Evolution
Evolution, a process of change through time, is what links together the
enormous
diversity of the living world. A lot of evidence is present that
indicates that
the earth has had a very long history and that all living
things arose in the
course of that history from earlier, more simpler forms.
In other words, all
species have descended from other species and all living
things share common
ancestors in the past. Basically, organisms are what they
are because of their
history. Today there are many theories and possibilities
related to evolution
which contribute to our understanding of the process.
Our planet was born 4.6
billion years ago as a great cloud of dust and gas
condensed into a sphere. As
gravity pulled this great cloud tightly together,
heat from great pressure and
radioactivity melted the planet’s interior and
most of its mass. For millions
of years after this, strong volcanic activity
all over the planet shook the
earth’s crust. At the same time, the earth was
showered by a very strong
meteor shower. From studying volcanoes, it is known
that eruptions pour out
carbon dioxide, nitrogen, and other gases. It is also
known that meteorites
carry water, in the form of ice, and many carbon
containing compounds. That
might suggest that the combination of volcanic
activity and a constant shower of
meteorites released the gases that created
the Earth’s atmosphere. Geologists
believe that the earth’s early atmosphere
contained water vapor, carbon
monoxide, carbon dioxide, hydrogen, and
nitrogen. It also may have contained
ammonia and methane. It did not contain
oxygen, which is the main reason why the
Earth could not have supported
life. As for oceans, they couldn’t have existed
at first because the Earth’s
surface was extremely hot. But about 3.8 billion
years ago, the Earth’s
surface cooled enough for water to remain a liquid on
the ground.
Thunderstorms wet the planet for many years and oceans began to
fill. This is
known because the earliest sedimentary rocks have been dated to
that time
period. Miller and Urey were two scientists who attempted to explain
the
origin of life on Earth without referring to any supernatural events.
They
performed an experiment that suggests how the Earth’s atmosphere might
have
formed. Miller mixed "atmospheric" gases (hydrogen, methane,
ammonia,
and water vapor) in a sterile glass container and charged them with
energy by
adding electric sparks to them. The electric sparks resembled
lightning at the
time of the Earth’s formation. After about a week, the
mixture turned brown
and was found to contain amino acids. This organic
compound produced in this
experiment was efficient in knowing how the Earth’s
early atmosphere formed.
That is because it was successful in producing
some of the building blocks of
nucleic acids under geologically relevant
conditions. A question that puzzled
scientists was how could all this have
started in the first place. It is noted
that amino acids and nucleic acids
stick to the structures of clay crystals. By
being held together in a regular
pattern on clay crystals, these molecules
combine to form proteins and
polynucleotides. Other researchers not that some
kinds of RNA can join amino
acids into protein chains without help from protein
enzymes. Some forms of
RNA can copy themselves and can actually edit other RNAs
by adding and
deleting nucleotides. These experiments support another hypothesis
that RNA,
rather than DNA, functioned as life’s first information storage
system.
According to this hypothesis, life based on RNA have started when
RNA
fragments began to copy and edit themselves and assemble proteins. As
time
passed, these RNAs could have evolved to the point where they produced
protein
enzymes that took over the work of bringing about chemical reactions.
Later,
storing genetic information could have similarly been passed on to
DNA. In this
way, over thousands of years, RNA, DNA, and proteins could have
evolved into the
complex system that characterizes life today. Discovering
that RNA can act as a
catalyst, makes it easier to imagine how life began.
According to Bruce M.
Alberts, "One suspects that a crucial early event
was the evolution of an
RNA molecule that could catalyze its own
replication". That makes it very
obvious why it is possible that RNA was the
first molecule that could replicate.
These molecules then diversified
into a group of catalysts that could assemble
ribonucleotides in RNA
synthesis or accumulate lipid-like molecules to form the
first cell
membranes. This clearly suggest how the first membranes could have
formed.
Fox and his co-workers attempted to find an answer, to the origin
of
membranes and prokaryotes, in their laboratories. They heated amino
acids
without water and formed long protein chains. As water was added and
the mixture
cooled down, small microspheres were formed. These seemed to
accumulate certain
compounds inside them. They also attracted lipids and
formed a lipid-protein
layer around them, as mentioned above. Assuming that
the microspheres combined
with self-replicating molecules, we are looking at
a very ancient organism. This
is what might have happened 3.8 billion years
ago as the first membranes and
prokaryotes were forming. As for eukaryotic
cells, according to Lynn
Margulis’s hypothesis, they arose from what is
called a symbiont relationship.
