DEFINITION OF LIFE: All groups of living organisms share several key characteristics or functions: order, sensitivity or response to stimuli, reproduction, adaptation, growth and development, regulation, homeostasis, and energy processing. When viewed together, these characteristics serve to define life.
Life abounds on earth in many forms. But what
is life? Firstly, the origin of life on earth is one of the greatest mysteries of
science. Are we alone in the universe? May or may not be, but we cannot give a
definitive answer to that question.
Logically we presume that since life evolved on
earth due to certain conditions, it should also evolve on other planets orbiting
other stars in the Universe where similar conditions are present. But till date
we even do not know how life came about on earth.
The Stanley Miller experiment merely proves
that amino acids evolved on the primordial earth, but it does not say how those
amino acids combined to form the varieties of life around us. A lot has to be
understood yet about life and unless that mystery is solved, we cannot say
anything with certainty.
The earth is about 4.5 billion years old.
Scientists estimate that 200 million years after, that is about 4.3 billion
years ago life evolved on earth. The oldest known fossils on the earth are however
only 3.7 billion years old. That leaves a 600 million year window for which we
do not have any information.
During this time it is possible that life may
have evolved but has been repeatedly extinguished by catastrophic collisions
with asteroids and comets. However, we have not found any evidence for that so
far.
Life is supposed to have first begun in
volcanically active hydrothermal environments on land and at sea. Some
microorganisms thrive in the scalding highly acidic hot springs environments
like those found today in Norway and Yellowstone in the USA. Therefore such
organism may have formed on planets with similar environments in other parts of
the Universe.
However, this is merely a hypothesis without
any supporting proof. This is the problem with life, as we do not know how
inanimate became animate; it becomes very difficult to say if it exists on
other planets.
What are in the ingredients for life on Earth?
It requires a steady energy source like the
Sun, organic compounds and water. There are some elements that are crucial for
life. They are Carbon, Hydrogen, Oxygen, Nitrogen and Phosphorous.
Now a planet so close to the Sun cannot
actually have Carbon and Nitrogen. Both the elements become solid only under
very cold temperatures that are not present on Earth. Carbon is rare at the
earth’s surface and chemically bonds more with iron than rock. So how did the
Carbon found on the earth’s surface get there?
Water now covers about 70% of the Earth’s
surface but how much of it was there 4 billion years ago for life to evolve?
How did life emerge on earth?
Scientists believe that life emerged from a
Primordial soup. In 1952 as a University of Chicago graduate student Stanley
Miller performed his famous experiment along with Harold Urey, a Nobel winner
in Chemistry.
They injected Methane, Ammonia and water vapour
into an enclosed glass container to simulate lightning. Amino acids the
building blocks of Proteins soon formed in the container. They realized that
this process would have created the way for formation of life.
Scientists now believe that earths early
conditions had a different chemical makeup from the one used by Miller in their
experiment. The experiment gave rise to a new scientific field named prebiotic
or abiotic Chemistry.
Some scientists think that molecules for life
may have been produced outside the earth and was brought to earth by meteorites
or comets. In 1969 the Murchison meteorite that fell in Australia contained
dozens of different amino acids.
In 2019, a team of researchers in France and
Italy reported finding extra-terrestrial organic material preserved in the 3.3
billion old sediments of Barberton, South Africa. They suggested micro
meteorites as the likely source. Further evidence came in 2022 from the samples
of Asteroid Ryugu brought back by a Japanese mission. It had over 20 different
amino acids.
Chirality means mirror image,
non-superimposable molecules. An experiment in 1953 pointed out that without
Chirality, large biological molecules like proteins would be unable to form
structures that could be reproduced.
But how can inanimate matter combine in a
certain way to develop to be a composite organism in the first place, where
each part becomes special for a particular job? It is very easy to say that some
chemical reaction produced Amino acids as the Miller experiment has shown but,
then how do those amino acids combine and formed the DNA, RNA and Proteins?
Is it not silly to say that they evolved? How
did that happen? Again taking for granted that proteins and RNA and DNA have formed,
how did they conveniently combine to make complex organisms?
The problem here is that there are too many
questions without any answers except if we exercise our sheer imagination which
is not based on any proof.
The Soviet Biochemist A I Oparin has advance
his theory for evolution of life. In it he says
1. There is no fundamental difference between a living
organism and lifeless matter. The complex combination of manifestations and
properties characteristic of life must have arisen as a part of the process of
the evolution of matter.
2. Taking into account the then recent discovery
of methane in the atmospheres of Jupiter and the other giant planets, Oparin suggested
that the infant Earth had possessed a strongly reducing atmosphere,
containing methane, ammonia, hydrogen and water vapor. In his opinion, these were
the raw materials for the evolution of life.
3. In Oparin's formulation, there were first only
simple solutions of organic
matter, the behaviour of which was governed
by the properties of their component atoms and the arrangement of these atoms
into a molecular structure.
Gradually though, he said, the resulting growth and
increased complexity of molecules brought new properties into being and a new
colloidal-chemical order developed as a successor to more simple relationships
between and among organic chemicals. These newer properties were determined by
the interactions of these more complex molecules.
4. Oparin posited that this process brought biological
orderliness into prominence. According to Oparin, competition, speed of cell
growth, survival of the fittest, struggle for existence and, finally, natural
selection determined the form of material
organization characteristic of modern-day living things.
Even the
simplest of unicellular organism has a complexity of structure and function
that staggers the mind and is removed from the beginnings of life by a
genealogy extending for millions and millions of years.
Pasteur’s
unequivocal experiments have given us irrefutable proof that living organisms,
no matter how simple they might be, cannot be generated from organic matter. Yet
this might have happened in the millions and millions of years of evolution. It
is impossible by any experiments to prove Biogenesis but looking to the life
around us it looks like a reasonably consistent logical necessity.
The
conditions on the earth in the last 2 billion years have undergone radical
alterations and perhaps Biogenesis may not be possible in the current
conditions. Even if it does evolve, the innumerable predatory organisms that
exist on the earth would quickly destroy any such new organism.
So, is life a new property of Organic matter
acquired in the course of evolution or is it something that which resulted from
the organization of organic matter?
There is every reason to believe that a
certain period of earth’s history must have been completely sterile. Proteins
containing Nucleic acids are the only constituents of the organisms which grow
and reproduce by self-duplication or by replication. Neither viruses nor genes
have the ability to duplicate or replicate themselves unless implanted in a
cell or nucleus. So are these Nucleo proteins living or non-living?
Till Louis Pasteur conducted his brilliant
experiments which were published in 1862, most scientists believed that microorganisms
automatically got generated from matter. Pasteur conclusively proved that in
every case the growth of organism is due to contamination and there is no fresh
generation of anything.
There are two fundamental theories based on
the conception of continuity of life. 1. The theory of Cosmozoa along with the
closely related theory of Panspermia and 2. Preyers theory of eternity of life.
Preyer starts from the empirical theory that
all organisms are derived from similar other organisms, and questions “Is not
the problem of the origin of life based on a wrong assumption that the living
must have come sometime from the non-living? “Why not consider the living to be in eternal
existence like other inorganic matter?”
But if a living thing has only come from a
living substance then that substance must have also existed at the time when
earth was still a molten mass. Preyer insists that the living did exist in that
molten mass. He says “If we rid ourselves of the idea wholly arbitrary and
unsupported by facts that Protoplasm can exist only in its current composition,
and of the old convenient prejudice that at first there was only inorganic
substance, we can without fear take the next bold step, discard altogether all belief
in a primary origin, and recognize the timelessness of the current of life. “
