Evolution of Life forms - A Theory

THEORIES ON THE ORIGIN OF LIFE

1. Theory of special creation states that the life was created by supernatural power in the form which has not undergone any change. It was given by Father Suarez. God created life in six days from materia prima and man was created by Him on the sixth day. According to this theory, earth is about 4000 years old.

2. Theory of catastrophism was given by Cuvier, according to which after a gap of certain period ( called age), the world undergoes a catastrophe (sudden calamity) which kills almost all the living organisms and then God creates a new generation or new life from inorganic matter.

3. Theory of biogenesis (i.e. life from life, omnis vivum ex. vivo) was proved by Redi, Spallanzani and Pasteur independently. They disproved (refuted) theory of spontaneous generation (abiogenesis). Francesco Redi (1668) proved that flies could not arise from putrefying meat without their eggs. Spallanzani (1767) demonstrated that putrefaction of meat is due to microbes in the air and it can be prevented by boiling and sealing the meat in air tight containers. Pasteur gave a definite proof of life arising from pre-existing life using microbes and sterilization methods. He performed "swan neck flask" experiment.

4. Cosmozoic theory (Theory of panspermia) given by Richter (1865); Helmholtz (1884), Arrhenius (1908) suggested that life reached the earth from some heavenly body through meteorites. Panspermia (primitive form of life, as suggested by Arrhenius, 1908) consisted of spores or seeds (sperms) and microbes that existed throughout universe and produced different forms of life on this earth.

Abiogenic or Chemical Origin of Life

Majority of the scientists are ofthe opinion that life originated from inanimate matter. Since the theory of abiogenic origin or chemical evolution of life is the only one that provides an explanation, which can be tested, most scientists have tentatively accepted it.

Oparin-Haldane Hypothesis

1. Alexander I. Oparin (1894-1980), a Russian biochemist, and J.B.S. Haldane (1892-1964), a British scientist, put forward the concept that the first living organism evolved from non-living material. They also suggested that the sequence of events that might have occured. In 1923, Oparin postulated that life originated on Earth at some point of time in the remote past, and under the conditions no longer observed. In his book, The Origin of Life (1938), Oparin submitted" abiogenesis first, butbiogenesis eversince". Oparin's theory is known as primary abiogenesis.

2. According to Oparin and Haldane (1929), spontaneous generation of early molecules might have taken place ifthe earth once had more reducing atmosphere compared to the present oxidising atmosphere. Oparin and Haldane agreed that the primeval Earth contained little, if at all, oxygen. Perhaps, in the primitive atmosphere oxygen in the free gaseous state was virtually absent. Therefore, no degradation of any organic compound arising in the primeval Earth could have taken place.

3. As there was no ozone layer in the atmosphere, any absorption of UV radiations, that is lethal to our present lives, was not possible in the primeval Earth.

4. The early gas cloud was rich in hydrogen, being present in the combined form in methane (CH₄), ammonia (NH₃) and water vapour (H₂O).

5. Moreover, the atmospheric water vapour along with early gas cloud condensed into drops of water and fell as rain that rolled down the rock surfaces and accumulated to form liquid pools and oceans. In the process, erosion of rocks and washing of minerals (e.g., chlorides and phosphates) into the oceans were inevitable. Thus, Haldane's hot dilute soup was produced and the stage was set for combination of various chemical elements.

6. Atmospheric chemicals and those in water produced small precursor molecules, like amino acids, sugars, nitrogenous bases etc. These precursor molecules then combined resulting in the appearance of proteins, polysaccharides and nucleic acids.

7. The energy sources for such reactions of organic synthesis were the UV radiations (solar radiation), cosmic rays, electrical discharges (lightning), intense dry heat (volcanic eruption) and radioactive decay of various elements on the Earth's surface. Once formed, the organic molecules accumulated in water because their degradation was extremely slow in the absence of any life or enzyme catalysts. Such transformation is not possible in the present oxidising atmosphere because oxygen or micro organisms will decompose or destroy the living particle that may arise by mere chance.

Experimental Evidence for Abiogenic Molecular Evolution of Life

Harold C.Urey (1893 -1981), an astronomer, accorded the first adequate recognition of Oparin-Haldane's view on the origin of life in 1952.