Lynn Margulis believed that mitochondra
were originally independent prokaryotic
aerobic individuals, living on a
symbiont relationship with another prokaryote.
The aerobic prokaryote was
enclosed by the bacterium’s cell surface membrane
in the process of
endocytosis, which is made easy by the absence of a cell wall
in the
bacterium. The aerobic prokaryote wasn’t digested but continued to
function
inside the other cell. The host cell received energy that the
aerobic
prokaryote released. The mitochondrion that was forming had
everything it
wanted, taking it from its host. A similar process occurred
later with the host
cell and photosynthetic prokaryotes. This evidence
explains the symbiotic theory
for the origin of the four Eukaryotic kingdoms,
which are the Protista, Fungi,
Animalia, and Plantae. Jean Baptiste de
Lamarck had his own proposal of
evolution. It was not really accepted because
his evidence, which was not very
convincing, was not very supporting.
According to his belief, evolution is
supposed to produce "higher" organisms,
with human beings at its
ultimate goal. Lamarck’s theory included inheritance
of acquired
characteristics, meaning that an organism’s lifestyle could bring
about
changes that it passed on to its offspring. An example would be the
fact that
Lamarck believes Giraffes have long necks because their
ancestors stretched
their necks because their ancestors stretched their necks
to browse on the
leaves; and that this increase in length was passed on to
succeeding
generations. This seemed unreasonable because people had been
cutting off tails
of many dogs but they never resulted in an offspring born
without a tail for
that same reason. Therefore, Lamarck’s idea cannot be
correct, mainly because
these changes do not affect the genetic material.
Change happens in genetic
material only when games are involved. In 1858,
Charles Darwin introduced a
theory of evolution that is accepted by almost
all scientists today. His theory
states that all species evolved from a few
common ancestors by natural
selection. Another British scientist, Alfred
Wallace, introduced an identical
theory at about the same time. But Darwin’s
theory was better developed and
more famous. Darwin’s and Wallace’s concept
was based on five premises: 1)
there is stability in the process of
reproduction 2) in most species, the number
of organisms that grow, survive,
and reproduce is small compared to the number
initially produced 3) in any
population, there are variations that are not
produced by the environment and
some are inheritable 4) which individual will
grow and reproduce and which
will not are determined to a significant degree by
the interaction between
these chance variations and the environment 5) given
enough time, natural
selection leads to the accumulation of changes that
differentiate groups of
organism from another. Darwin’s theory of natural
selection is really the
process of nature that results in the most fit organisms
producing offspring.
There has been experimental evidence for this process,
attempting to prove it
correct. Darwin observed that wild animals and plants
showed variations just
as domesticated animals and plants did. He filled his
notebooks with records
of height, weight, color, claw size, tail length, and
other characteristics
among members of the same species. He also observed that
high birthrates and
a shortage of life’s necessities forced organisms into a
constant "struggle
for existence," both against the environment and
against each other. Plant
stems grow tall in search of sunlight, plant roots
grow deep into the soil in
search of water and nutrients. All that evidence is
what supported Darwin’s
theory about natural selection. Peppered moths provide
an example of natural
selection in action. Peppered moths spend most of their
time resting on the
bark of oak trees. In the beginning of the nineteenth
century, the trunk of
most peppered moths in England were light brown speckled
with green. There
were always a few dark-colored moths around, but light colored
moths were the
most common. Then, the Industrial Revolution began in England and
pollution
stained the tree trunks dark brown. At the same time, biologists
noticed that
dark-colored moths were appearing. The evolutionary hypothesis
suggested that
birds were the main reason. Birds are the major predators of
moths. It is a
lot harder for birds to see, catch, and eat moths that blend in
with the
color of the tree bark than it is for them to spot moths whose color
makes a
strong contrast with the tree trunks. The moths that blend in with
their
background are said to be camouflaged. As the tree trunks darkened,
the
dark-colored moths were better camouflaged and harder to spot, having a
better
condition for survival. This hypothesis was not enough, and more
experiments had
to be made. A British ecologist, called Kettlewell, prepared
another test for
this hypothesis. He placed equal numbers of light and dark
colored moths in two
types of areas. In one area, trees were normally
colored. In the other area,
they were blackened by soot. Later on, he
recaptured, sorted, and counted all
the moths he could, which were marked
earlier by him. Kettlewell found that in
unpolluted areas, more of his
light-colored moths had survived. Kettlewell
showed by his experiments that
the moths that were better camouflaged had the
higher survival rate. In
conclusion, when the soot darkened the tree trunks in
an area, natural
selection caused the dark-colored moths to become more
common.
