Theories for Origin of Life

1.0Theory of Special Creation

The greatest supporter of this theory was Father Suarez. This is a mythology-based theory.

This theory has three connotations-

All living organisms (Species or types) that we see today were created as such.

The diversity was always the same since creation and will be the same in future.

The Earth is about 4000 years old.

All these ideas were strongly challenged during the nineteenth century based on observations of Charles Darwin, Wallace etc. They believed that life forms varied over the periods of time.

From fossils records and their dating, we can conclude that Earth is very old, not thousands of years as was thought earlier but billions of years old.

2.0Cosmic Panspermia Theory

Some scientists believe that life came from outer space. Early Greek thinkers thought units of life called spores were transferred to different planets including Earth.

'Panspermia' is still a favourite idea for some astronomers.

3.0Theory of Spontaneuos Generation

For a long time, it was also believed that the life came out of decaying and rotting matter like straw, mud, etc. spontaneously.

This hypothesis was supported by ancient Greek philosophers. They believed that the mud of river Nile could give rise to fishes, frogs, crocodiles etc when warmed by light rays.

Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life and not from dead matter.

He showed that in pre-sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from killed yeast.

4.0Theory of Biogenesis

Proposed by Harvey & Huxley. They stated "Omnis vivum ex ovo or vivo", which means "New life can originate from pre-existing life." 

Experiments of Francesco Redi, Lazzaro Spallanzani, and Louis Pasteur etc supported the theory of biogenesis and disproved the abiogenesis. Experiment of Louis Pasteur is most renowned among all of these. However, this theory did not answer how the first life-form came on Earth.

5.0Oparin - Haldane Theory (Modern Theory)

Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (e.g. RNA, protein, etc.) and that formation of life was preceded by chemical evolution, i.e., formation of diverse organic molecules from inorganic constituents.

Oparin's theory was published in his book 'ORIGIN OF LIFE'.

First life originated in sea water, so water is essential for origin of life.

Oparin-Haldane theory is studied in two parts – Chemogeny & Biogeny.

Chemical Evolution (Chemogeny)

•  Early Earth had free atoms of all those elements which are essential for the formation of protoplasm (C, H, O, N etc.). Hydrogen was maximum among all of them.
• Due to the high temperature, hydrogen reacted with oxygen to form water and no free oxygen was left, which made the atmosphere. Hydrogen also reacted with nitrogen and formed ammonia.
• Hence, water and ammonia were probably the first inorganic compounds formed on Earth. Methane (CH₄) was the first organic compound.
•  As the Earth cooled down, the water vapor fell as torrential rain, to fill all the depressions and form primitive oceans. Meanwhile, molecules continued to react with each other and formed various simple and complex organic compounds.
• Now, the water of oceans has become a rich mixture of macromolecules/ complex organic compounds. Haldane called it hot dilute soup/ prebiotic soup.
• Hence, the possibilities of life were established in the water of primitive oceans because these macromolecules (Proteins, polysaccharides, fats/lipids, nucleic acids) form the main components of protoplasm.

Harold Urey & Stanley Miller Experiment:

In 1953, S.L. Miller, an American scientist, created similar conditions at laboratory scale which were thought to be on primitive Earth.

He took CH₄, NH₃, H₂ and water vapor at 800°C in a large flask (spark flask).

He created electric discharge by using two tungsten electrodes as a source of energy to raise the temperature inside spark flask to 800° C.

He observed the formation of simple amino acids like glycine, alanine, and aspartic acid.

In similar experiments other scientists observed, formation of sugars, nitrogen bases, pigments, and fats.

Analysis of meteorite content also revealed similar compounds indicating that similar processes are occurring elsewhere in space. With this limited evidence, the first part of the conjectured story i.e. chemical evolution was more or less accepted. 

Diagrammatic representation of Miller’s experiment

However, we have no clear idea about how the first self-replicating metabolic capsule of life arose, but we consider it to be a result of some mutations in the nucleic acids.

Biological Evolution (Biogeny)

(i) Origin of Protobionts:

Macromolecules which were synthesized abiotically in primitive oceans later came together and formed large colloidal drop-like structures named protobionts.

(ii) Origin of protocells (Eobionts):

Non-Replicating Nucleic Acid Self-Replicating Nucleic Acid.

Self-Replicating Nucleic Acid + proteins Nucleoproteins

Nucleoproteins were the first sign of life.

(iii) Origin of first cellular form (Prokaryotes):

As a result of mutation protocells became more complex and efficient to use the materials available in the surrounding medium and evolved into prokaryotic cells.

• This first cellular form of life did not possibly originate till about 2000 million years ago. (2 bya).
• The first living beings were single celled bacteria like prokaryotes with naked DNA. All life forms were in water environment only. 

Note:

Evolution of Oxygen

Liberation of free oxygen by cyanobacteria was a revolutionary change in the history of Earth.

Origin of Eukaryotic Cell

Nucleus, mitochondria, and other cell organelles developed in the cell and the cell became metabolically more active. These free living unicellular eukaryotic organisms are thought to have first appeared about 1.5 billion years ago in the primitive ocean.

6.0A Brief Account of Evolution

About 2000 million years ago (mya) the first cellular forms of life appeared on Earth.

By the time of 500 mya, invertebrates were formed and became active.

Jawless fishes probably evolved around 350 mya.

Sea weeds and few plants existed probably around 320 mya.

The first organisms that invaded land were plants. They were widespread on land when animals invaded land.

Fish with stout and strong fins could move on land and go back to water. This was about 350 mya. In 1938, a fish caught in South Africa happened to be a Coelacanth which was thought to be extinct.

These Coelacanth or lobefins evolved into the first amphibians that lived on both land and water. There are no specimens of these left with us. However, these were ancestors of modern day frogs and salamanders.

Lobefins

The amphibians evolved into reptiles. They lay thick shelled eggs which do not dry up in sun unlike those of amphibians. Again, we only see their modern day descendents, the turtles, tortoises and crocodiles.

Synapsids were the mammal like early reptiles which gave rise to mammals.

Sauropsids were the lizard like early reptiles which gave rise to different dinosaurs, modern reptiles and birds.

Sauropsids

In the next 200 million years or so, reptiles of different shapes and sizes dominated on Earth.

Giant ferns (pteridophytes) were present, but they all fell to form coal deposits slowly.

Some of the land reptiles went back into water to evolve into fish like reptiles probably 200 mya (e.g. Ichthyosaurs).

About 65 mya, the dinosaurs suddenly disappeared from the Earth. We do not know the true reason. This may have happened because (i) Climatic changes killed them or (ii) Meteorites’ collisions killed them. Moreover, some say that most of them evolved into birds. The truth may live in between.

Small sized reptiles of that era still exist today.

The first mammals were like shrews. Their fossils are small sized.

Mammals were viviparous and protected their unborn young inside the mother's body. Mammals were more intelligent in sensing and avoiding danger at least.

When reptiles came down mammals took over this Earth.

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