Origin of Life (Part - 2) - Notes, Botany, Class 12 PDF Download

Origin of Life

BIOLOGICAL EVOLUTION

(i) Origin of Protobionts and Nucleoprotein (Coacervates) Macromolecules which were synthesized abiotically in primitive ocean later came together and formed large colloidal drop like structures named as Protobionts (Later called coacervates by oparin, Fox called them Microsphere and Deamer called them vesicles). Each protobiont was a cluster of macromolecule. They contain proteins, nucleic acids, lipids, polysaccharides etc. They grew by absorbing molecules from their environment. They could divide by budding like bacteria, many chemical reactions including the decomposition of glucose took place inside the protobionts. The sun provide energy for chemical reaction. According to oparincoacervates were the first sole living molecules which gave rise to cell .

 (ii) Origin of protocells [Eobiont] : The first living form named protocell originated in the primitive oceans.

The protocell were clusters of nucleo proteins which formed by composition of nucleic acids and enzymetic proteins. Nucleoproteins had the property of self duplication. Nucleo proteins were first sign of life. The protocell represented the beginning of life.

From protocells or eobionts few core of nucleoproteins gets seperated free in oceans and became inactive but when they enter in another eobionts they became active so virus like structures were formed.

Origin of virus like structure is an example ofretrogressive evolution(complex to simple). While evolution which inhibits earth surface was progressive means simple to complex.

Important 

(1) Khorana (1970) artificially synthesized 77 nucleotide RNA molecule out side a living cell which suggests that probably RNA was the premordial genetic meterial rather than DNA.

Zaug, Thomas cech and Altman described that some RNA molecules have enzymatic activity hence probably the RNA enzymes called ribozymes were able to replicate the premordial RNA . The discovery of RNA molecule working as enzyme has also changed our thinking about origin of life. It is now believed that about 4 billion years ago earth was an 'RNA world' in which RNA molecule carried out all the process of life without the help of either protein or DNA. By this discovery evolution is named as RNA world.  

 (2) It is estimated that life originated about 3.9 billion years ago as protocell (eobionts) in precrambian era which was anaerobic heterotrophic.

(iii) Origin of Prokaryotes – As a result of mutation the protocells became more complex and efficient to use the materials available in the surrounding medium and condensed themselves into prokaryotic cells. Thus the first living being were prokaryotic, like bacteria they were single celled and consisted of naked DNA. Nutritionally they were chemoheterotrophs (saprotrophs), respiration wasanaerobic.

(iv) Origin of Autotropism – It includes the origin of chemosynthesis and photosynthesis.

(a) Origin of chemosynthesis : Due to continue withdrawal of organic molecules by chemoheterotrophs organic material decreased in oceans.

Before the organic material disappeared in sea, new modes of nutrition developed, one of them was chemosynthesis.

The organism which perform chemosynthesis are called as chemoautotrophs. They were anaerobic and synthesise organic molecules from inorganic material. The energy was obtained by oxidising inorganic materials present in the sea.

Such mode of nutrition is found in Bacteria e.g. sulphur bacteria, iron bacteria, nitrifying bacteria. 

(b) Origin of Photosynthesis : After some time bacteriochlrophyll developed in some autotrophic bacteria like organism. They could absorb solar energy and convert it into chemical form. These organism called photoautotrophs.

They utilise solar energy for synthesizing organic compounds. This process is called photosynthesis.

They were anaerobic and utilised hydrogen from sources other than water like H2S. Therefore, no oxygen was evolved and atmosphere remained reducing.

This stage of photosynthetic autotrophism is represented by planktonic sulphur bacteria of today.

The Bacterio chlorophyll formed true chlorophyll by molecular changes. Such organism which were bearing true chlorophyll are similar to today's Cyanobacteria (B.G.Algae).
They utilized water as hydrogen donor and evolved oxygen.

 Oxygen revolution – During the origin and evolution of life liberation of free O2 by blue green algae like prokaryotes due to photosynthesis was a revolutionary change in the history of earth. It is called oxygen revolution.

It includes important changes like –

(1) Atmosphere of earth changed from reducing to oxidising, hence possibilities of further chemical evolution and abiogenesis finished, because chemical evolution always take place in reducing environment.

(2) Free O₂ oxidized CH₄ and NH₃ to form gases like CO₂, N₂ and H₂O.

(3) Accumulation of free O₂ formed a layer of O₃ (ozone) above the atmosphere of earth. Which started absorbing most of the U.V. rays of sunlight.

Origin of Eukaryotic cell – About 2.7 billion years ago conditions became suitable for aerobic respiration with the release of free O₂.

Aerobic respiration yields about 20 times more energy then anaerobic respiration hence the prokaryotes adapted themselves for aerobic mode of respiration.

Nucleus, mitochondria and other cell organelles developed in the cell and thus free living eukaryotic cell like organism originated about 1.5 billion years ago in the primitive ocean.

Organic Evolution – 

1. Though life originated by chemical evolution on primitive earth, was later replaced by organic evolution.

2. Organic evolution states "Descent with modification". i.e. the present day complex organism have evolved from earlier simpler organism by small but gradual changes which have occurred over millions of years.

3. Though living organisms show great diverstiy in size, structure, function, behaviour etc. they also show basically similar metabolic processes indicating common ancestory.

Special Points :

1. Evolution up to formation of coacervates termed as chemical evolution, in which complex organic compound were formed which were essential for formation of cellular structure.

2. Evolution from coacervates to simple cell structure known as biological evolution.

3. From simple cell to recent..... evolution is called organic evolution, in which organism developed structures and modified them by which they became more adaptive in their changing environment.

4. First protein which is formed during evolution in primitive oceans were not structural.

5. First nucleic acid which was formed in primitive oceans from combination of nucleotides, did not have power of replication. They obtained power of replication later by mutation.

6. Evolution term intorduced by - Herbert Spencer.

7. # What is evolution ?
The word evolution means to unfold or unroll or to reveal hidden potentialities. Evolution simply means an orderly change from one condition to another.

8. # Stellar Evolution – Changes in planets and stars from their origin to death.

9. # Inorganic evolution – The matter, elements change in time called Inorganic evolution.

10. Evolution is a slow but continuous process which never stop-Buffon.

11. Dollo's law - It states that evolution is irreversible.

12. According to Theodosius Dobzhansky (1973), nothing in biology makes sense except in the light of evolution.

13. Scala nature or ladder of nature :– Aristotle represented the evolution of complex organism from simple organism in the form of a ladder which is called Scala Nature or Ladder of nature. He kept simple organism at the bottom of ladder and complex organism at the top of this ladder. George cuvier studied the evolutionary history of organism by studying fossils and rejected Scala nature.

14. The history of life actually comprises two events :

(i) The origin of life
(ii) Evolution of life [the mechanism involved in the changes of living organisms through time]

15. Two great themes of evolutionary biology :

(i) The diversity of life, including both the differences and similarities.
(ii) The characteristics of organism, both adaptive and non-adaptive.

16. For origin of life, at least three conditions needed to have been fulfilled.

(i) There must have been a supply of replicators i.e. self  producing molecules.
(ii) Copying of these replicators must have been subject to error via mutation.
(iii) The system of replicators must have required a perpetual supply of free energy and partial isolation from the general environment.

17. Cosmology– Study of universe.

18. Biological evolution is also known as biogeny.

19. Unit of evolution is population.

20. Oparin's theory also known as primary abiogenesis.

21. Evolutionary biology– Study of history of  life forms on earth.