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Human has been in search of possible
extra terrestrial life for quite a while and has invested considerable
effort in it. But how life began on our home planet earth is still
riddled with mysteries! Different theories have been framed to solve
this mystery. They only give us a few possibilities. These possibility apart,
the evidence obtained so far tells us about when the earliest form of
life existed. Yes, existed. But not pretty certain about exactly when
it did originate.
Evidence indicates that all plants and animals alive today have neither
popped up from nowhere nor dropped from the sky altogether. And even
when the earliest form of life did originate it took thousands of
million years to evolve into the forms we see now around us. All the
plants and animals, known and unknown to us, has come into being
through this step by step slow and gradual process. Called 'evolution',
this process has given birth to different forms of life at different
As life did evolve, however, creatures gradually developed hard body parts
some of which were preseserved over time that, in modern times, provided a
record of their existence in the form of fossils. A
study of the earth's age and the dating of fossils discovered from all
over the planet have also helped us graduate the time scale along which
life has flowed.
These fossils are the remains of prehistoric animals and plants or bear
some direct evidence of their existence. They may be bones or shells of
animals, imprints of leaves and stems of plants, or just trails or
borings made by worms in wet mud or sand that was later changed into
rock. But getting secured in such a dramatic way is only a game of
chance for the dead animals. There may still be many which did not
happen to be fossilized. This possible slippage may leave a gap in a
series of evidences.
The oldest fossils are probably those of algae, one of the simplest
form of plants. These date back to some 2700 million years. Meanwhile,
the oldest rocks discovered so far are about 3,000 million years old.
Even, these rocks do not represent the total age of the Earth. For, the
Earth is still older, about 5,000 million years old. This is revealed
by studying the expansion of the universe over our time scale.
So what about the fossils that can carry us farther back into the first
2000 million years of the Earth's history?
Only against a background of about 3,000 million years of life does the
process of evolution become clear. Science is concerned largely with
the question of how this evolution happened.
Studying the stages of evolution and dating the fossils we seem to be
certain about one thing. That the earliest living things on Earth must
have been very simple.
The first atmosphere of the Earth probably consisted mainly of methane
gas(CH4),water(H2O),and ammonia gas(NH3). These substances contain all
the chemical elements needed to make amino acids - the building blocks
of protein, which is the basis of life. Laboratory experiments have
already demonstrated that a mixture of these gases subjected to an
electric discharge will produce amino acids. But the planet is not like
the confined laboratory. And did the electric charge spark plugged the
Some scientists believe that lightning in the early atmosphere may have
assisted in bringing about such a reaction. Again it was not a single
lightning. What could be brought about in the laboratory in such with
an electric discharge in a few minutes had taken thousands of years to
take the final shape. Again from the amino acids to proteins is a long
and complex step. And it is a still longer one from proteins to the
simplest living cell. Once the cell comes up life got started. For it
is the cell that forms the building block of life. Though it took quite
a long period of time for even a single cell life to make their
appearance on this planet.
This discovery makes it possible to imagine a series of steps by which
the earliest life may have originated.
However, the odds are indeed many against having the proper
circumstances for the beginning of life in this manner. But the
conditions necessary for this development of life must have existed on
Earth for perhaps as long as 2000 million years and over enormous areas
of the Earth's surface. And there is no proof that life did originate
in this or any other particular way.
Irrespective of this controversy over the how and when of
the origin, we are certain about one thing that has made life live on
for so many years. We know, as surely life is born, it certainly dies,
sometime or the other. So what is it that makes life go on for years
together? Or, rather say, for some millions of years together? It is
the capability to reproduce itself, sometime before the death comes
along. This ability is so unique that it makes life differ so much from
the non-living ones. So between its birth and death it brings in
another life to carry on the journey for yet another bunch of years. So
life in every form has its parent and children. It's like a
parent-child chain that keeps life roll on for so long. As time passes
the chain gets longer and longer. And from the days of origin until
these days life rolls on through this long chain.
But doesn't the chain break? Yes, it does. And it does when some of the
children becomes too weak to carry on. But while it breaks. Some other
children make themselves stronger
and more intelligent to carry on. They keep on adjusting themselves
with the changing environments. Thus while some children carried on,
some died, never to be born again. And
in between the new ones were born. Scientists and historians mapped out
the forms of life those evolved, survived and died into extinction
And indeed, the chief ingredient of this process of evolution is -
Based on this time scale the origin of primitive form of life can be
said to have happened during the Precambrian Era. The oldest
fossils so far known come from the Pre-Cambrian age. They include
algae, fungi, jellyfish, worms, and some sponge-like animals. This
lasted till some 570 millions years ago, said to be the starting point
of the Paleozoic Era. This lasted for about 345 million years
and is subdivided into Cambrian, Ordovician, Silurian, Devonian,
Mississippian, Pennsylvanian and Permian periods. Following these came
the Mesozoic Era stretching for 160 million years. The Mesozoic
comprise Jurassic, Triassic and Cretaceous period. Following these came
the Cenozoic Era starting some 65 million years. This comprises
the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene and
the Recent period starting only about a million years ago and is still
True, the earliest form of life is believed to have originated in the
Pre-Cambrian era, life in our known form all started during the
Cambrian period. By the Ordovician periods and most of the Silurian,
many kinds of creatures thronged the shallow seas. These periods show a
wide variety of plant and animal life, all of which show considerable
advancement over creatures of the Pre-Cambrian and are far evolved from
the first living things.
Life in its first form made its appearance in the sea. Possibly there
were no living things on land then, and possibly none in the existing
streams and lakes. In those days the land areas must have been
unbelievably desolate wastelands of bare rock, sand, and mud, without
any single form of life.
The oldest well preserved land plants come from the upper Silurian of
Victoria, Australia. By Devonian time land plants had become common and
are known from such scattered areas across the world. Still most were
small and primitive. But by upper Devonian times there were forests of
scale trees in some areas. These trees, which became extinct during the
Paleozoic era, were the forerunners of those forming coal swamps of
The oldest undoubted fossil record of animal life on land from Devonian
times when arthropods, including spider like and tick like animals were
established. Several kinds of insects had appeared by the upper
Fishes were well established and flourishing by the Devonian times.
The Mesozoic times saw the real thrive of various reptiles, especially
the dinosaurs. Even crocodiles also appeared during the Mesozoic era.
Flying reptiles which form the earliest bird also appeared during this
time in the Jurassic age. The close of Mesozoic saw decline of
Cenozoic saw the plants and animals resembling those of today.
Flowering plants and mammals dominated in this era. Of all mammals man
belongs to the Primates and is the latest and most advanced addition in
the biological pyramid.
While various forms of life have emerged, sustained and gone into
extinction over the past, they are all the ancestors of any form of
life doing fine here on Earth. These, including us, are what we see
The forms of
Let us now see what types of life forms we see around us. Are they only
some creatures, intelligent or not, capable of moving around? Or, are
they only capable of growing? Or, are they capable of growing and
feeding themselves? Or, are they the forms in which all of these can be
Earlier it was the world of animals that was thought to be the only
living forms on this planet.
And it is a matter of only the last few decades that forms, other than
animals, came to be included in the world of living beings. And it was
till recently that the bacteria, the fungus, and some forms of algae
have come to be regarded as separate forms of life.
So based on this, today we know that there is a five-kingdom system
representing the living beings around us. This is as opposed to the two
kingdom system of plants and animals.
The animals, the bacteria, the fungus, the
algae, and, the plants.