Download Origin of Life - Academic Script

ORIGIN OF LIFE
[ Academic Script ]
Course Name
:
Zoology
2nd Year Under Graduate
Paper No. & Title
:
Z - 204B
Evolution and Behavior
Topic No. & Title
:
Topic - 2
Origin of Life on Earth
Lecture Title
:
Origin of Life
Introduction
The origin of life has been the most attracting and complicated
problem for biologists since time immemorial. What is life and in which
form the first living form arose on this earth? Several scientific and
non-scientific explanations have been given from time to time about
the nature of life- its origin, evolution and occurrence in the universe.
The concept of biochemical origin of life has become widely accepted
among all proposed theories.
Ancient and Medieval Beliefs
1. Theory of spontaneous generation or Abiogenesis
According to these theory, life has originated from non-living organic
matter abiogenetically, i.e., without the intervention of living things.
Epicurus (342-271 B.C.) advocated the origin of worms and several
other animals from the soil or manure by the action of warmth of the
sun and air. The views of Aristotle (384-322 B.C.) were even more
abiogenetic postulating that not only worms and insects, but also fish,
frogs and mice emerge spontaneously from dew, rotten slime,
manure, dry wood, sweat, meal etc. Eels originated from sea mud,
frogs and salamanders evolved from coagulated slime and butterflies
from cheese.
2. Theory of Panspermia of Comozoic Theory
According to this theory the life is distributed throughout the cosmos
in the form of resistant spores of living forms, the cosmozoa. These
reached the earth accidentally from some other planet and on getting
favourable conditions for life these developed into organisms.
3. Biogenesis
The physician Francesco Redi (1621-1697) was the first scientist to
remove the beliefs of spontaneous generations. He cooked few fishes
and placed them in three jars. Out of these three, one was left
uncovered, the other was covered with parchment paper and the last
was covered with fine muslin. In a few days maggots appeared only in
the first jar and not in the others. He concluded that the flies arise
from the eggs laid down by the parent flies in the uncovered jar. Thus
maggots cannot appear spontaneously.
Modern Hypothesis of Origin of Life
Or
Biochemical Origin of Life
The modern hypothesis of origin of life was formulated by Haeckel,
who considered that the most primitive organisms would have been
generated spontaneously at some time from some inorganic matter as
a result of formative action of some special external physical forces as
electrical charges, ultra-violet-light and corpuscular radiations of
radioactive elements.
But the hypothesis was in such a crude form that it needed further
explanation and experimental support. The genius work of Haldane,
A.I. Oparin, Stanley Miller, and H. Urey and others has provided
evidences in favour of this hypothesis. The first scientific account of
the origin of life was given by Oparin (1922) in the book ‘Origin of
Life’. It is believed that origin of life was a gradual process and that
life did not come into existence suddenly as a consequence of a
combination of whole series of chance occurrences. It has been
described in the following steps:
(A) Origin of Earth and its Primitive Atmosphere
The earth is presumed to have originated about five billion or six
billion years ago either as a part broken off from the molten mass of
sun or by gradual condensation of interstellar dust from which our
entire solar system is presumed to be formed.
Initially, it was a fiery spinning ball of hot gases and vapours of
various elements. Gradually, through hundreds of millions of years,
the gases condensed into a molten core and different elements got
stratified according to their density. The heavy elements like iron and
nickel etc. sank to the centre and formed the solid core of the earth,
the lighter elements such as silicon, aluminium formed the middle
shell, while the lightest ones like helium, hydrogen, oxygen, nitrogen
and
carbon
etc.
flowed
to
the
surface
and
formed
gaseous
atmosphere.
The original temperature of earth was about 5000-60000C. At this
high temperature elements like hydrogen, oxygen, carbon, and
nitrogen could not exist in free state. These combined either among
themselves or with metals forming oxides, carbides and nitrides. Thus
carbon was found as dicarbon (C2), cyanogens (CN), methane (CH4)
and as metal carbides. Nitrogen was present in combination with
metals to form nitrides. Similarly oxygen formed oxides and hydrogen
combined with oxygen, carbon and nitrogen forming water (H2O),
methane (CH4), ammonia (NH3) and cyanide. All these compounds
were present in gaseous state and water as superheated steam due to
high temperature of earth. These formed the atmosphere of the
primitive earth.
