Life is unique and complex Organization of chemical molecules which can express itself chemically. How life originated on earth is still not having suitable answer. Different biologists have proposed different theories for the origin of life. The major theories for the origin of life are:
*Theory
of special creation
*Theory
of spontaneous generation or Abiogenesis
*
Theory of biogenesis
*
The cosmozoic theory or theory of Panspermia
*
The catastrophic theory
* Modern theory or Biochemical theory or Oparin- Haldane theory
* Theory of special creation
This is the oldest view regarding origin of life
and was proposed by Hebrew et. al. According to this theory, life was created
by some supernatural power of God. According to Spanish priest father Suarez (1548-1671)
world was created in six natural days.
The Earth and heaven were created in
the first day.
The sky and water were created on
the second day.
The land and plants were created on
the third day.
The sun, moon and stars were created on
the 4th day.
The fishes and birds were created
on the 5th day.
The Man and animals were created on
the 6th day.
According to Bible, Adam was the first man
molded from clay and Eve was the first woman formed from one of his ribs. God
infused life into them.
According to Hindu mythology; Lord Brahma, the
creator created the world and various forms of life from different parts of his
body. God, devil, and human were formed from head; Goat from mouth; birds from
chest and other animals and plants from hairs. Manu and Shradha were first man
and woman created by God on the earth.
This view has no scientific
support. Thus, it is not accepted.
The
process of formation of living things from nonliving materials in a spontaneous
manner is called as abiogenesis or spontaneous generation of life. It was proposed by Von Helmont (1577-1644). According to him Mice was formed by keeping
moist, dirty shirt with wheat grains in an earthen pot in a dark place for 21
days.
Ancient
Chinese, Babylonian, Egyptian, Greeks, etc. were in support of this theory.
Aristotle, Thales, Anaximenes, Anaxiander, Plato, Empedocles etc were in
support of this theory.
Aristotle believed that insects were developed
from morning dew and rotting manure. Crabs and salamanders were developed from
wet earth and slime and tapeworms from excreta of animals.
Ancient
Egyptians believe that frogs, snakes and crocodiles were developed from the mud
of Nile River.
Anaximenes
proposed the air as sole cause of life.
William
Harvey a famous physiologist despite his view that all life came from eggs although
believe that lower animals like worms and insects might also arise by
spontaneous generation.
This
view is widely accepted up to the middle of 17th century. There is
no scientific Support for this view. Thus, it is not accepted nowadays.
*Theory of biogenesis
The process of formation of life from
pre-existing life is called as Biogenesis. According to it; life can originate
only from pre-existing life i.e. Omnis
vivum ex ovo or vivo.
This theory cannot explain how the first life form came into existence on
Earth. It only proves that life cannot be formed spontaneously. Scientists
performed experiments to show life cannot originate spontaneously and to show
life originates only from pre-existing life. The theory of spontaneous
generation was experimentally disproved by many scientists of 17th, 18th and
19th century. They proved that life is formed only from pre-existing life.
Scientists like Francisco Reddi, Lazzaro Spallanzani and Louis Pasteur experimentally
challenged the view of abiogenesis.
*Redi’s experiment:
Italian
physician Francesco Redi (1668) performed an experiment. In which he took meat
and cooked it so that no organisms were left alive. He placed the meat in three
jars; one jar was covered with parchment; one was covered with muslin cloth and
one was left open. The meat decayed in all the jars and flies were attracted
towards all the three jars. He observed that maggots developed only in the open
jar though the flies visited all the jars. The flies entered only in the open
jar and laid eggs which produced maggots. This confirms that maggots arise from
eggs and not from the decaying meat (non-living matter). From this experiment he
proved that life originates only from pre-existing life and life does not
originate from non-living things.
 |
| Francesco Redi |
 |
| Redi's Experiment |
An Italian scientist Lazzaro Spallanzani (1765)
performed an experiment in which he poured hay infusion in 8 bottles and boiled
all of them. Four of them were loosely corked, while other four were made
airtight. After few days he found that there was thick growth of microbes in
all loosely corked bottles, but no micro-organisms were found in the air tight bottles.
He concluded that air contains microbes and new micro-organisms arise from
pre-existing microorganisms. The air is the source of contamination. From this
experiment; he concluded that life originates only from pre-existing life and
does not originate from nonliving things.
 |
| Lazzaro Spallanzani |
 |
| Spallanzani's Experiment |
*Louis Pasteur’s experiment:
Louis Pasteur French biologist(1864) Performed an experiment in which he took a flask with curved S-shaped neck almost half filled with water, sugar and yeast. The content of Swan neck shaped flask was boiled and it is left undisturbed for several days. There appeared no organisms even after several days. After that the neck of the bottle was broken. After 24 hours, he noticed microorganisms in the flask. From this experiment, He concluded that life cannot originate spontaneously; originates only from Pre-existing life.
 |
| Louis Pasteur |
 |
| Pasteur's Experiment |
*
The cosmozoic theory or theory of Panspermia:
It
was proposed by Ritcher(1865) and was supported by Arrhenius. It states that
life came on the earth from some other planet in the form of seeds or spores
called as Panspermia or Cosmozoa. It does not explain the mechanism by which
panspermia survived in adverse conditions like extreme high temperature and
radiation of interplanetary space during its migration. It is also called as
interplanetary theory.
*
The catastrophic theory
It
was proposed by George Cuvier, the father of modern paleontology, and states
that there have been several creations, each preceded by a catastrophe due to
some major geological changes. Each catastrophe destroyed previous life and new
life was created after that. There is no support for this theory and is not
accepted.