NIOS BIOLOGY NOTES CH-1 : ORIGIN AND EVOLUTION OF LIFE AND INTRODUCTION TO CLASSIFICATION - PART 1

1. The planet earth came into existence sometime between 4 and 5 billion years ago.Life evolved on planet earth about 3.5 billion years ago. Since then, approximately 15 million different species of organisms have evolved

2. Origin of life means the appearance of simplest primordial life from nonliving matter.

3. Evolution of life means the gradual formation of complex organisms from simpler ones.

4. The widely accepted theory is the Chemosynthetic theory of origin of life, proposed by A.I. Oparin

5. Chemosynthetic Theory Life might have originated at first on earth through a series of combinations of chemical substances in the distant past and it all happened in water.

6. Chemosynthetic Process
-The earth originated about 5 billion years ago.
- It was initially made up of hot gases and vapours of various chemicals.
- Gradually it cooled down and a solid crust was formed.
-The early atmosphere contained ammonia (NH3), water vapour (H2O), hydrogen
(H2), methane (CH4). At that time there was no free oxygen. This sort of atmosphere (with methane, ammonia and hydrogen) is still found on Jupiter and Saturn
-Heavy rains fell on the hot surface of earth, and over a very very long period the water bodies appeared that still contained hot water.
- Methane and ammonia from the atmosphere dissolved in the water of the seas.
- In this water, chemical reactions occurred and gave rise to amino acids, nitrogenous bases, sugars and fatty acids which further reacted and combined to give rise to biomolecules of life such as proteins and nucleic acids.

7. Probable stages in the origin of life

#1. First stage : The sources of energy were the ultraviolet rays or electric discharge (lightening) or heat. Either alone or a combination of these energy sources caused reactions that
produced complex organic compounds (including amino acids) from a mixture of ammonia (NH3), methane (CH4), water (H2O) and hydrogen (H2). The amino acids
are the building blocks of proteins which are the main components of protoplasm.

NOTE : Stanley Miller and Harold C. Urey in 1953 set up an experiment with an air-tight apparatus  in which four gases (NH4, CH4, H2 and H2O) were subjected to an electric discharge for one week. On analyzing the liquid, they found a variety of organic substances in it, such as amino acids, urea, acetic acid, and lactic acid

#2. Second Stage : Simple organic molecules combined to form large molecules which included peptides (leading to the formation of proteins), sugars, starch and fat molecules.

#3. Third stage : The large molecules of different kinds combined together to form multi-molecular heaps or complexes. Some simple fat molecules arranged themselves around this
molecular complex in a sort of membrane. It was observed in the laboratory experiments that when such complexes reached a certain size they separated from the surrounding solution in the form of what were termed “coacervate drops” of microscopic size, moving in the liquid with a definite boundary (coacervate means “heap” referring to the combining together of the molecules). Coacervate like aggregates were probably the precursors of the first living cells. Some sort of “metabolism” could occur within these coacervates with synthesis of certain substances and breakdown of others. The latter (i.e. breakdown reactions) could provide energy.Some of the earliest formed proteins might have acted like enzymes and would have
affected the rates of reactions. It is also believed that RNA molecules might have shown enzymatic activity in the “primordial soup” of chemical compounds. Such molecules have been termed ribozymes.