With the gradual fall in temperature, some of the gases liquefied and
some
of
the
liquids
changed
into
solids.
Superheated
steam
condensed into water and resulted in rain. The rain drops on
approaching the superheated earth immediately evaporated and
returned into the atmosphere. This cycle continued for millions of
years due to which earth surface become cool enough to hold water
and the large water bodies or lagoons, seas and oceans came into
existence. The oceanic water contained atmospheric ammonia and
methane which got dissolved and washed down with the water,
dropped from the atmosphere in the form rain. Mineral rocks also
dissolved leading to the accumulation of mineral and salts in the
oceanic water. Therefore, primitive oceans were alkaline.
Therefore about three billion years ago or afterwards, the earth had a
solid crust, frequently punctured by eruptions of molten rocks
(volcanoes) and at places filled with hot boiling sea water. The sea
water contained dissolved ammonia, methane, some minerals and
salts. The primitive atmosphere of earth was devoid of free oxygen
but rich in ammonia, methane and hydrogen, as found in the present
atmosphere
of
Jupiter,
Saturn,
Uranus
and
Neptune.
Other
investigators have concluded that the principal constituents were CO,
CO2, N2 and H2 (Abelson 1966). The earth was also bombarded with
high energy radiations from the sun.
(B) Chemical Origin of Life
Oparin suggested that from these simple compounds more and more
complex organic
compounds were
formed gradually
under
the
influence of electric charges, ultraviolet rays or corpuscular radiations.
Step 1. Origin of Organic Compounds
a) Formations of hydrocarbons- When the temperature of earth
surface cooled down to 1000ºC or even lower, a variety of simple
hydrocarbons both saturated and unsaturated were formed possibly
by the following methods:
the combinations of highly reactive free radicals CH and CH2:-
CH + CH → HC≡CH
Acetylene
CH2 + CH2 → H2C=CH2
Ethylene
CH2 + CH2 → CH4 + C
Methane
2CH4 → HC≡CH +3H2
Acetylene
Metal carbides reacted with steam:
CaC2 + H2O→HC≡CH + CaO
Acetylene
3Fe4C5 +16 H2O → C15H32 + 4 Fe3O4
Thus,
primordial
atmosphere,
contained
mixtures
of
ammonia,
hydrogen and other elements, i.e., it was reducing atmosphere.
b) Formation of oxy-and hydroxyl derivatives of hydrocarbonsSaturated and unsaturated hydrocarbons reacted with superheated
steam and formed oxy-and hydroxyl derivatives-aldehydes, ketones
and acids.
HC≡CH + H2O → CH3CHO
Acetylene
Acetaldehyde
c) Formation of Carbohydrates- The hydroxyl derivatives, also gave
rise to small chain compounds of C, H and O. These first formed
compounds must have been glucose and fructose. Their condensation
formed the disaccharides (sugar) and polysaccharides (starch).
d) Formation of fatty acids and glycerol- The condensation and
polymerisation of the aldehydes and ketones, and their oxidation gave
rise to fatty acids. In fatty acids, percentage of oxygen was less and
had long straight chains of carbon. It is supposed that in early oceans,
glycerol and fatty acids might have combined to form the fats.
Condensation
CH3CHO+ CH3CH
CH3CHOHCH2CHO
Acetaldehyde
Aldol
In polymerization the molecules became interlinked forming complex
derivatesPolymerization
CH3COOH + C2H5OH
CH3COOCH3CH2 + H2O
Acetic acid Ethyl alcohol
Ethyl acetate
Oxido-reduction
2 CH3CHO + H2O
CH3COOH + C2H5OH
Acetaldehyde
Acetic acid
Ethyl alcohol
Polymerization
2HCHO
CH2OHCHO
Formaldehyde
Glycol-aldehyde
Oxido-reduction
2CH2OHCHO + H2O
2CH2OHCH2OH or 2CH2OHCOOH
Glycol aldehyde
Glycol
Glycolic acid
CH2OHCOOH +NH3
CH2NH2COOH + H2O
Glycolic acid
Glycine (amino acid)
e) Formation of amino acids and proteins- Hydrocarbons, ammonia
and water under the influence of freely available energy reacted with
each other to form amino acids. The polymerisation of amino acids
gave rise to long chains of polypeptide chains-the protein molecule.
