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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