Evolution is one of the aspects which are full of mysteries. No one has been successful to prove the method of evolution of living organisms. Till date, there are only many hypothesis related to evolution.
Among such hypothesis, Oparin and Haldane Theory is also the one. This is taken as the most modern and convincing theory of the origin of life. This theory is also called as the Theory of Chemical Evolution. Russian Biochemist Alexander Ivanovich Oparin (1923 A.D.) and an Englishman J.B.S. Haldane (1928 A.D.) independently gave the “biochemical theory of origin of life”. They proposed that the origin of life occurred along the origin and evolution of the earth and its atmosphere.
According to Oparin and Haldane Theory, life evolved in the oceans during a period when the atmosphere was reducing. This reducing atmosphere contained H2, H2O, NH3, CH4 and CO2, but no free O2. Oparin provided a biochemical explanation of the origin of life in his book “The Origin of Life on Earth” which was published in the year 1936 A.D. This theory can be explained under three different steps. They are:
1. Chemical Evolution (Chemogeny): This step includes the following sub-steps:
i) Formation of Simple Compounds: About 4.2 billion years ago, the conditions on the earth were favorable for the chemical evolution. The atmosphere of the early earth composed primarily of methane (CH4), ammonia (NH3), water (H2O), carbondioxide (CO2) etc. These existed in gaseous state due to high temperature. Water existed as vapour. The water vapour condensed and resulted in the cooling of the earth and water bodies like rivers, lakes and oceans came into existence.
ii) Formation of Organic Compounds: The organic molecules are created in oxygen-less atmosphere through the action of sunlight. They are alcohols, aldehydes, amino acids, glycerol, fatty acids, simple sugars, purines and pyrimidines. They are formed under the influence of ultraviolet rays, radiations, electricity of thunderstorms and volcanic activities. The organic compounds formed in the hot water of ocean had been described as “the broths” or “primordial soup” or “the hot dilute soup” by Haldane.
iii) Formation of Complex Organic Compounds: Simple organic molecules combined to form complex organic compounds like polysaccharides, fats, proteins, nucleotides, nucleosides, nucleic acids etc. The formation of protein molecules can be considered as a landmark in the event of origin of life. The formation of nucleic acids was another important event in the transformation of non-living things to living things.
2. Biological Evolution (Biogeny): Biogeny consists of the following sub-steps:
i) Formation of Coacervates: The complex organic compounds of oceanic soup were accumulated due to intermolecular attraction and formed large and highly organized colloidal systems. These were named coacervates. Formation of coacervates was termed as coacervation. The coacervates acquired the ability to exchange energy and could grow. They grew in size as a result of dissolving of absorbing substances in the surrounding water. After attaining the maximum size, they multiplied by breaking down into smaller droplets. Coacervates are considered as the living molecules which gave rise to the life. In other words, the coacervates are described as the intermediates between the molecules and the organisms.
ii) Formation of Primitive Life: The coacervates contained biologically important macromolecules such as proteins, lipids, nucleic acids, nucleoproteins and other organic and inorganic substances. Later, a thin limiting membrane was developed around the coacervates. Some of the proteins of coacervates started working like enzymes for both destructive as well as synthetic reaction. Therefore, the coacervates started absorbing organic substances from oceanic soup and became anaerobic heterotrophs or protocells or eobionts. Eobionts developed into prokaryotes. The first “cell like” structures with division power wer called “eobionts” or “pre-cells”. Oparin called them “protobionts”. Eobionts originated about 3800-4200 million years ago. The prokaryotes evolved about 3.8 billion years ago. They were very simple in structure and without a definite nucleus. These early prokaryotes were chemoheterotrophs and were anaerobes.
3. Modifications of Life (Cognogeny): This step contains the following sub-steps:
i) Origin of Autotrophs: Primitive autotrophs developed from heterotrophs. The primitive autotrophs used chemical energy for synthesis of carbohydrates. They are called chemoautotrophs. Later some autotrophic bacteria synthesized bacteriochlorophyll. They trapped solar energy but did not produce oxygen, because they did not use water. The first aerobic photosynthetic organisms were probably some cyanobacteria-like organisms which evolved 3300-3500 million years ago. They were the earliest oxygen producing photoautotrophs. With the evolution of chlorophyll molecules, oxygen was released as a byproduct of photosynthesis and this changed the earth’s atmosphere from reducing to oxidizing one. Therefore, all possibilities of further chemical evolution were finished. In 1968 A.D., a 3.2 billion years old fossil of such a blue-green algae named Archaeo-spheroids barbertonensis was reported from the continent of Africa.
ii) Origin of Eukaryotic Cells (True Nuclear Cell): Aerobic respirations yield about twenty times more energy in a biological system. The prokaryotes, therefore, gradually modified to adapt the aerobic mode of respiration. They developed a true nucleus, mitochondria and other various cell organelles. Thus, free living eukaryotes originated in the ocean probably about 1.6 billion years ago. These eukaryotes were like unicellular organisms of today. Cellular organisms then gathered to form colonies and finally gave rise to diverse forms of life.