Researches and several findings have proved that until now, earth is the only planet where existence of life is possible. Though, at present times, many more researches are still going on to look forward that whether life on other planets is possible or not. Whatever may be the result, but the fact cannot be denied that earth is the first planet where life existed. Have we ever thought that how life originated in the earth? What was the first living being and how did it evolved in earth surface?
Therefore, to prove about the origin of life, different scientist has given their ideas through observation and experiments. Among many views, some ideas where rejected whereas some views on origin of life were accepted.
Special creation theory: according to this theory, life on the earth was created by the strong power of his almighty, god. The holy books of different religions have clearly engraved on this fact of origin of life. As per the Hindu mythology, God Brahma has created the entire universe and all the living beings present here whereas according to Bible or Christianity, life is created within six days and the first man to exist on the earth is Adam. Likewise, many more believes are there by other religions on existence of life. But scientifically, this theory is not accepted because there are no particular evidences that support this theory.
Theory of spontaneous generation or abiogenesis: according to this theory life originated spontaneously from the non- living matter. This theory was given by Aristotle and was also supported by Von Helmont. He explained that if a dirty cloth soaked in water containing grains can generate rat in 14 to 15 days. But this theory was also completely rejected by many other scientists like Pasteur, spallanzani and F. Redi.
Theory of panspermia/ cosmozoic theory: this theory was given by Richter that tells that life has come from other planet in the form of spore.
Theory of catastrophism: according to this theory, life was already present on the earth in different form due to the destruction by catastrophes.
Theory of biogenesis: this theory tells that life was created from some preexisting form of life on the earth. This theory was supported by different scientist who gave different experiments to prove biogenesis of life.
Redi’s experiment: he is an Italian scientist who gave an experiment to prove biogenesis and disapprove theory of spontaneous generation. He took 3 jars with containing meat. The first jar was completely covered with cloth, second jar was also covered with cloth but with holes in the cloth and third jar was left open. After some days, he noticed that no changes are there in the closed jar, whereas in the second jar some maggots were there outside the cloth and in the open jar, lots of maggots can be noticed. Therefore, he proved that these maggots came from the eggs that have been laid in the jar.
Spallanzani ‘s experiment: he was the other scientist who disapproved theory of spontaneous generation by giving an experiment. He took a jar and added some yeast in it. Along with the yeast, he also added some sugar and boiled the mixture for some time. Then he left the jar undisturbed for about 2-3 months. After the certain time period, when he checked for the changes, he found no bacteria growing inside the jar. It is because, no air was present there, hence so growth of bacteria was noticed.
Pasteur’s experiment: he was the French scientist, he also explained biogenesis theory through an experiment. He took a swan neck flask, and added yeast, sugar and water to it. He boiled the mixture and left it for 4-5 hrs. He noticed nothing in the flask but when he broke the neck of the flask in such a way that the mixture came in contact with the air, after sometime he noticed the growth of bacteria inside it. It is because air contains some living microbes or bacteria that when came in contact with the mixture, resulted in growth of bacteria.
These were some of the experiments done by different scientist to prove that life came from living form and not from the non-living substances.
Theory of biochemical origin of life: this theory was given by Oparin and Haldane. According to this theory, origin of life in the earth is a slow and gradual process that originated 3.7 bya. This theory also clearly explains that:
After these above theories and experiments, it was Miller and Urey who correctly gave an experiment to prove the origin of life.
Miller and Urey experiment: they used different chemicals, energy and apparatus to come to an exact solution. Firstly, they used different molecules that were present in the primitive earth such as: methane, ammonia, hydrogen and water. All these gases were placed in a closed system and continuous electric current was supplied to it. After a week they noticed that 10-15% of carbon has been converted to organic form and a less percent of carbon have combined to give amino acid. Therefore this experiment proved that the reducing condition on the primitive earth when there was abundance of hydrogen and no oxygen or ozone after series of chemical reactions resulted in the presence of life on the earth.
Image Reference: www.fas.org
Chemogeny: it means life begin due to the chemical reactions that took place in the reducing environment.
1. Formation of simple compound: Hydrogen combines with oxygen, methane and cyanide. Due to high temperature of the earth water was present in the form of vapor, but slowly due to several reactions it condensed to form rain. Slowly, years after reactions resulted in cooling of earth and hence different water bodies were formed.
2. Formation of organic compound: as the temperature cooled down, condensation, polymerization, oxidation and reduction took place thus giving simple organic compound. Such as: purines, pyramidines, fatty acid, glycerol and so on. These reactions took place in presence of energy obtained from thunder, radiation and volcanic activities.
3. Formation of complex compound: the simple molecules that were formed slowly combined to give larger and complex compounds like proteins, nucleic acid, fats etc.
When phospholipids were formed, they got assembled, thus double layer of phospholipids acquired inclusions, i.e. droplets and this was the next step toward the life on earth. After chemical evolution, protocell were formed and these protocell are the self assembled membrane bound system that includes different macromolecules, hence resulting in biological evolution in which different compounds and structures were formed. Two model for protocell was proposed; coacervates and microsphere. Experiments were also done by maintaining the primitive condition and this experiment was done by different scientist. Oparin obtained coacervates that were large enough to be viewed under the microscope.
1. Formation of coacervates: these are the organized colloidal systems that were formed due to the attraction and accumulation of complex organic compounds. They had the ability to exchange energy and grow to the particular size. Some of the properties noticed by Oparin from the coacervates prepared in the lab maintaining the primitive condition are:
Formation of microsphere: the experiment on microsphere was done by Sydney F. Fox. He took protenoids and boiled with H2O,thus microsphere were obtained. Some of the properties of microsphere are:
2. Formation of primitive life: simple prokaryotes were formed from organic and inorganic substance. Chemoheterotrophs and anaerobes were the first life forms that developed from the coacervates.
3. Origin of autotrophs: primitive autotrophs used chemical energy for synthesis of their food, hence called chemoautotrophs that were developed from heterotrophs. Then there were photoautotrophs, oxygen producing organism named cynobacteria.
4. Origin of eukaryotic cell: when oxygen became available in the environment, it made way for aerobic respiration. Slowly prokaryotes developed different cell organelles and adapted aerobic mode of respiration and then free living eukaryotes originated.
This way different chemical reaction marked the initiation of life in the earth and slowly from simple organism, many complex organisms were formed. Still, after all these experiments and observations, there was a controversy on what came first, RNA, DNA or the Protein. But lately several hypothesis has proved that RNA has ability to store and transfer the information. It assists in making protein and it also worked as an enzyme. Though single stranded RNA can hardly survive, in fact it is unstable, therefore double stranded RNA came into existence and ribose was replaced with deoxyribose. Still, it is difficult to figure out and prove regarding what came first because all the three of these molecules are important for life and it is impossible to get on a conclusion for what has come first.
Still, we are now presently well aware on the existence of life and surely its importance on the earth. Therefore, what we are now, is all due to changes that took place long time back.