|Why do people have to die?
|Question Date: 1999-04-26|
Wow! You sure know how to ask the tough questions!
In my opinion, I think the main reason why life
must end for any organism - plant, animal or
bacteria - can be found in Isaac Newton's Second
Law of Thermodynamics, which says that all
organized systems tend to fall into disrepair with
time. So, no matter how much effort you put into
maintaining your health, the organized systems in
your body that you depend upon (nervous system,
circulatory system, respiratory system, etc) will
start to fail you eventually. No process which
requires energy to maintain itself can last
forever. Machines break down, life ends, even the
sun in our solar system is slowly dying out. Some
reasons why our bodies cannot exist forever are
very obvious. Cells in our body are "programmed"
to die after a certain time. Sometimes they are
replaced, but sometimes they are not. For
example, skin cells are constantly dying and being
shed, but they are continuously being replaced by
new skin cells just beneath the old ones. Our
brain cells are also dying, very slowly, but
unlike skin cells, brain cells cannot be replaced.
How cells are triggered to die, what controls
how long different cells live, why certain cells
can be replaced and others cannot, where in our
DNA all this information is stored...all these
questions are being studied by scientists but are
still unanswered. The fact remains that as we age
our body slowly goes into disrepair: our bones and
muscles weakens, our immune system is less able to
protect us from disease, we tire easier, our
senses become less acute (hearing fades, eyesight
becomes poor, etc.), we heal
slower, etc. Can
you think of any advantages to having cells that
are programmed to die, if they are replaced with
new cells? (How do you think a caterpillar turns
into a butterfly?)
In a general sense, a
organism's life span is related to its size. This
holds pretty well for mammals, at least (a group
that includes humans). For example, a mouse or a
hamster lives less than a year while a dog can
live up to 15 years and a human over 100 years.
This is because the larger you are, the slower
your metabolism runs. Your metabolism is the
general rate at which your body's systems work. A
mouse's heart beats 6 times per second and a mouse
can digest a meal in a few minutes while a human's
about once per second and it takes
us several hours to digest a meal, so we say a
human has a slower metabolism. The slower your
metabolism runs, the less energy you spend
maintaining it and the longer it will last. As
humans, we have managed to increase our average
lifespan a great deal (from less than 40 years in
the 1600's to 70 years or more today). By
protecting ourselves from disease, improving our
food supply, and using medical technology we have
removed some of the major threats to our health.
We have also added a few new ones!
scientists might be able to remove the information
in our genes that tells cells when to die, or
somehow add information that will enable cells in
our body to repair themselves or prevent damage
from occuring. One recent discovery suggests that
oxidative damage (damage caused by oxygen) to
specific cells (motor neurons) may play a large
roll in the aging process in humans. We have a
special enzyme in our cells that helps prevent
oxidative damage, and perhaps we can use this
enzyme to slow down the aging process. So we may
be able to maintain our eyesight and muscle
strength a little longer. But until we figure out
how to get around Newton's Second
Thermodynamics, I believe our bodies will always
"have to die" at some point. I am sure some people
would disagree with me.
A scientist answering this from a scientific
perspective might say that cells can only divide a
certain number of times before they don't work
quite right any more.In other words, it is as if
they have a built-in clock that eventually runs
out. Even if we are not growing, cells in our
body die and need to be replaced by new cells, so
the process of cell division is still very
important. This is controversial, but I think
there is evidence to support this idea of limited
On the other hand, even as a
scientist, I am not sure that science has the best
answer to this question!
I'll acknowledge that death of humans is an
emotional topic--just last month I literally
watched my grandfather die and a few years
earlier, my father. Although it would be nice if
nobody would die, death is an obvious reality.
Some reasons for dying have no rational
explanation (i.e. Colorado). So I'll stick to the
ecological reasons for the concept of death and
not the process or the philosophical meaning. I'm
sure your class has heard the Disney explanation
of the "circle of life" and there is an element of
truth behind the marketing. Imagine that you have
a virus or a bacteria that causes no disease but
where the individuals cannot be killed. Although
the size of any individual seems to be harmless,
any individual requires resources such as space or
energy. As the population grows and grows, they
consume these resources to the detriment of all
other life (remembering that energy cannot be
created or destroyed). Resources are not
available for other life forms. Some time in the
future, the virus or bacteria will cover the
planet and will have crowded out all other life.
Death is necessary to allow resources to cycle
between organisms so that different SPECIES can
continue to survive rather than just INDIVIDUALS.
Tough question... why does any living thing die?
Scientists and philosophers alike have pondered
this one for ages. Consider a finely built clock.
Its builder tends it daily with loving care,
rewinding, oiling and polishing. He passes on his
love of the clock to his son, who tends it daily
and eventually even has to begin to replace parts.
It's not because they were damaged, but just that
friction and gravity eventually wear down the
metal. The son passes on the tending to his
daughter, who passes it on and so on and so forth.
Finally, 8 generations later, there are no more
repairs possible, the mainspring has just finally
given out. Not through neglect, not through
damage, but just use. It's the same for any
machine. It's the same for a piece of granite--
eventually it turns to sand.
