Life and its Origin
Evolution
Evolution is defined as a slow and progressive change resulting into complexity from simple and lower organisms which has taken place during millions of years on the earth.
Theories of origin of life
- Theory of special creation
- Theory of cosmogeny
- Theory of abiogenesis
- Theory of biogenesis
- Theory of biochemical origin of life
Theory of special creation
This theory was proposed by some spiritual people. According to this view, all the existing plants and animals were created by some supernatural power or God. The first man was Adam and woman Eve. However, according to Hindu mythology, God Brahma created various forms of life as such in one stroke. The first man was Manu and woman Shraddha.
As the theory has no any scientific evidence, presently this theory is not accepted. However, it was accepted by a Spanish Priest Father Saurez.
Theory of cosmogeny
This theory was proposed by philosopher - Ritcher and supported by Arrhenius.
According to this theory, life came on the earth from another planet in the form of small spores, called panspermia. When the planet collided with the earth, panspermia came on the earth and developed in different forms of life.
As this theory has no scientific proof, this theory is not accepted.
Theory of abiogenesis
At early \(19^{th}\) century, both scientists and philosophers believed that living things could arise from inanimate substance. Example: Frogs, toads, snakes were believed to be created from mud. Parasites, beetles, flies arose from sweat and manures.
As this theory has no any scientific evidences, this theory is not accepted.
Theory of biogenesis
This theory states that the life was originated not from non-living substances but from the living organisms which were already present in the atmosphere in pre-existing forms. When they get a suitable condition or medium, they grow and multiply. Many scientists established the theory of biogenesis which some experiments which are as follows:
Redi's Experiment
To prove this idea, Redi made an experiment using two jars. He kept the lumps of boiled meat in both jars. One jar was left uncovered and another was covered with the gauze or muslin cloth and others with parchment paper. The experiment was left for few weeks.
Observation: After few days, Redi observed fly maggots only in the uncovered or open jars where flies could lay their eggs on meat.
Conclusion: On the basis of the above observation, it could be stated that the fly maggots were originated not from inanimate substances but from their pre-existing forms which might be present already in the atmosphere.
Spallanzani Experiment
To his experiment, he set up two sets of experiments using flasks. He poured hay infusion in all flasks. They were boiled to make them sterilized. The experiment was left for few days.
Observation: After few days, he observed thick growth of microorganism in the opened flasks but no microorganisms were observed in the sealed flasks.
Conclusion: On the basis of the above observation it could be stated that the microorganism were created not from inanimate hay infusion but from the pre-existing forms which were present already in the atmosphere.
Pasteur's Experiment
In this experiment he used a swan-necked flask. He kept hay infusion in the swan-necked flask and made it sterilized by boiling. The mouth of the flask was closed. He left the experiment for 18 months.
Observation: Even for over 18 months, Pasteur did not find any microorganism in the flask. After that he broke off the swan neck from some flasks, exposing the hay infusion to air. After some time, microbes appeared in the flasks.
Conclusion: On the basis of the above observation, Pasteur concluded that the microorganisms were created not from inanimate hay infusion but from their pre-existing forms which might be present in the atmosphere.
Theory of biochemical origin of life
It is the modern and widely accepted concept of the origin of life. The primitive atmosphere of the earth had water, methane, ammonia and cyanamide that underwent a series of chemical reactions in the ocean. As a result, mixture of biologically important macromolecules like protein, sugar, nucleic acids etc. were formed which finally developed life on the earth.
This is a complex phenomenon, Oparin grouped this theory in three steps:
a) Chemogeny (synthesis of simple organic molecules)
It involves the synthesis of simple organic molecules from inorganic substances. This process is sub-divided into the following sub-steps:
i. Formation of water, ammonia, methane and cyanamide
Elements like carbon, hydrogen, nitrogen and oxygen were most abundant on the primitive earth in the beginning. As the earth surface gradually cooled, these elements reacted with each other to form water, ammonia, methane and cyanamide
\( C + 2H_{2} \rightarrow CH_{4} \) (methane)
\( H_{2} + \frac{1}{2}O_{2} \rightarrow H_{2}O \) (water)
\( N_{2} + 3H_{2} \rightarrow 2NH_{3} \) (ammonia)
ii. Formation of hydrocarbons
It is presumed that when the temperature of the earth's surface was considerable cooled to 1000°C or even low, the highly reactive free radicals - CH and -CH₂ condensed to form varieties of saturated and unsaturated hydrocarbons.
