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V SEMESTER ZOOLOGY
ORIGIN OF LIFE ON EARTH
EARLY THEORIES
Many theories were formulated to explain the origin of life before modern times. They
are special creation, cosmozoic theory, spontaneous generation and biogenesis.
A. Special Creation theory:
It is the oldest theory. Special creation theory states that all living organisms are
created by super natural power. It is a religious view. Opening chapter of genesis says
that on the 3rd day of creation, God brought forth living creatures, first plants, then
fish and birds, land mammals, and finally man. According to Hindu mythology, life
was the creation of Brahma, the supernatural being. Special creation theory was
discarded as it did not stand in scientific arguments.
B. Cosmozoic theory:
According to this theory, life did not arise in our planet. But life was descended from
other planet where life existed previously. Helmholtz (1884) said that micro-organisms
from space came on earth along with meteorites and comets and then evolved into
higher organisms in water. Recently a piece of Martian rock has been recovered from
Antarctica. NASA has discovered fossils of bacteria-like organisms on this rock,
suggesting that life could have come on earth from outer space. Francis Crick and
Laslie Orgel argued that all organisms have molybdenum in their bodies. Plants
require it in nitrogen metabolism as the enzymes nitrogenase and nitrogen reductase
are catalysed by molybdenum. But molybdenum is a rare element on earth,
amounting to only 0.2% of all elements. Cosmozoic theory has recently been extended
to include the concept that some planet with primitive life must have collided with
earth and seeded it with life. Proponents of this theory are Richter (1865), Helmholtz
(1884) and Arrhenius (1908). Explorers like Eric von Daniken and Zecharia Sitchin go
a step forward in proposing that the universe has several planets having intelligent life
that is capable of space travel and that they have landed on earth in the past and
seeded it with life. They provide evidences of extraterrestrial visitations of earth in the
past from the writings on clay tablets recovered from Mesopotamia, which carry
accounts of extraterrestrial visitations and details of solar system, galaxies etc.
C. Spontaneous generation theory:
Ancient Greek philosophers, namely, Thales, Anaximander, Aristotle and scientists of
the medieval period, like Harvey, Newton and Needham believed that life regularly
originated from the non-living things. They believed that worms and frogs can appear
spontaneously from mud and that maggots can appear in meat and mice in refuse
spontaneously. The theory was accepted universally for about 2000 years.
Italian scientist, Francisco Redi (1690) was first to refute this theory and
experimentally demonstrated that maggots did not appear if cooked fish and meat
were placed in covered containers. He kept fish and meat in some open jars and some
in jars covered with muslin cloth and showed that maggots appeared in open jars but
not in covered jars. Redi published his results in his book, Experiments on the
generation of insects. But people still believed that microorganisms developed by
spontaneous generation. This concept was refuted by another Italian L. Spallanzani
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(1780), who showed that boiled broth if sealed did not develop micro-organisms.
However, people thought that by boiling he removed the “vital force” necessary for life.
Louis Pasteur (1860) used flasks whose neck was drawn like swan's neck to trap
micro-organisms and dust. The boiled broth kept in such flasks did not rot, while
broth kept in open flasks started rotting. This experiment conclusively proved that
spontaneous generation theory was wrong.
D. Biogenesis theory (Life begets life):
It was proposed by Francisco Redi, Richter, and Louis Pasteur. According to this
theory life arose from pre existing life. Redi practically proved that life never arose from
non living matter. Maggots will arise from decaying organic meat if it contains eggs
only. If eggs are absent on such meat, never maggots will arise. Even though
biogenesis explains the formation of living organism from pre existing life, it never
explains the origin of first living organism. But it laid the foundation for proposing the
theories of organic evolution.
MODERN THEORY
This theory was simultaneously proposed by a Russian biochemist, A.I.Oparin (1923)
and the British biologist, J.B.S.Haldane (1928). Later Oparin published the book, The
Origin of Life in 1936. Oparin explained the origin of life from non living materials
through the chemical evolution of life. Oparin’s theory is called naturalistic theory or
theory of coacervates.
OPARIN’S HYPOTHESIS:
The approximate age of earth is 4,600 million years. Whereas the age of first life is
3,500 million years.
FORMAION OF SOLAR SYSTEM
The origin of sun, earth and other planets is explained by cosmic evolution. It was
proposed by Sir James Jeans. According to him, before 4,600 million years ago, the
nebula was a ball like mass. It contained high temperature and everything is in the
form of a gas. One star was attracted by the nebula. As the star speedily collided with
nebulous mass, the outer atmosphere was broken into nine pieces. These pieces were
sprayed in the different distances. As these pieces moved away they gradually lost the
temperature. These nine pieces were formed into planets. The remaining nebulous
mass is being considered as the sun. As the earth began to cool, the cosmic dust was
condensed. Hence on earth solids, liquids and gases are co existing.
