Modern Theory on Origin of Life

8/10/2019 Modern Theory on Origin of Life

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Modem views regarding the origin of life include chemical evolution and biological

evolution:

A. Chemical Evolution (Chemogeny):

1. The Atomic Phase:

Early earth had innumerable atoms of all those elements (e.g., hydrogen, oxygen,

carbon, nitrogen, sulphur, phosphorus, etc.) which are essential for the formation of

protoplasm. Atoms were segregated in three concentric masses according to their

weights, (a) The heaviest atoms of iron, nickel, copper, etc. were found in the centre of

the earth, (b) Medium weight atoms of sodium, potassium, silicon, magnesium, alu-

minium, phosphorus, chlorine, fluorine, sulphur, etc. were collected in the core of the

earth, (c) The lightest atoms of nitrogen, hydrogen, oxygen, carbon etc. formed the

primitive atmosphere.

2. Formation of Inorganic Molecules:

Free atoms combined to form inorganic molecules such as H2(Hydrogen),

N2(Nitrogen), H20 (Water vapour), CH4(Methane), NH3(Ammonia), C02(Carbon

dioxide). Hydrogen atoms were most numerous and most reactive in primitive

atmosphere.

First hydrogen atoms combined with all oxygen atoms to form water and leaving no free

oxygen. Thus primitive atmosphere was reducing atmosphere (without free oxygen)

unlike the present oxidising atmosphere (with free oxygen).

Hydrogen atoms also combined with nitrogen, forming ammonia (NH3). So water and

ammonia were probably the first molecules of primitive earth.


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3. Formation of Simple Organic Molecules (Monomers):

The early inorganic molecules interacted and produced simple organic molecules such

as simple sugars (e.g., ribose, deoxyribose, glucose, etc.), nitrogenous bases (e.g.,

purines, pyrimidines), amino acids, glycerol, fatty acids, etc.

Torrential rains must have fallen. As the water rushed down, it must have dissolved

away and carried with it salts and minerals, and ultimately accumulated in the form of

oceans. Thus ancient oceanic water contained large amounts of dissolved NH3, CH4,

HCN, nitrides, carbides, various gases and elements.

CH4+ C02+ H20 > Sugars + Glycerol + Fatty Acids

CH4+ HCN + NH3+ H20 > Purines + Pyrimidines

CH4+ NH3+ C02+ H20 > Amino Acids

Some external sources must have been acting on the mixture for reactions. These

external sources might be (i) solar radiations such as ultra-violet light, X-rays, etc., (ii)

energy from electrical discharges like lightning, (iii) high energy radiations are other


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sources of energies (probably unstable isotopes on the primitive earth). There was no

ozone layer in the atmosphere.

A soup-like broth of chemicals formed in oceans of the early earth from which living

cells are believed to have appeared, was termed by J.B. Haldane (1920) as prebioticsoup (also called hot dilute soup). Thus the stage was set for combination of various

chemical elements. Once formed, the organic molecules accumulated in water because

their degradation was extremely slow in the absence of any life or enzyme catalysts.

Experimental Evidence for Abiogenic Molecular Evolution of Life:

Stanley Miller in 1953, who was then a graduate student of Harold Urey (1893-1981) at

the University of Chicago, demonstrated it clearly that ultra-violet radiation or electricaldischarges or heat or a combination of these can produce complex organic compounds

from a mixture of methane, ammonia, water (stream of water), and hydrogen. The ratio

of methane, ammonia and hydrogen in Millers experiment was 2:1:2.

Miller circulated four gases methane, ammonia, hydrogen and water vapour in an air

tight apparatus and passed electrical discharges from electrodes at 800C. He passed

the mixture through a condenser.

He circulated the gases continuously in this way for one week and then analysed the

chemical composition of the liquid inside the apparatus. He found a large number of


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simple organic compounds including some amino acids such as alanine, glycine and

aspartic acid. Miller conducted the experiment to test the idea that organic molecules

could be synthesized in a reducing environment.

Other substances, such as urea, hydrogen cyanide, lactic acid and acetic acid were alsopresent. In another experiment Miller circulated the mixture of the gases in the same

way but he did not pass the electric discharge. He could not get the significant yield of

the organic compounds.

Later on many investigators have synthesized a great variety of organic compounds

including purines, pyrimidines and simple sugars, etc. It is considered that the essential

building blocks such as nucleotides, amino acids, etc. of living organisms could thus

have formed on the primitive earth.

4. Formation of Complex Organic Molecules (Macromolecules):

A variety of amino acids, fatty acids, hydrocarbons, purines and pyrimidine bases,

simple sugars and other organic compounds accumulated in the ancient seas. In the

primeval atmosphere electrical discharge, lightning, solar energy, ATP and

polyphosphates might have provided the source of energy for polymerisation reactions

of organic synthesis.