Urey asked his student Stanely L. Miller, a biochemist, to replicate the primordial atmosphere as propounded by Oparin and Haldane.

Miller (1953) made the first successful simulated experiment to assess the validity of the claim for origin of organic molecules in the primeval Earth's conditions.

Concept Builder

1. Mllier sealed in a spark chamber a mixture of water (H₂O), methane (CH₄), ammonia (NH₃) and hydrogen gas (H₂). He made arrangement for insertion of two electrodes to provide electrical energy (simulation of lightning) to the spark chamber. CH₄, NH₃, H₂ were in 2 : 1 : 2 ratio and water vapour at 800ºC. Electric sparks of 75,000 volt was provided to the mixture.

2. The spark chamber was connected to another flask with arrangement for boiling water (provision for evaporation). The other end of the spark chamber was connected to a trap by a tube that passed through a condenser (an arrangement for condensation and collection of aqueous solution, equivalent to rain and Haldane's soup). The trap, in turn, was connected with the flask for boiling water (arrangement for circulation).

3. The control apparatus contained every arrangement except that it was devoid of energy source.

4. After eighteen days, significant amount of the simple organic compounds (monomers), such as amino acids and peptide chains began to appear in the aqueous sample of the experimental set. Amino acids found were alanine, glycine and aspartic acid. Therefore, the obvious inference was that abiotic synthesis of organic monomers occurred in the simulated experimental condition. By analogy, such synthesis could have occurred in the primitive atmospheric condition. Later on many scientists repeated Miller's experiment using slightly different starting materials and UV radiation or other energy sources. All of them could successfully synthesise amino acid and related compounds. With hydrogen cyanide (HCN), even adenine and other nitrogen bases were produced.

Abiotic synthesis of biomolecules is studied under following headings:

1. Chemogeny: Synthesis of organic molecules by chemical reactions.

2. Biogeny: Formation of self replicating biomolecules in broth (primordial hot soup or warm little pond).

3. Cognogeny: Evolution of various forms of life or diversification of existing groups.

Enclosing the Prebiotic Systems

The experiments of Miller and other scientists demonstrate that prebiotic molecules could have been formed under the conditions which most likely existed on early Earth.

Still, the formation of prebiotic soup of small molecules does not necessarily lead to the origin of life.

For origin of life, atleast three conditions needed to have been fulfilled:

1. There must have been a supply of self-replicators i.e., self-producing molecules.

2. Copying of these replicators must have been subject to error via mutation.

3. The system of replicators must have required a perpetual supply of free energy and partial isolation from the general environment.

The high temperature prevailing in early Earth would have easily fulfilled the second condition, that is, the requirement of mutation. The thermal motion would have continually altered the prebiotic molecules.

Concept Builder

The third condition, partial isolation, has been attained within aggregates of artificially produced prebiotic molecules.

These aggregates called protobionts can separate combinations of molecules from the surroundings; maintain an internal environment but are unable to reproduce.

Two important protobionts are coacervates and microspheres.

Oparin (1924) observed that if a mixture of a large protein and a polysaccharide is shaken, coacervates form.

Their interiors, which are primarily protein and polysaccharide, with some water, become separated from the surrounding aqueous solution.

The later has much lower concentration of proteins and polysaccharide.

Oparin's coacervates also exhibits a simple form of metabolism.

As these coacervates do not have lipid outer membranes and cannot reproduce, they fail to fulfil the requirement as a candidate of probable precursors of life.

Microspheres were formed when mixtures of artificially produced organic compounds were mixed with cool water.

If the mixture contains lipids, the surface of the microspheres consists of a lipid bilayer, reminiscent to the lipid bilayer of cell membranes.

Sydney Fox (1950) obtained protenoid microspheres.

There is considerable discussion among biologists as to how the first cells may have evolved.

The discovery made in the 1980's that RNA can act like enzyme to assemble new RNA molecules on an RNA template raised the interesting possibility that Coacervates may not have been the first step in the evolution of life.

Perhaps the first macromolecules were RNA molecules, and the initial steps on the evolutionary line were ones leading to more complex and stable RNA molecules.

Later, the stability might have been improved by surrounding the RNA within a coacervate.

Still other scientists reject the notion of 'RNA world', entirely, pointing out that some RNA components are too complex to have been present on the primitive earth.