The different steps in the formation of first living organism are explained below.
A. Environment of early earth:
According to this theory, life developed in the primitive atmosphere of earth by simple
chemical reactions that took place as the earth started cooling down from the hot ball
that it was when it got detached from the sun. Organic compounds, namely,
carbohydrates, fats and proteins, consist of mainly Carbon, Oxygen, Hydrogen and
Nitrogen. These elements were present in the primitive atmosphere of earth, when the
atmosphere cooled down to 5000-6000 degrees about 4600 million years ago. As the
earth’s temperature came down to about 1000oC, the elements started combining with
one another to form larger molecules such as carbon dioxide, carbon monoxide, water,
ammonia, methane, hydrogen, acetylene etc.
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There was no free oxygen at this time and the atmosphere was highly reducing. As the
water vapors were formed along with different gases in a highly turbulent atmosphere
and as the temperature dropped down further, condensation of the vapors and the
gases brought down heavy rains, which created oceans. Hot metal carbides on earth’s
surface reacted with rain water producing acetylene gas that filled the atmosphere.
B. Production of polymers:
Ammonia donated NH2 to acetic acid to produce the simplest amino acids like
Glycine. As all amino acids have basically the same structure, they must have been
formed by the simple chemical reactions taking place in the primordial broth, and
since carbohydrates and fatty acids are also chains of carbon, hydrogen and oxygen,
these organic compounds also could have been easily formed by simple chemical
reactions.
Aminoacids must have naturally formed polypeptide chains by amide bonding to form
complex proteins in the high energy and high temperature oceans of the primitive
earth. Such protein globules must have formed protein aggregates or poteinoid
Microspheres, which were called Coacervates or Protobionts by Oparin. Whether
such protein globules can be called livings things is debatable, even though they could
grow in size by absorbing protein molecules from the substrate and then divide by
binary fission and probably had enzymatic activity too.
Lipids must have been naturally formed in water and coated the surface of
coacervates, transforming them into Eubionts, which could maintain their shape,
grow and multiply by fission due to this new primitive cell membrane around them. If
eubionts could grow and reproduce, they must have developed primitive enzymes and
some sort of replicating genetic material to store information for proteins.
C. Origin of nucleic acids:
Nucleoproteins are difficult to form in natural conditions but various kinds of protein
chains inside the eubionts must have made more and more complex combinations to
give rise to nucleoproteins and eventually to simple RNA. This is likely to have
happened owing to energy filled atmosphere and high energy available for chemical
reactions at that time. Cairns & Smith (1971), however, believe that the earliest genes
were minerals associated with proteins, capable of self-replicating and coding for
useful proteins. Nucleic acids as coding material came later and being more efficient
were selected out against minerals. It is now believed that RNA was the first genetic
material evolved (Crick, 1968, Watson et al., 1986). RNAs have also been found to act
as enzymes that catalyze their own replication. DNA might have been formed by RNA
with the help of some kind of reverse transcriptase enzyme. Such organisms appeared
about 4000 m.y.a and were much like today's PPLOs (Pleuropneumonia-like
organisms). We can call them Progenotes, the ancestors of all life that developed
later. They were slow growing and slowly replicating heterotrophs that contained only
a few genes and few proteins and a limiting plasma membrane. They fed on the
readymade organic molecules abundantly present in the primordial broth. Ability to
synthesize organic substances required by the cell using enzymes perhaps evolved
later.
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Progenotes then gave rise to more complex Prokaryotes, such as bacteria which
contained free DNA and a cell-wall. Fossils of cyanobacteria from 3500 million year
old rocks have been recovered.
Prokaryotes apparently gave rise to Eukaryotes, when DNA got enclosed in a nuclear
membrane about 1600 m.y.a. Thus the single-celled organisms evolved, which
gradually gave rise to all life on earth.
Evidences supporting biochemical origin of Life
Publication of Oparin’s book created a kind of stir in scientific community and many
thought that if life could originate naturally in oceans by simple biochemical reactions,
it should be possible to produce it in laboratory by recreating the primitive atmosphere
of earth. Biochemical origin theory appeared to be very sound and hence several
scientists tried to test the theory by experimentation. S.L. Miller, a research student
of the Nobel laureate Prof. Harold C. Urey was assigned the project in 1953 to design
an experiment in which conditions of primitive earth could b recreated.