The hot dilute soup- The synthesis of carbohydrates, fats, and amino
acids and other complex organic substances probably occurred in sea.
This had been described by Haldane as ‘The hot dilute soup’,
containing molecules of these organic substances in abundance.
Such organic molecules could not accumulate today because of
enormous number of micro-organisms that use and degrade them as
food. But on the primitive earth, with the absence of micro-organisms,
such molecules could accumulate in layers on mud or clay, which
provide surface for continuous reactions.
The energy required to complete the processes were available fromUV radiation from sun, ionizing radiation (protons, electrons and Xrays), electric discharge (lightening) and heat.
Step 2. Formation of Macromolecules or Complex Organic Molecules
In the hot organic soup the molecules of simple organic substances
came together in increasing number, collided, reacted and aggregated
forming new molecules of increasing size and complexity resulting in
the formation of more and more complex organic compounds like
polysaccharides, fats, proteins, purines, pyrimidines, nucleosides and
nucleotides.
Specially, sugar molecules combined to form polysaccharides like
starch, cellulose and glycogen, etc. the fats were formed by the action
of fatty acids with glycerol. A large number of amino acid molecules
became linked together forming long polypeptide chains. Some of
them acted as primitive enzymes and speeded up the rate of
formation of specific molecules. The formation of protein molecules
can consider a land mark in the origin of life.
Step 3. Formation of Nucleic Acid
Purine-sugar-phosphate and pyrimidine-sugar-phosphate were formed
in the beginning. These compounds are called nucleotides. These
nucleotides linked together in different combination to form nucleic
acids. Thus formation of nucleic acid was an important event for the
origin of life.
Step 4. Formation of Nucleoproteins
The protein molecules and nucleic acid thus formed, later aggregated
and united to form giant molecules of nucleoproteins. It is believed
that these nucleoproteins must have characteristics of a free living
gene. A number of such genes might have aggregated to form a large
unit, called the protovirus which is comparable to the present day
virus.
All the events upto the formation of nucleo-proteins and other
complex macromolecules have been under chemical evolution of life.
Further events in the origin of living forms are described under
biological evolution of life.
(C) Biological Evolution
Step 5. Formation of Primary Organisms
The giant organic molecules had a tendency to aggregate in various
combinations to form large colloidal particles. These were named
‘coacervates’. These coaservates contained more than one type of
proteins, nucleoproteins and other organic and inorganic molecules in
various combinations. These appeared as distinct bodies suspended in
the surrounding water medium. In the mean time some of the fatty
acids like lecithin and cephalin became organised to form limiting
membrane around the coaservates. Formation of surface membrane
resulted in the accumulation of various substances inside the
coaservates. Certain internal reactions like catalysis and absorption
could also occur independently inside them. At this stage some
proteins differentiated as enzymes. Naturally, then coaservates with
enzymes were able to complete some of those reactions, which
otherwise require more energy or a very high temperature. These
changes might have resulted in the formation of first ever cell.
Natural selection must have operated at this level and the coaservates
with more favourable internal arrangements became more abundant.
The nucleoproteins are sticky in nature. Therefore, these accumulated
extensive shells of nutrients around themselves. These with nutritive
shells proved more successful and became abundant and finally led to
the development of cell.
The first living cell- The coacervate with nucleoprotein molecules
surrounded by nutrient shells and the limiting membranes are
believed to be the first living cell.
Early cell types - Among the various cell types that appeared during
early period, the two are of particular significance:
(i)
Moneran type- In some cells the nucleoproteins were loosely
aggregated and embedded in the cell substance. Such cells
are called moneran type and are regarded to be the
ancestors of bacteria and blue-green algae.
(ii)
Protistan type- In some cells the nucleoproteins were
condensed into
a
central mass
surrounded by a fine
membrane. The nucleoproteins in such cells were separated
from the cell substance and were called protistan type.
Step 6. Problem of Energy Sources
These early cell types might have been akin to our present day
microbes, competing for raw materials available from the sea water.