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S.W. Fox of the University of Miami has demonstrated that if a nearly dry mixture of

amino acids is heated, polypeptide molecules are synthesized. Similarly simple sugars

could form polysaccharides and fatty acids could combine to produce fats. Amino acids

could form proteins, when other factors were involved.

Thus the small simple organic molecules combined to form large complex organic

molecules, e.g., amino acid units joined to form polypeptides and proteins, simple sugar

units combined to form polysaccharides, fatty acids and glycerol united to form fats,

sugars, nitrogenous bases, and phosphates combined into nucleotides which polymer-

ized into nucleic acids in the ancient oceans.

Sugar + Sugar -> Polysaccharides

Fatty Acides + Glycerol -> Fats

Aminoacids- + Aminoacids > Proteins

Nitrogenous Bases + Pentose Sugars + Phosphates > Nucleotides

Nucleotides + Nucleotides > Nucleic Acids


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Which came First RNA or Protein?

The RNA first Hypothesis:

In the early 1980s three scientists (Leslia orgel, Francis Crick and Carl Woese)

independently proposed the RNA World as the first stage in the evolution of life in

which RNA catalysed all molecules necessary for survival and replication. Thomas Ceck

and Sidney Altman shared Nobel Prize in chemistry in 1989 because they discovered

that RNA can be both a substrate and an enzyme.

If the first cells used RNA as their hereditary molecule, DNA evolved from an RNA

template. DNA probably did not evolve as a hereditary molecule un tills RNA based life

became enclosed in membrane. Once cells evolved DNA probably replaced RNA as the

genetic code for most organisms.

The Protein First Hypothesis:

A number of authors (for example Sidney Fox, 1978) claimed that a protein catalytic

system must have developed before a nucleic acid replicative system. Sidney Fox had

shown that amino acids polymerized abiotically when exposed to dry heat to form

proteinoids.

Cairns-Smiths Hypothesis:

It was proposed by Graham Caims-Smith, according to which both proteins and RNA

originated at the same time.

Formation of Nucleoproteins:

The giant nucleoprotein molecules were formed by the union of nucleic acid and protein

molecules. These nucleoprotein particles were described as free living genes.

Nucleoproteins gave most probably the first sign of life.

B. Biological Evolution (Biogeny):

Conditions for the Origin of Life:For origin of life, at least three conditions are needed.

(a) There must have been a supply of replicators, i.e., self-producing molecules.

(b) Copying of these replicators must have been subject to error through mutation.


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(c) The system of replicators must have required a continuous supply of free energy and

partial isolation from the general environment.

The high temperature in early earth would have fulfilled the requirement of mutation.

1. Protobionts or Protocells:

These are at least two types of fairly simple laboratory produced structures Oparins

coacervates and Foxs microspheres which possess some of the basic prerequisites of

proto cells.

Although these structures were created artificially, they point to the likelihood that non-

biological membrane enclosures (proto cells) could have sustained reactive systems for

at least short periods of time and led to research on the experimental production of

membrane bound vesicles containing molecules, i.e., proto cells.

(i) Coacervates:

The first hypothesis was proposed by Oparin (1920). According to this hypothesis early

proto cell could have been a coacervate. Oparin gave the term coacervates. These were

non-living structures that led to the formation of the first living cells from which the

more complex cells have today evolved.

Oparin speculated that a proto cell consisted a carbohydrates, proteins, lipids and

nucleic acids that accumulated to form a coacervate. Such a structure could have

consisted of a collection of organic macromolecules surrounded by a film of water

molecules.

This arrangement of water molecules, although not a membrane, could have functioned

as a physical barrier between the organic molecules and their surroundings. They could

selectively take in materials from their surroundings and incorporate them into their

structure.


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Coacervates have been synthesized in the laboratory. They can selectively absorb

chemicals from the surrounding water and incorporate them into their structure. Some

coacervates contain enzymes that direct a specific type of chemical reaction.

Because they lack a definite membrane, no one claims coacervates are alive, but they doexhibit some life like characters. They have a simple but persistent organization. They

can remain in solution for extended periods. They have the ability to increase in size.

(ii) Microspheres:

An another hypothesis is that early proto cell could have been a microsphere. A

microsphere is a non-living collection of organic macromolecules with double layered

outer boundary. The term microsphere was given by Sydney Fox (1958-1964).

Sidney Fox demonstrated the ability to build microspheres from proteinoids.Proteinoids are protein like structures consisting of branched chains of amino acids.

Proteinoids are formed by the dehydration synthesis of amino acids at a temperature of

180C. Fox, from the University of Miami, showed that it is feasible to combine single

amino acids into polymers of proteinoids. He also demonstrated the ability to build

microspheres from these proteinoids.

Fox observed small spherical cell-like units that had arisen from aggregations of

proteinoids. These molecular aggregates were called proteinoid microspheres. The first

non-cellular forms of life could have originated 3 billion years back. They would have

been giant molecules (RNA, Proteins, Polysaccharides etc.).