UREY AND MILLER’S EXPERIMENT
Miller and Urey experimentally proved the formation of complex molecules in the
laboratory. They supported Oparin’s hypothesis.
Miller made an apparatus to synthesize complex materials, amino acids. Miller’s
apparatus contains two rounded glass flasks and glass tubes. The rounded flasks are
connected by glass tubes. In one flask, Miller took fresh water. He took C, H, N and
phosphorus in one connected tube. In the another flask, Miller took marine water. He
boiled the fresh water. Hence steam was produced. The steam was passed over the
elements and finally mixture was sent into marine water flask. Miller and Urey
observed ammonia, methane and phosphates in the marine water flask. These
chemicals sent into a glass tube, which was connected to electrodes. The electrodes
produced sparks. Then the chemical mixture was cooled and sent into the ‘u’ shaped
tube. After one week, Urey and Miller observed amino acids in the ‘u’ tube. Miller
succeeded in obtaining amino acids like glycine, alanine and aspartic acid in the
trapped liquid. The results were published on May15, 1953 in Science, Volume 117,
page 528, under the title, “A production of aminoacids under possible primitive earth
conditions.”
Several other workers repeated this experiment and obtained almost all amino acids.
Bahadur (1954) obtained almost all aminoacids by subjecting a mixture of
paraformaldehyde, ammonia and ferric chloride to strong sunlight. Pavlovskaya and
Pasynskii (1959) exposed a mixture of gases to ultraviolet radiation and obtained
alanine, glycine, aspartic acid and glutamic acid. Lowe, Rees & Markham (1963)
synthesized many organic compounds by heating liquid hydrogen cyanide and
aqueous ammonia to 90oC for 18 hours. Abelson (1966) found that a mixture of CO,
N2, and H2 would react to form hydrogen cyanide and when subjected to ultraviolet
radiation formed aminoacids glycine, alanine, serine, aspartic acid and glutamic acid.
That aminoacids formed naturally in the primordial oceans was supported by William
Schopf of Harvard University, when in 1967 he detected 22 aminoacids in the rock
formations that were 3000 million years old.
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Protein-first hypothesis
If aminoacids were formed naturally in the oceans, could they also polymerise
naturally to form protein microspheres. Calvin (1965) thought that this could happen
under three possible conditions: first, if they are dehydrated and subjected to heat.
Second, if aminoacids are absorbed in clay or minerals and third, if they reacted with
cyanide or phosphate compounds. Sydney Fox (1957) heated a mixture of 20 amino
acids and obtained long chains of polypeptides, which immediately formed
microspheres in water. They were equivalent to coacervates of Oparin and were called
protenoids. Later in 1965, she placed a mixture of dry aminoacids in a block of hot
lava and obtained an amber-colored liquid, which when diluted with hot salt solution
formed microspheres. Sydney Fox believed that microspheres exhibited some
properties of cells, such as they divided by fission, were covered by a double layer of
non-fatty membrane and in the presence of zinc they split ATP to obtain energy.
If peptides were formed spontaneously in the primitive oceans, could they duplicate
themselves or synthesize new ones in the absence of genetic material. Steinman &
Moser (1967) experimentally proved that peptide production could be stimulated by
peptides already present in the substrate, without participation of nucleic acids.
Perhaps some of them acted as enzymes.
Gene-first hypothesis
Formation of nucleotides is perhaps the most unlikely event to envisage in the natural
conditions of primitive earth as it requires catalytic action of several enzymes to bind
together nucleoproteins, sugars and phosphates. But several workers have indicated
that it could have happened naturally on the young earth. Ponnamperuma (1965)
produced ribose and deoxyribose sugars by irradiating a mixture of methane,
ammonia and water. He also indicated that formaldehyde in aqueous solution under
suitable condition condensed to form simple sugars. Schramm (1965) experimentally
demonstrated that polynucleotides were formed when phosphorus-containing
compounds were present under water-free conditions. Oro (1965) and Ponnamperuma
(1965) indicated that adenine is readily formed from hydrogen cyanide in special
conditions and that purines and pyrimidines could have been formed under nonbiological conditions due to strong ultraviolet radiation.
Horowitz 1959) proposed that life began in the form of “naked genes” which had the
capability of self-duplication and mutation and ability to influence environment to
ensure supply of materials for the cell. There is no concrete evidence to prove it but
Muller (1965) and Sagan (1965) suggested that probably there were some
polynucleotide chains that could act as enzymes.
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