Evolution

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