\( CH + CH \rightarrow C_{2}H_{2} \) (acetylene)
\( CH_{2} + CH_{2} \rightarrow C_{2}H_{4} \) (ethylene)
\( CH_{2} + CH_{2} \rightarrow CH_{4} + C \) (methane)
iii. Formation of oxy and hydroxyl derivatives of hydrocarbons
These hydrocarbons further reacted with super heated steam. Due to unusual bonding property of their carbons, oxy and hydroxy derivatives of hydrocarbons such as aldehydes and ketones were formed.
\( C_{2}H_{2} + H_{2}O \rightarrow CH_{3}CHO \) (acetaldehyde)
iv. Formation of carbohydrates, amino-acids and fatty acids
Due to the condensation, polymerization, oxidation and reduction reactions of above molecules, complex macro-molecules such as simple sugar, amino-acids and fatty acids were formed.
\( CH_{4} + H_{2}O \rightarrow \) Sugars \( (C_{6}H_{12}O_{6}) \)
\( CH_{4} + H_{2}O + NH_{3} \rightarrow \) Amino acids
v. Formation of purines, pyrimidines and nucleotides
The above molecules again reacted and aggregated together in hot water (ocean) and formed new molecules like purines, pyrimidines and nucleotides.
b) Biogeny
This step involves the formation of complex and self-reproducing biological life. This can be described under the following sub-steps:
i. Formation of nucleic acids
It is presumed that sugar, phosphate, purines and pyrimidines combined together to form nucleotides or nucleoproteins.
ii. Formation of coacervates
The nucleoproteins along with other macromolecules of primordial soup in the ocean aggregated together and bound by a water layer.
iii. Formation of primary organisms
These grew in size reproduced and multiplied and resulted to the formation of anaerobic heterotrophs or proto-cells or first cells.
Prokaryotic cells: These cells are of without definite nuclei like monera. Monera evolved into bacteria, cyanobacteria and blue green algae (Prokaryotes)
Eukaryotic cells: These cells are with definite nuclei like protista. Protista gave rise to eukaryotes that evolved into Protozoa, Metazoa and Metaphyta.
Note: What is coacervate?
Coacervates is a colloidal substance which contains nucleoprotein bounded externally by glycolipid.
c) Cognogeny
This step involves diversification in Protozoa, Metazoa and Metaphyta. As the food material got shortage in the primitive sea, the organism started to change their feeding habits.
Chemo-autotropism: The energy required in the autotropism for the synthesis of the organic molecules was solely from anaerobic breakdown is called chemo-autotropism.
\( 6CO_{2} + 12H_{2}S \rightarrow C_{6}H_{12}O_{6} + 6H_{2}O + 12S \)
Photosynthesis: The presence of chlorophyll enabled the organisms to synthesize carbohydrates using water in place of sulphur (\( H_{2}S \)) such as blue green algae. This process is called photosynthesis.
\( 6CO_{2} + 6H_{2}O \xrightarrow{\text{solar energy}} C_{6}H_{12}O_{6} + 6O_{2} \)
Miller and Urey Experiment
Miller and Urey designed an apparatus of glass flasks. The apparatus contained two glass chambers-water chamber and gas chamber. The gas chamber contained gases like methane, ammonia and hydrogen in the ratio of 2:1:2. The whole set up was run for a week.
Observation: With the experiment, a series of chemical reactions took place in the gas chamber. As a result, various products were formed, condensed and collected in tube.
Result: Chemical analysis of products indicated the presence of alanine, aspartic etc.
Conclusion: The result of the above experiment suggested that electric discharges produced during lightning in the primitive atmosphere of the earth containing \( NH_{3} \), \( CH_{4} \), \( H_{2} \) and water vapour might have resulted in the formation of amino acids and other organic compounds naturally on the primitive earth.