Experimental Evidence by Urey & Miller (1952)

Simulation experiment made in spark–discharge apparatus with H₂, NH₃ & CH₄ in ratio of 2 : 1 : 2 connected to the flask with boiling water on one hand and condenser on the other hand to cool water vapour to liquid form.

Fig.3.1: Urey & Miller Experiment

Provision of electric spark through tungsten electrodes, supply of radiation and heat energy were made.

Experiment run continuously for 18 days resulted in formation of glycine, alanine and aspartic acid.

Calvin Melvin (1958) irradiated CO₂ and H₂O directly in a cyclotron and obtained formic acid, succinic acid and oxalic acid.

Origin of life in this prebiotic soup must have needed 3 conditions to fulfill–

• Formation of replicators i.e., self duplicating molecules.
• Replications must have been subjected to error via mutations.
• System of replicators must have required a perpetual source of energy and partial isolation.

These could be explained in further steps :

4^th Step :

The complex molecules in prebiotic soup aggregated to form complex colloidal droplet as prebionts (or protobionts) to fulfill the 3rd (above said) condition of partial isolation. Such aggregates have been artificially produced, which maintain internal environment but are unable to reproduce.

Oparin (1924) called such protobiont as coacervates, Sydney fox (1950) called them microsphere.

Coacervates (protein + polysaccharide + some water) of Oparin showed some metabolism, but had no lipid memberane and  couldn’t reproduce, hence could not be precursor of life. Microspheres formed by mixing  artificially made organic compounds with cool water also had a lipid bilayer. Fox obtained these bodies (1-2 μm) by heating a mixture of few dry amino acids at 130–180ºC and  later cooling them in water, this resembled the size & shape of a coccoid bacteria and could constrict in a process that resembled budding in bacteria. The drawback is that they have limited diversity.

5^th Step :

Nucleic acids are the one to fulfill the 1st requirement i.e. of replicators in prebiotic stage. The experiment yielded purines & pyrimidines which could have formed the nucleotides by random polymerization. But within organisms enzymes (protein) control this process which is formed by transcription→ translation. How could this system have evolved if protein needed nucleic acids to form and nucleic acids required protein to catalyse its replication ? This “chicken–and–egg” puzzle of gene first or metabolism first has an answer if RNA (r-RNA) is considered to have formed first since it has both catalytic and informational properties. The first genetic code would have based on RNA that catalysed its own replication and other chemical reaction e.g., formation of lipid like molecule, proteo-lipid membrane, proteins etc.

One of such formed proteins (enzyme) did play as reverse transcriptase to form DNA from RNA. Then DNA took over the role of genetic material (the blue print) of life.

6^th Step :

The earliest cell (1st living organism) formed were anaerobic heterotrophic prokaryotes. Horowitz (1945) and Orgel (1973) believed that naked gene or present day virus-like form were formed first which then synthesized the likeable molecules around itself from the available raw material in the sea water and then covering by lipid membrane. They obtain energy by anerobic breakdown of organic molecules absorbed from surroundings.

7^th Step :

Before the exhaustion of available organic molecules some heterotrophs might have evolved the property of synthesizing organic molecules first by chemosynthesis and then by photosynthesis.

Chlorophyll like molecule was evolved to trap the sunlight energy, these were the present day Cyanobacteria (blue green algae) like prokaryotes. Emergence of O₂ in photosynthesis was a turning point that changed the reducing atmosphere of earth to oxidising.

8^th Step :

To protect itself from the hostility of O₂ and utilize its potential for generating higher amount of energy more efficient system with membranous organelles i.e., eukaryotes were evolved. There are 2 theories :

• Gradualistic theory or theory of independent origin– As per Oparin’s idea the membranous organelles gradually developed around the specific molecules to form mitochondria, chloroplast and other organelles. Such isolation made the eukaryotes more efficient and so aerobic metabolism of 1 mole of glucose could yield 36 mol. ATP.
• Symbiotic Theory: (Margulis-1964)– believes in formation of eukaryotes by symbiotic association among various kinds of independently existing prokaryotes.Chloroplast and mitochondria like small efficient autotrophic prokaryotes entered the large, less efficient heterotrophic prokaryotes for better protection and supply of raw material, hence all getting benefitted from each other. It became more acceptable after the discovery of DNA and ribosomes inside mitochondria & chloroplasts.