Evolution, a process of change through time, is what links together the enormousdiversity of the living world. A lot of evidence is present that indicates thatthe earth has had a very long history and that all living things arose in thecourse of that history from earlier, more simpler forms. In other words, allspecies have descended from other species and all living things share commonancestors in the past.
Basically, organisms are what they are because of theirhistory. Today there are many theories and possibilities related to evolutionwhich contribute to our understanding of the process. Our planet was born 4. 6billion years ago as a great cloud of dust and gas condensed into a sphere. Asgravity pulled this great cloud tightly together, heat from great pressure andradioactivity melted the planets interior and most of its mass.
For millionsof years after this, strong volcanic activity all over the planet shook theearths crust. At the same time, the earth was showered by a very strongmeteor shower. From studying volcanoes, it is known that eruptions pour outcarbon dioxide, nitrogen, and other gases. It is also known that meteoritescarry water, in the form of ice, and many carbon containing compounds. Thatmight suggest that the combination of volcanic activity and a constant shower ofmeteorites released the gases that created the Earths atmosphere.
Geologistsbelieve that the earths early atmosphere contained water vapor, carbonmonoxide, carbon dioxide, hydrogen, and nitrogen. It also may have containedammonia and methane. It did not contain oxygen, which is the main reason why theEarth could not have supported life. As for oceans, they couldnt have existedat first because the Earths surface was extremely hot. But about 3. 8 billionyears ago, the Earths surface cooled enough for water to remain a liquid onthe ground.
Thunderstorms wet the planet for many years and oceans began tofill. This is known because the earliest sedimentary rocks have been dated tothat time period. Miller and Urey were two scientists who attempted to explainthe origin of life on Earth without referring to any supernatural events. Theyperformed an experiment that suggests how the Earths atmosphere might haveformed.
Miller mixed “atmospheric” gases (hydrogen, methane, ammonia,and water vapor) in a sterile glass container and charged them with energy byadding electric sparks to them. The electric sparks resembled lightning at thetime of the Earths formation. After about a week, the mixture turned brownand was found to contain amino acids. This organic compound produced in thisexperiment was efficient in knowing how the Earths early atmosphere formed.
That is because it was successful in producing some of the building blocks ofnucleic acids under geologically relevant conditions. A question that puzzledscientists was how could all this have started in the first place. It is notedthat amino acids and nucleic acids stick to the structures of clay crystals. Bybeing held together in a regular pattern on clay crystals, these moleculescombine to form proteins and polynucleotides. Other researchers not that somekinds of RNA can join amino acids into protein chains without help from proteinenzymes.
Some forms of RNA can copy themselves and can actually edit other RNAsby adding and deleting nucleotides. These experiments support another hypothesisthat RNA, rather than DNA, functioned as lifes first information storagesystem. According to this hypothesis, life based on RNA have started when RNAfragments began to copy and edit themselves and assemble proteins. As timepassed, these RNAs could have evolved to the point where they produced proteinenzymes that took over the work of bringing about chemical reactions. Later,storing genetic information could have similarly been passed on to DNA. In thisway, over thousands of years, RNA, DNA, and proteins could have evolved into thecomplex system that characterizes life today.
Discovering that RNA can act as acatalyst, makes it easier to imagine how life began. According to Bruce M. Alberts, “One suspects that a crucial early event was the evolution of anRNA molecule that could catalyze its own replication”. That makes it veryobvious why it is possible that RNA was the first molecule that could replicate. These molecules then diversified into a group of catalysts that could assembleribonucleotides in RNA synthesis or accumulate lipid-like molecules to form thefirst cell membranes. This clearly suggest how the first membranes could haveformed.
Fox and his co-workers attempted to find an answer, to the origin ofmembranes and prokaryotes, in their laboratories. They heated amino acidswithout water and formed long protein chains. As water was added and the mixturecooled down, small microspheres were formed. These seemed to accumulate certaincompounds inside them.
They also attracted lipids and formed a lipid-proteinlayer around them, as mentioned above. Assuming that the microspheres combinedwith self-replicating molecules, we are looking at a very ancient organism. Thisis what might have happened 3. 8 billion years ago as the first membranes andprokaryotes were forming. As for eukaryotic cells, according to LynnMarguliss hypothesis, they arose from what is called a symbiont relationship.