Kettlewell’s work is considered to be a very good classic
demonstration of
natural selection in action. All organisms share biochemical
details. All
organisms used DNA and RNA to carry information from one
generation to another
and to control growth and development. The DNA of all
Eukaryotic organisms
always has the same basic structure and replicates in
the same way. The RNAs of
various species might act a little differently, but
all RNAs are similar in
structure from one species to the next. ATP is an
energy carrier that is also
found in all living systems. Also many proteins,
such as cytochrome c, are also
shared by many organisms. This molecular
evidence has made it possible to make
precise comparisons of the biochemical
similarities between organisms.
Scientists also noticed that embryos of
many different animals looked so similar
that it was hard to tell them apart.
Embryos are organisms at early stages of
development. These similarities show
that similar genes are present. The fact
that early development of fish,
birds, and humans is similar shows that these
animals share a common
ancestor, who had a particular gene sequence that
controlled its early
development. That sequence has been passed on to the
species that descended
from it. In the embryos of many animals the limbs that
develop look very
similar. But as the embryos mature, the limbs grow into arms,
legs, flippers
that differ greatly in form and function. These different
forelimbs evolved
in a series of evolutionary changes that altered the structure
and appearance
of the arm and leg bones of different animals. Each type of limb
is adapted
in a different way to help the organism survive in its
environment.
Structures like these, which meet different needs but
develop from the same body
parts, are called homologous structures. This is
all additional evidence of
descent from a common ancestor. There are other
theories for the origin of
species including special creation and panspermia.
Special creation involves
humans. Many people believe that humans were
created by God; so the theories of
evolution go against their religions
especially why they do not see God’s
hands in the process. As for panspermia,
it suggests that life could have
originated somewhere else and came to us
from space. This might be possible but
there is actually no supporting
evidence for it. Paleontology has also played a
big role in the study of
evolution. Over the years, paleontologists have
collected millions of fossils
to make up the fossil record. The fossil record
represents the preserved
history of the Earth’s organisms. Paleontologists
have assembled great
evolutionary histories for many animal groups. An example
would be looking at
probable relationships between ancient animals whose
evolutionary line gave
rise to today’s modern horse. The fossil record also
tells us that change
followed change on Earth. Scientists can use radioactivity
to determine the
actual age of rocks. In rocks, radioactive elements decay
into
non-radioactive elements at a very steady rate. Scientists measure this
rate of
radioactive decay in a unit called a half-life. A half-life is the
length of
time required for half the radioactive atoms in a sample to decay.
Each
radioactive elements has a different half-life. Carbon-14 is
particularly useful
because it can be used to date material that was once
alive. Because carbon-14
is present in the atmosphere, livings things take it
into their bodies while
they’re alive. So the relative amount of carbon-14 in
organic material can
tell us how long ago this material stopped taking in new
carbon into its system.
That was the time it died. Then, a graph is used
to determine the time. This is
the way scientists can deduce the approximate
age of materials based on a simple
decay curve for a radioisotope. In
organisms, variations in specific molecules
can indicate phylogeny; and
biochemical variations can be used as an
evolutionary clock. Phylogeny is the
line of evolutionary descent. Biochemistry
can be used to support other
evidence about revolutionary relationships, and it
can be very simple.
Scientists study similar molecules in different species and
determine how
much difference there is between the molecules. The more
difference there is,
the longer the time-span since the two species shared a
common ancestor. The
most commonly used substances in this technique are
hemoglobin , cytochrome
c, and nucleic acids. Hemoglobin is suited to studying
closer related
organisms that contain hemoglobin. Cytochrome c has been used to
compare
groups that are more different. The results from comparative
biochemistry
lone do not prove anything, but they confirm data found using other
methods.
Together, they become convincing. Today, the theory of evolution is
generally
considered to be the most important fundamental concept in the
biological
sciences. Nearly all scientists support it. However, large numbers of
people
opposed the theory when it was introduces. Still, some people do not
accept
it today.
Bibliography
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Fourth Edition.
The United States of America. Harcourt Brace College
Publishers; 1995. Pages
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Evolution: Theory and
Evidence," Chapter 48,"Natural Selection," Chapter
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York,
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Chapter 34, "
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