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.
1. Arms, Camp. Chapter 17, “Evolution and Natural Selection,”
Chapter 21, “Origin of Life.” Biology. Bonnie Boehme.
Fourth Edition. The United States of America.
Harcourt Brace College Publishers; 1995. Pages 352-372, 440-455
2. Curtis, Barnes. Chapter 46, ” Evolution: Theory and Evidence,”
Chapter 48,”Natural Selection,” Chapter 49 “On the Origin of Species.” Biology. Sally Anderson. Fifth Edition. New York, USA; Worth; 1989:
3. Roberts, M.B.V. Chapter 34, ” Evolution in Evidence,”
Chapter 35,”The Mechanism of Evolution,” Biology. Fourth Edition. Surrey, UK; Nelson; 1986: Pages 560-616
4. The World Book Encyclopedia. World Book, inc. London: v.6;
5. Encyclopedia Britannica. Encyclopedia Britannica, inc. Chicago: v.4;
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