So what about
living things? Well, I can't answer "why humans
HAVE to die," but we can think about why human
(and other ) living bodies "wear out." In many
ways, an animal's body is a machine -- the heart
muscle pumps, the blood veseels and capillaries
are the hoses, the muscles and bone are the
pistons, the liver and kidneys are the filters,
etc, etc... All of these physically wear down or
clog up. That is one reason why bypass and other
heart surgeries are so common. One really
interesting aspect of "aging" that was recently
discovered has to do with the DNA in each cell of
our bodies. Without getting too detailed, each
chromosome has lots of special sequences called
TELOMERES at each of its ends. These sequences
ensure that the genes on the ends of the
chromosomes are faithfully replicated during each
cell cycle. But the consequence is that some
telomeric sequence is lost in each round.
Eventually, with time, replication is no longer
complete and important genes may not be replicated
or expressed properly. So, even the DNA can "wear
This was one of the main questions
scientists wanted to know about Dolly, the sheep
that was cloned. Do you remember what they did?
First, the scientists took a sheep oocyte (egg)
and they removed the nucleus. Then, they were able
to put the nucleus (DNA) of another sheep into
that egg, and then stimulate development. The
result was Dolly. So even though Dolly is herself
only a few years old based on her birthdate, her
DNA is actually much older (it is Dolly's age plus
the age of the donor). So how old is
really? And will she "age" much faster? Scientists
are looking at this question pretty closely and
the answer is still unknown.
Biologists look at two types of answers to a
question like this. One is how exactly does it
happen. The other is why a particular thing
(mechanism) might have evolved. Let's tackle the
first question first.
We replace old,
damaged or dead cells through cell division
(mitosis). Some cells, like nerve and muscle
cells, never divide. That's why when we have an
injury to our brain, spinal cord, or heart (what
is the heart made of?), these parts do not recover
well. Other cells die or are damaged through
normal use. As we get older, all of our cells
divide less. This means that our parts start to
"wear out" because the cells that make them up
aren't replaced as fast. Each cell is programmed
to only divide a certain number of times. Cells
that don't have this built-in message to stop
dividing cause tumors or cancer because they
become unspecialized (no longer do their proper
jobs) and at the same time they divide out of
control, taking over the space of normal cells.
Things with fast metabolic rates (which you can
tell by their heart rate) seem to "wear out"
sooner, so shrews are dead of old age in 2 years
while tortoises may be over 100.
So why do
we have a "timer" on our cell division? Why
doesn't each person live forever? When we want to
answer a question like that, we look around at
what lives for a long time and what doesn't.
Among animals, things that are big and/or smart
tend to live longer. (why?)Let's do a thought
experiment. What if every person who had ever
been born were alive today? Things would be a bit
crowded, right? In fact there wouldn't be food,
water, or space for all of us. Now let's say we
take anyone who died from a cause not related to
aging out of the picture. That would still leave
a huge number of people, more than the earth could
support. There certainly wouldn't be room to add
Let's scale this way back to
the individual level, where selection takes place.
Things that survive and reproduce leave more
offspring that have some of the same traits that
made them successful. The recipe for the traits
are in the genes they pass on. Why don't we see
animal species where an individual keeps on having
offspring and neither parent or offspring die of
In a species that reproduces
sexually (whether plant or animal), a parent gives
its offspring half its genes. (Where does the
other half come from?) So from an evolutionary
standpoint, having two children alive is as good
as being alive yourself if it comes to passing
genes. The math shows you will leave more genes
by having 3 offspring and dying than by living.
The offspring will have to compete with the
parents for resources unless there's some other
place to go where there are more. Plus, a random
danger like a falling tree could wipe out the
whole gene collection if only one individual had
them. If the genes are in two individuals, only
half would be lost. So maybe having offspring,
then dying means you leave more genes than you
would by hanging around and competing for
resources. And if other problems, like injuries
and disease also make you less and less able, your
two offspring might do better than you.
the really long-lived things, we have to look at
plants, which might live thousands of years. Even
the longest lived animals such as people,
tortoises, parrots, and elephants don't live
anywhere near that long. How are plants an
animals different? Well a plant might send
thousands of seeds long distances away, presto, no
competition with its "parent" plants. Plus, if a
part of a plant is detroyed, a new piece can grow
in its place, so there's less need to "start over"
with a whole new plant. Last, few plants actually
survive the sprouting and early growth part of
their lives, so it makes sense for the successful
gene carrier (parent) to hang around rather than
take the long shot that two of its seeds can
successfully replace it.
Many cultures have
struggled with the question "why do we die?" and
come up with other answers, which may involve one
or more gods or spirits. Many people find comfort
in the non-science explanations. In science we
can only look at processes that obey natural laws.
This doesn't mean other explanations are wrong.
They're just not science.
I don't think anyone really understands
(natural) death in a scientific way. Why cells die
and why living things age and die is still a
mystery as far as I know.
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