Origin of life

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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 stages.
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 process. 

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. 

The stages:

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 along side.
And indeed, the chief ingredient of this process of evolution is - time.

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 going.

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 Pennsylvanian times.

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 Devonian times. 

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 reptiles.

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 around.
The forms of life:
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 found? 

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.

 pre historic animals animals of our time | the plants