Lynn Margulis believed that mitochondra were originally independent prokaryoticaerobic individuals, living on a symbiont relationship with another prokaryote. The aerobic prokaryote was enclosed by the bacteriums cell surface membranein the process of endocytosis, which is made easy by the absence of a cell wallin the bacterium. The aerobic prokaryote wasnt digested but continued tofunction inside the other cell. The host cell received energy that the aerobicprokaryote released. The mitochondrion that was forming had everything itwanted, taking it from its host.
A similar process occurred later with the hostcell and photosynthetic prokaryotes. This evidence explains the symbiotic theoryfor the origin of the four Eukaryotic kingdoms, which are the Protista, Fungi,Animalia, and Plantae. Jean Baptiste de Lamarck had his own proposal ofevolution. It was not really accepted because his evidence, which was not veryconvincing, was not very supporting.
According to his belief, evolution issupposed to produce “higher” organisms, with human beings at itsultimate goal. Lamarcks theory included inheritance of acquiredcharacteristics, meaning that an organisms lifestyle could bring aboutchanges that it passed on to its offspring. An example would be the fact thatLamarck believes Giraffes have long necks because their ancestors stretchedtheir necks because their ancestors stretched their necks to browse on theleaves; and that this increase in length was passed on to succeedinggenerations. This seemed unreasonable because people had been cutting off tailsof many dogs but they never resulted in an offspring born without a tail forthat same reason. Therefore, Lamarcks idea cannot be correct, mainly becausethese changes do not affect the genetic material. Change happens in geneticmaterial only when games are involved.
In 1858, Charles Darwin introduced atheory of evolution that is accepted by almost all scientists today. His theorystates that all species evolved from a few common ancestors by naturalselection. Another British scientist, Alfred Wallace, introduced an identicaltheory at about the same time. But Darwins theory was better developed andmore famous. Darwins and Wallaces concept was based on five premises: 1)there is stability in the process of reproduction 2) in most species, the numberof organisms that grow, survive, and reproduce is small compared to the numberinitially produced 3) in any population, there are variations that are notproduced by the environment and some are inheritable 4) which individual willgrow and reproduce and which will not are determined to a significant degree bythe interaction between these chance variations and the environment 5) givenenough time, natural selection leads to the accumulation of changes thatdifferentiate groups of organism from another.
Darwins theory of naturalselection is really the process of nature that results in the most fit organismsproducing offspring. There has been experimental evidence for this process,attempting to prove it correct. Darwin observed that wild animals and plantsshowed variations just as domesticated animals and plants did. He filled hisnotebooks with records of height, weight, color, claw size, tail length, andother characteristics among members of the same species. He also observed thathigh birthrates and a shortage of lifes necessities forced organisms into aconstant “struggle for existence,” both against the environment andagainst each other.
Plant stems grow tall in search of sunlight, plant rootsgrow deep into the soil in search of water and nutrients. All that evidence iswhat supported Darwins theory about natural selection. Peppered moths providean example of natural selection in action. Peppered moths spend most of theirtime resting on the bark of oak trees. In the beginning of the nineteenthcentury, the trunk of most peppered moths in England were light brown speckledwith green.
There were always a few dark-colored moths around, but light coloredmoths were the most common. Then, the Industrial Revolution began in England andpollution stained the tree trunks dark brown. At the same time, biologistsnoticed that dark-colored moths were appearing. The evolutionary hypothesissuggested that birds were the main reason. Birds are the major predators ofmoths.
It is a lot harder for birds to see, catch, and eat moths that blend inwith the color of the tree bark than it is for them to spot moths whose colormakes a strong contrast with the tree trunks. The moths that blend in with theirbackground are said to be camouflaged. As the tree trunks darkened, thedark-colored moths were better camouflaged and harder to spot, having a bettercondition for survival. This hypothesis was not enough, and more experiments hadto be made. A British ecologist, called Kettlewell, prepared another test forthis hypothesis.
He placed equal numbers of light and dark colored moths in twotypes 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 allthe moths he could, which were marked earlier by him. Kettlewell found that inunpolluted areas, more of his light-colored moths had survived.