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Microspheres can be formed when proteinoids are placed in boiling water and slowly

allowed to cool. Some of the proteinoid material produces a double-boundary structure

that encloses the microsphere. Although these walls do not contain lipids, they do

exhibit some membrane like characteristics and suggest the structure of a cellular

membrane.

Microspheres swell or shrink depending on the osmotic potential in the surrounding

solution. They also display a type of internal movement (streaming) similar to that

exhibited by cells and contain some proteinoids that function as enzymes. Using ATP as

a source of energy, microspheres can direct the formation of polypeptides and nucleic

acids. They can absorb material from the surrounding medium.

They have the ability of motility, growth, binary fission into two particles and a capacity

of reproduction by budding and fragmentation. Superficially, their budding resembles

with those of bacteria and fungi.

According to some investigators, microspheres can be considered first living cells.

2. Origin of Prokaryotes:

Prokaryotes were originated from proto cells about 3.5 billion years ago in the sea. Theatmosphere was anaerobic because free oxygen was absent in the atmosphere.

Prokaryotes do not have nuclear membrane, cytoskeleton or complex organelles. They

divide by binary fission. Some of the oldest known fossil cells appear as parts of

stromatolites. Stromatolites are formed today from sediments and photosynthetic

prokaryotes (mainly filamentous cynobacteria blue green algae).


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3. Evolution of Modes of Nutrition:

(i) Heterotrophs:

The earliest prokaryotes presumably obtained energy by the fermentation of organic

molecules from the sea broth in oxygen free atmosphere (reducing atmosphere). They

required readymade organic material as food and thus they were heterotrophs.

(ii) Autotrophs:

Due to rapid increase in the number of heterotrophs the nutrient from sea water began

to disappear and gradually exhausted. That led to the evolution of autotrophs. These

organisms were capable of producing their own organic molecules by chemosynthesis or

photosynthesis.

(a) Chemoautotrophs:

Drop in temperature stopped synthesis of organic molecules in the sea water. Some of

the early prokaryotes got converted into chemoautotrophs which prepared organic food

by using energy released during certain inorganic chemical reactions. These anaerobic

chemoautotrophs were like present anaerobic bacteria. They released CO2in the

atmosphere.

(b) Photoautotrophs:

Evolution of chlorophyll molecule enabled certain protocells to utilize light energy and

synthesize carbohydrates. These were anaerobic photoautotrophs. They did not use

water as a hydrogen source. They were similar to present day sulphur bacteria in whichhydrogen sulphide split into hydrogen and sulphur. Hydrogen was used in food

manufacture and sulphur was released as a waste product.

Aerobic photoautotrophs used water as a source of hydrogen and carbon dioxide as

source of carbon to synthesize carbohydrate in the presence of solar energy. The first

aerobic photoautotrophs were cyanobacteria (blue green algae) like forms which had

chlorophyll. They released oxygen in the atmosphere as the by product of

photosynthesis. The main source of genetic variation was mutation.

Oxygen Revolution:

As the number of photoautotrophs increased, oxygen was released in the sea and

atmosphere. Free oxygen than reacted with methane and ammonia present in the

primitive atmosphere and transformed methane and ammonia into carbon dioxide and

free nitrogen.


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CH4+ 202- > CO2+ 2H2O

4NH3+ 3O2 > 2N2+ 6H2O

The oldest fossil belonging to blue green algae, named Archaeospheroides

barbertonensis which is 3.2 billion years old. Oxygen releasing prokaryotes first

appeared at least 2.5 billion years ago.

4. Formation of Ozone Layer:

As oxygen accumulated in the atmosphere, the ultraviolet light changed some of oxygen

into ozone.

2O2+ O2- > 2O3

The ozone formed a layer in the atmosphere, blocking the ultraviolet light and leaving

the visible light as the main source of energy.

5. Origin of Eukaryotes:

The eukaryotes developed from primitive prokaryotic cells about 1.5 billion years ago.

There are two views regarding the origin of eukaryotes.

(i) Symbiotic Origin:

According to Margulis (1970-1981) of Boston University, some anaerobic predator host

cells engulfed primitive aerobic bacteria but did not digest them. These aerobic bacteria

established themselves inside the host cells as symbionts. Such predator host cells

became the first eukaryotic cells.

The predator host cells that engulfed aerobic bacteria evolved into animal cells while

those that captured both aerobic bacteria and blue-green algae became eukaryotic plant

cells. The aerobic bacteria established themselves as mitochondria and blue green algae

as chloroplasts.


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(ii) Origin by Invagination:

According to this view cell organelles of eukaryotic cells might have originated by

invagination of surface membrane of primitive prokaryotic cells.

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Modern Theory on Origin of Life

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