NPTEL – Basic Courses – Basic Biology

Lecture 3: Origin of Life (Part-I)

Introduction: Study of living organisms such as plants, animals and human etc is the
active area of life science. Now question is how you will define “LIFE”. Life is defined
as “the ability of an organism to reproduce, grow, produce energy through chemical
reactions to utilize the outside materials”. But scientists and philosophers have tried to
understand two important questions related to life

1. How life originated on earth?

2. How different kinds of organisms are formed in the world?
So first the question is how earth formed and how its internal structure support the life?
evidences suggest that earth and other planets in solar system came to existence around
4.5-5 billion years ago. Earth originally had two components: solid mass lithosphere and
the surrounding gaseous envelope atmosphere. Once the temperature of primitive earth
cooled down below 1000C, liquid components known as hydrosphere.

The formed earth consists of three parts as given in Figure 3.1. These parts are as follows:
1. Baryosphere: it is the central core of the earth. It is filled with molten magma with
large quantity of iron and nickel. Baryosphere has two zones: inner core region (~800
miles radius) and outer core region (~1400miles radius).

2. Pyrosphere: it is the middle part of the earth, also known as mantle. It is ~1800 miles
in thickness and mainly consists of silica, magnese and magnesium.

3. Lithosphere: it is the outermost region of the earth, also known as crust. It is 20-25
miles in thickness and mainly has silica and aluminium.

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Figure 3.1 : Cross section of Earth

Now the next question, what will be the pre-requisite of life on earth? There are
multiple conditions on earth to support the life on earth. These are as follows:

Primitive earth with little or no oxygen. The earth original had a reducing
environment due to presence of hydrogen and hydrogen compounds with water
(such as CH4) and ammonia (NH3). Due to gravitional forces, these gases remains
within the atmosphere of primitive earth. The reducing environment of
primitive earth will help to synthesize organic compounds from interaction of
inorganic substances.

Inorganic raw material for origin of life: Inorganic material in the earth interact to
form organic material required to to produce life.

Energy source. The energy source on primitive earth came from the following
sources:
Solar radiation
Electric discharge
Volcanic eruptions

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Heat
Cosmic Rays
Radioactive Decays

Infinite time: As per estimate it tooks almost 1 billion years from the formation
of earth to appearance of life. Such a lon time is needed for chemical reactions to
occur without the help of enzyme.

NOW COMING TO OUR FIRST QUESTION? HOW LIFE ORIGINATED ON

EARTH?
Six major theories are proposed to explain the origin of life on earth. These theories are
as follows:
1. Theory of Special Creations: The theory of special creation is proposed that life on
earth is created by a supernatural power, the GOD. According to the Christian belief, god
has created the universe, planet, animal,plant and human in six natural days. Similar
beliefs are also been proposed by other religion as well. There are believes in the theory
of special creation. These points are as follows:

A. All living organisms were created same day [NO DIFFERENCE IN THEIR
APPEARANCE].

B. They were created in the present form [NO EVOLUTION].

C. Their bodies and organs are fully developed to meet the requirement to run the life
[NO ADAPTATION]

OBJECTIONS TO THE THEORY OF SPECIAL CREATION:

It was purely based on religious belief.

There was no experimental evidences to support the assumptions.

The age of different fossils proves that living organism appear on earth in
different time frame.

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2. Theory of spontaneous generations: The theory of spontaneous generation or

abiogenesis assumes that non-living material in a spontaneous manner give rise to life.
There are several observations supporting this theory, which are as follows:

Hair of horse tail dipped in the water gives rise to horsehair worm, gordius.

Fly larvae develops on rotten meat.

In ancient Egypt, it was believe that frog, snake, crocodiles in the mud of nile
river warmed with sun.

Van Helmont claimed that he can produce mice from the dirty shirt and hadful of
wheat grains kept in dark cupboard in 3 weeks.

EVIDENCES AGAINST THE THEORY OF SPONTANEOUS GENERATION:

Theory of spontaneous generation was criticized by Lazzaro spallanzani, Francisco redi
and Louis Pasteur. These great scientists performed well designed scientific experiments
to disprove the theory of spontaneous generations.

1. Redi’s experiment: Francisco redi did conclusive and well designed experiment to
disapprove the theory of spontaneous generation. He placed meat and fishes in 3 separate
jars. Jar No. 1 was left open, No. 2 was covered with gauze and the third one was covered
with paper. The meat/fishes decayed in all three jars and attracted flies. In Jar No. 1, flies
entered and layed eggs which eventually gave birth new larvae. Whereas in Jar No. 2,
flies couldn’t be able to enter and no larve was found inside the jar. But flies laid eggs on
gauze that produced larvae. This has conclusively proves that organisms arise from the
pre-existed organism rather than non-living matter.

2. Spallanzani’s Experiment: The experimental setup is given in the Figure 3.3. In the
designed experiment to test the validity of theory of spontaneous generation. In this
experiment, spallanzani has prepared animal or vegetable broth and boiled them for
several hours and then either remained open or sealed immigately. These broth remained
free from microorganism growth. He concluded that high temperature boiling had killed
all microorganisms and in the absence of microorganism life could not appear. The broth
left open or exposing of sealed broth shows growth of microorganisms.

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Figure 3.2 : Redi’s Experiment to disprove the theory of abiogenesis.

3. Louis Pasteur Experiment: In another conclusive experiment, Pasteur had designed

experiment in a flask with “S” shaped curve tube (Figure 3.4). He took hay infusion in
the flask and boiled for several minutes. After cooling, the steam condensed into the
lower part of tube and act as barrier to stop the entry of microorganisms. No life appeared
in the flask for several months. Analysis of condensed water indicates appearance of
microorganism in the neck of the tube. Breaking of “S” tube allowed the growth of
microorganisms in the flask.

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3. THEORY OF CATASTROPHISM: This is the extension of the theory of

special creation. This theory assumes that life is originated by the creation and it is
followed by catastrophe due to geographical disturbances. Each catastrophe destroyed the
life completely whereas each creation forms life different from the previous one. Hence,
each round of catastrophe/creation is responsible for evolvment of different types of
organisms on earth. The critism of the current theory is same as previous one, No
scientific experiment to support the hypothesis and mostly be based on imaginery
concepts.

Figure 3.3 : Spallanzani’s Experiment to disprove the theory of abiogenesis.

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4. THEORY OF COSMOZOIC: This theory was put forward by Richter and

strongly supported by Arrhenius. The theory assumes that life was present in the form of
resistant spores and appeared on earth from other planet. Since the condition of earth was
supporting the life, these spores grew and evolved into different organisms. This theory
was also known as “theory of panspermia or spore theory”. The theory initially got the
support from the fact that fossils of microorganism were found in meteorites in 1961. But
no mechanism is known about the transfer of spores from other planet or whether these
spores could survive the journey in space. The absence of life forms on any planet except
earth and no details about the spores, its origin and mechanism of crossing interplanetary
space and reaching earth. In addition, this theory doesn’t add much into the fundamental
details about origin of life. No scientific experiment were given to support the theory. As
a result, the hypothesis didn’t receive much attention.

Figure 3.4 : Pasteur’s Experiment to disprove the theory of abiogenesis.

5. THEORY OF ETERNITY OF LIFE: This theory assumes that life had no
beginning or end. It believes that life has ever been in existence and it will continue to be
so ever. It further believe that there is no question of origin of life as it has no beginning
or end. The theory is also known as steady state theory. The main objection against the
proposed theory that it could not be able to explain; evidences support that initially earth
forms and then life appeared on it. Where life exist before the formation of earth?

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