Kettlewellshowed by his experiments that the moths that were better camouflaged had thehigher survival rate. In conclusion, when the soot darkened the tree trunks inan area, natural selection caused the dark-colored moths to become more common. Kettlewells work is considered to be a very good classic demonstration ofnatural selection in action. All organisms share biochemical details. Allorganisms used DNA and RNA to carry information from one generation to anotherand to control growth and development.
The DNA of all Eukaryotic organismsalways has the same basic structure and replicates in the same way. The RNAs ofvarious species might act a little differently, but all RNAs are similar instructure from one species to the next. ATP is an energy carrier that is alsofound in all living systems. Also many proteins, such as cytochrome c, are alsoshared by many organisms.
This molecular evidence has made it possible to makeprecise comparisons of the biochemical similarities between organisms. Scientists also noticed that embryos of many different animals looked so similarthat it was hard to tell them apart. Embryos are organisms at early stages ofdevelopment. These similarities show that similar genes are present. The factthat early development of fish, birds, and humans is similar shows that theseanimals share a common ancestor, who had a particular gene sequence thatcontrolled its early development.
That sequence has been passed on to thespecies that descended from it. In the embryos of many animals the limbs thatdevelop look very similar. But as the embryos mature, the limbs grow into arms,legs, flippers that differ greatly in form and function. These differentforelimbs evolved in a series of evolutionary changes that altered the structureand appearance of the arm and leg bones of different animals.
Each type of limbis 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 bodyparts, are called homologous structures. This is all additional evidence ofdescent from a common ancestor. There are other theories for the origin ofspecies including special creation and panspermia. Special creation involveshumans. Many people believe that humans were created by God; so the theories ofevolution go against their religions especially why they do not see Godshands in the process.
As for panspermia, it suggests that life could haveoriginated somewhere else and came to us from space. This might be possible butthere is actually no supporting evidence for it. Paleontology has also played abig role in the study of evolution. Over the years, paleontologists havecollected millions of fossils to make up the fossil record. The fossil recordrepresents the preserved history of the Earths organisms.
Paleontologistshave assembled great evolutionary histories for many animal groups. An examplewould be looking at probable relationships between ancient animals whoseevolutionary line gave rise to todays modern horse. The fossil record alsotells us that change followed change on Earth. Scientists can use radioactivityto determine the actual age of rocks. In rocks, radioactive elements decay intonon-radioactive elements at a very steady rate. Scientists measure this rate ofradioactive decay in a unit called a half-life.
A half-life is the length oftime required for half the radioactive atoms in a sample to decay. Eachradioactive elements has a different half-life. Carbon-14 is particularly usefulbecause it can be used to date material that was once alive. Because carbon-14is present in the atmosphere, livings things take it into their bodies whiletheyre alive.
So the relative amount of carbon-14 in organic material cantell 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 isthe way scientists can deduce the approximate age of materials based on a simpledecay curve for a radioisotope.
In organisms, variations in specific moleculescan indicate phylogeny; and biochemical variations can be used as anevolutionary clock. Phylogeny is the line of evolutionary descent. Biochemistrycan be used to support other evidence about revolutionary relationships, and itcan be very simple. Scientists study similar molecules in different species anddetermine how much difference there is between the molecules.
The moredifference there is, the longer the time-span since the two species shared acommon ancestor. The most commonly used substances in this technique arehemoglobin , cytochrome c, and nucleic acids. Hemoglobin is suited to studyingcloser related organisms that contain hemoglobin. Cytochrome c has been used tocompare groups that are more different.
The results from comparativebiochemistry lone do not prove anything, but they confirm data found using othermethods. Together, they become convincing. Today, the theory of evolution isgenerally considered to be the most important fundamental concept in thebiological sciences. Nearly all scientists support it.
However, large numbers ofpeople opposed the theory when it was introduces. Still, some people do notaccept it today. Bibliography1. 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. Pages352-372, 440-455 2.
Curtis, Barnes. Chapter 46, ” Evolution: Theory andEvidence,” Chapter 48,”Natural Selection,” Chapter 49 “Onthe Origin of Species. ” Biology. Sally Anderson. Fifth Edition.
New York,USA; Worth; 1989: Pages 961-973, 991-1029 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 WorldBook Encyclopedia. World Book, inc. London: v.
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