With gradual increase in the early cells and consequent decline in the
natural source of food in the ocean, this delimited central mass of
nucleoproteins constituted the nucleus.
Early living organisms (the ancestors of monerans and protista)
evolved different methods for obtaining food. These are:
(i)
Parasites- These forms started living within the bodies of
living cells and obtained their food from them. Monerans,
viruses and few protista are parasitic.
(ii)
Saprophytes-
Some
organisms
started
drawing
their
nourishment from the bodies of dead and decaying cells.
Many bacterial groups adopted this method.
(iii) Predator- This was animal-like way of eating in which one
organism eats another in whole or in parts and obtained its
food in this manner.
(iv) Chemosynthesizers- This method includes the utilization of
energy from the inorganic molecules. Certain primitive forms
could utilize compounds of sulphur, iron and nitrogen, etc.
and make them undergo chemical reactions. The energy
liberated this way is used in the synthesis of organic
molecules. Certain bacteria living today still subsist on this
mode of feeding.
(v)
Photosynthesis- The process of chemosynthesis could not
become popular due to limited resources. Therefore, an
independent method was evolved to utilize the never ending
solar energy, which is known as photosynthesis.
Organisms which developed photosynthetic capacity were the first
plant-like types and in due course evolved into the plant kingdom.
Step 7. The Oxygen Revolution
With the increase of photosynthesizing organisms, oxygen was
liberated in the sea and then into the atmosphere. This free oxygen
then reacted with methane and ammonia present in the primitive
atmosphere and transformed them into CO2 and free N2.
CH4 + 2O2
→ CO2 + 2H2O
NH3 + 3O2
→ 2N2 + 6H2O
These events ultimately transformed the ancient atmosphere with
plenty of free oxygen. The modern atmosphere does not contain
methane and ammonia but is mainly formed of water vapours, carbon
dioxide and molecular nitrogen, hydrogen and large quantities of free
molecular oxygen. Free oxygen finally lead to the evolution of aerobic
mode of respiration which yielded more energy on oxidation of food
stuffs compared to anaerobic respiration.
Experimental Evidences
The modern theory of origin of life is based on certain evidences
collected from the spectroscopic analysis of light coming to the earth
from sun and other planets and also from experiments conducted in
artificial environment.
(i) The earth, sun and other planets like Mars, Saturn, etc. are
presumed to have originated at the same time from the local
condensations within the cosmic dust of more or less uniform
composition. The spectroscopic analysis of the light coming to earth
from sun and other planets combined with chemical analysis of
meteorites that fall on to the earth suggest that the six elements
essential for life i.e. hydrogen, carbon, oxygen, nitrogen, sulphur and
phosphorus are among the most abundant throughout the solar
system even today. So these must have been abundant on the newly
formed earth. Moreover, the atmosphere of Uranus and Neptune is
believed to possess hydrogen and helium gases together with
methane gas and the atmosphere of Jupiter and Saturn consists of
large amounts of ammonia together with methane, hydrogen and
helium. These findings suggest that the primitive atmosphere of earth
might have also consisted of hydrogen, helium, methane and
ammonia and that there was no free oxygen and nitrogen in its
atmosphere.
(ii) Experiments conducted by Stanley Miller (1953) and Harold Urey
at the University of Chicago showed that amino acids like glycine and
alanine and even more complex organic compounds can be formed in
vitro when the mixture of water vapours, methane, ammonia and
hydrogen gases in a closed chamber is exposed to electric discharge
for few days.
From this simple experiment Miller suggested that electric discharges
produced during lightening in the early atmosphere of earth containing
ammonia, nitrogen, methane, hydrogen and water might have
resulted in the formation of amino acids and other complex organic
compounds which are the component of living system.
Pavlovskay and Pasynskii (1959) subjected the above mixture of
gases to sparking as well as to the ultraviolet rays and reported the
synthesis of alanine, ß-alanine, glycine, aspartic acid and glutamic
acid etc.
(iii) In 1968 synthesis of a DNA molecule consisting of 77 nucleotides
in the test tube by Dr. Hargobind Khorana has further supported the
possibility of such a synthesis of genes by the union of nucleotides in
nature. Dr. Khorana won Nobel Prize for the synthesis of gene in
1970.