’s syndrome is a type of progeria (rapid aging) in which
children undergo rapid senescence and die during their second
decade. One component of this syndrome is rapid telomere loss.
• The storm petrel lives 30 years, which is one of the longest life
spans for a bird. Unlike those of almost all other animals, the
telomeres of the storm petrel do not shorten with age.
The correlation between telomere loss and aging does not
mean that telomere loss causes aging. Telomere loss could be an
effect rather than a cause of aging. Furthermore, the loss of telomere
nucleotides is not the only cellular change that accompanies
aging. Aging is also correlated with oxidative stress. Oxygen gas
(O 2) is very reactive. When they come in contact with water, oxygen
molecules can produce highly dangerous molecules such as peroxide
ions and superoxide radicals. Cells have enzymes that protect
them from these dangerous molecules. Superoxide dismutase
changes superoxide radicals into peroxide, and catalase changes
peroxide into water. Antioxidants (such as some of the vitamins)
also inactivate oxygen radicals. Flies that have a more active form
of superoxide dismutase, and worms that produce high levels of
cellular antioxidants, have longer life spans. Without such protective
enzymes and antioxidants, a cell would quickly die. Oxygen
gas quickly kills all anaerobic cells, which do not have protective
enzymes. But even these enzymes and antioxidants cannot protect
a cell perfectly.
There are some cells in an animal body that remain eternally
young. Among these are:
Stem cells. While most embryonic stem cells differentiate into
tissues and organs, some cells remain undifferentiated and genetically
young. In children and adults, these adult stem cells retain
the ability to divide and differentiate at a later time, as part of the
process of healing and regeneration in damaged tissue. Most adult
stem cells have only a limited ability to differentiate. For example,
satellite cells in muscle tissue can differentiate only into various
kinds of muscle cells. Nervous tissue, even in the brain, contains
some stem cells that can differentiate into new nerve cells. Bone
marrow stem cells produce new red and white blood cells. If scientists
understood the biochemical differences between stem cells
and other cells of the body, they might be able to transform mortal
cells into immortal ones; at this time, however, there is no promise
of such a breakthrough. Such cells, though genetically immortal,
would eventually die for other reasons, explained below. As stem
cells make up only a very small fraction of the cells of the body, the
body undergoes senescence and carries the stem cells down into
the abyss of death with it.
Germ cells. The cells involved in the production of eggs and
sperm also remain eternally young, not losing their telomeres, in
the body of an animal. These germ cells specialize very early in
embryonic development. Germ cells represent an immortal cell line
throughout all animal generations. The germ cells of any individual,
however, are housed within a mortal body, making their eventual
death as certain as that of stem cells.
Cancer cells. One of the problems with trying to make cells
immortal is that there is a fine distinction between immortality and
cancer on the cellular level. Cancer cells are cells that have lost
the ability to stop reproducing, and the ability to differentiate into
different kinds of cells with specific functions. This is why cancer
forms tumors of blob-like cells that keep spreading. Cancer cells
may be immortal but are within a mortal body, the death of which
they hasten.
It may be impossible for an organism to consist only of immortal
cells. First, cells appear to lose their immortality by differentiating
into specialized functions such as those of skin, muscle, and