The speculation around the origin of life on earth is one of the most sought-after mysteries. The theory of special creation according to which a higher being has created life in the form that we know today, has been around since time immemorial. However, this theory has not found acceptance in the scientific world. Ancient Greek thinkers like Aristotle, Plato, Thales, etc., advocated the theory of spontaneous generation from non-living matter. This is known as abiogenesis. However, this theory was discarded after individual experiments conducted by Spallanzani, Redi, and Pasteur. The theory of biogenesis attracted much attention. However, biogenesis still cannot explain how the first trace of life appeared on earth. This theory was also discarded.
So, how exactly did life begin on earth? Religious beliefs aside, science has advocated for the chemical basis of the origin of life. What led to the formulation of such a theory, and how did this origin take place? Let's explore here!
The earth is currently home to almost 8.3 million species. Considering such an immense variety and network of life on earth, one can’t avoid the question of how it all started.
These species differ from each other in terms of their cellular arrangement, cellular components, genetic makeup, reproductive abilities, mode of nutrition, mode of growth and development, etc. To add to this plethora of variations and diversities, energy is constantly being transferred through different lifeforms.
However, one thing remains common to all life forms - the cell. The cell itself consists of a plethora of different molecules including biomacromolecules and inorganic molecules. All functions are operated by biopolymers including the RNA, the DNA, proteins, carbohydrates, phospholipids, etc.
This led scientists to speculate on the chemical basis of the origin of life.
It is typically assumed that the precursors of these vital components were synthesized abiotically, accumulated, condensed into polymers, interacted mutually, and ultimately evolved into a self-sustaining system through the natural phenomena that took place on the pre-biotic, primitive earth.
This theory was especially advocated and experimentally proved by Oparin and Haldane.
According to this theory of the chemical basis of the origin of life, life on earth originates from abiological matter, specifically organic chemicals. Organic chemicals over time, under the influence of the forces of physics and chemistry, combined with geochemical forces, led to the formation of the first traces of life on the face of the earth.
A I Oparin in 1923 first proposed the primordial (or prebiotic) soup theory, according to which organic compounds were first created spontaneously from inorganic molecules in an oxygenless, reductive atmosphere, by the action of sunlight and lightning.
The compounds thus formed were dissolved in the primitive ocean, wherein they concentrated and polymerized, forming coacervate droplets
J B S Haldane (1928) published a paper at around the same time, in which he proposed that the earth’s prebiotic oceans would've formed a hot, dilute soup in which the organic compounds could’ve formed.
The Oparin-Haldane Theory is known as the Biochemical Theory for the Origin of Life. According to this theory, life on earth began as a series of chemical reactions −
The primitive earth’s atmosphere contained elements such as hydrogen, nitrogen, carbon, oxygen, sulfur, etc. These elements combined among themselves, forming simple inorganic substances such as ammonia, water vapor, etc.
The simple inorganic compounds gradually reacted together, in presence of heat, sunlight, lightning, etc., to give rise to simple organic substances such as sugars, amino acids, and nitrogenous bases.
These simple organic compounds entered the primitive oceanic environment and reacted to form more complex organic macromolecules such as nucleic acids, proteins, etc. These molecules formed the basis for the origin of life.
The macromolecules thus formed aggregated into membrane-enclosed droplets (aka coacervates) owing to intermolecular forces. If you think about it, these droplets do resemble some properties of living systems, such as organization, selective permeability, and use of energy.
In the next step, the coacervates probably gave rise to a more organized, membrane-bound, autonomous structure, which contained the selfreplicating RNA molecule. These assemblages are referred to as protocells.
The theory of biochemical evolution or the Oparin-Haldane hypothesis has been explored via laboratory experiments, wherein the most plausible conditions for synthesis, accumulation and polymerization of the organic monomers have been simulated.
The Oparin-Haldane hypothesis was tested by Stanley Miller and Harold Urey, in 1953.
They attempted to create a simulation of the early earth conditions as proposed by the Oparin-Haldane theory.
They set up a closed chamber containing water into which they introduced the molecules thought to exist in the early earth’s primitive atmosphere.
Methane, hydrogen, and ammonia were the primary gases in the primitive atmosphere. Electrical current was supplied to simulate the primitive lightning discharge.
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Within a few days, organic molecules such as amino acids, urea, lactic acid, glycolic acid, propionic acid, etc., were observed in the chamber.
Further chemical evolution resulted from the polymerization or condensation of these monomers into macromolecules like peptides, polysaccharides, etc.
Thus, the Miller-Urey experiment demonstrated that some biological molecules, like amino acids and other organic acids, may have arisen abiotically, under primitive earth’s prevalent conditions.
Various speculative theories were put forward to explain the beginning of life on earth. Some theories that were put forward were abiogenesis, special creation, biogenesis, and panspermia. However, due to a lack of scientific basis, these theories were discarded. The theory of the chemical origin of life was proposed by Oparin and Haldane. This theory was experimentally tested by Miller and Urey, in 1953. According to this theory, life on earth began as a series of chemical reactions, where inorganic molecules reacted to form complex organic molecules. These molecules eventually were contained within a bilayer and formed the protocell. The protocells eventually evolved into reproducing organisms.
Q1. What is the theory of Panspermia?
Ans. According to this theory, life on earth actually originated from other extraterrestrial sources of life that could've been transported to earth via celestial bodies like meteorites.
Q2. How did the proposed formation of nucleic acids take place in prebiotic earth?
Ans. It is proposed that adenine was formed by the condensation of HCN, which was plentiful in the prebiotic atmosphere. The reaction is supposedly catalyzed by ammonia. Other bases were also synthesized similarly.
Q3. What is the LUCA in terms of evolution?
Ans. The Last Universal Common Ancestor is the RNA-based organism from which the archaea (and later the eukarya) and the eubacteria evolved.
Q4. How were the first biological membranes formed?
Ans. The first biological membranes were the result of the spontaneous assembly of phospholipids that originated in the primordial soup. These membranes enclosed the self-replicating RNA and other molecules.
Q5. What is the RNA World hypothesis?
Ans. The RNA world hypothesis proposes that RNA was the first genetic material and the first life form on earth, supported by its ability to store, transmit and replicate genetic information, along with the ability to act as a catalyst.
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