We see various life forms all around us, everyone of them have their differences and also some similarity. But the processes which are occurring inside their body for their survival are same, whether their organs are different. There is also a huge number of life forms around us which we can't able to see with our naked eyes. Yes! I'm talking about microorganisms. Scientists believe that the first living organism on the earth was a microorganism which have not a well defined cell ( they also called as prokaryotic cells ). After several genetic mutation those prokaryotic celled organism came in the form of eukaryotes and then after multicellular and they also becomes huge organisms with very much complicated body. But the phenomena is not very simple.
About 3.8 billion years ago when the temperature of the earth suddenly goes down to below 100℃ which makes the vapours ( gaseous H2O ) to water ( liquid H2O ) and at that time a great monsoon come over the whole earth. Which give the birth of several oceans and the blue colour of our planet. In those early oceans, the most strangest event occurred in the history of the earth, which led to the formation of the first organism on the earth. But still nobody knows, how those primitive organisms were able to survive in that extreme condition. This shows that the primitive organisms were much more developed than the organisms in today's world. That is why we still can't able to define that first organism. And how such complicated organism developed by itself.
Today we know that the terrestrial life form is based on organic compounds for example: your body is made up of an organic compound named ' Amino acid'. But the problem is that, protiens in today's organisms are incredibly complicated. Some of them are compost of hundreds of aminoacids which is arranged in a precise manner. This is impossible to believe that such complicated compounds were form by itself. So, there is no any organism on the earth which is developed by its own, they must evolved from other.
But what leads the early elements to build an organic compound which can do several life processes? And how that looks like? Unfortunately nobody knows. Possibly it was just a simple molecule, but that molecule differed from others because it have ability to make copy of its own. But scientists believe that they have a RNA or ribonucleic acid but even that is fairly complicated to be created by any accident.
Whatever the case, this first replicating compound was eventually replaced by DNA ( Deoxyribonucleic acid ). It wrapped itself into a protective shell ( called as 'cell wall' ) which leads the formation of the first prokaryotic cell on the earth which was similar to today's prokaryotic cells. DNA determines the exact appearance of every living organism. The protective cell of your immune system vigilantly look for unwanted visitor, growing of your nail, hair and height these all were determines by your DNA.
Reproduction
To reproduce offspring primitive organism copy its DNA in to a new shell by breaking its body in to two parts. But modern complicated organisms achieve that by sexual reproduction. In this type of reproduction two individual decide to join their DNA into one. Which makes their offspring a quite different from them. But prokaryotic cells reproduce by dividing itself as I mentioned above. Imagine, suddenly your body cut down into two and form two organisms. This type of reproduction were done by early cells and today's also. This process is categories in to asexual reproduction and also several ways of reproduction came under this for example: budding, fragmentation etc.
As I've described that during reproduction an organism copy its DNA to offspring. But this process is not perfect every time. There were several errors may be done during copying the DNA. A disobedient particle inside DNA finds itself in a wrong place or a certain segment of DNA is copied multiple times. These errors are called 'Genetic Mutation'. This phenomena had played a great role in evolution. Due to genetic mutation the offspring of an organism can becomes abnormal but some times these abnormality becomes a new quality of the organism and that quality again goes to the next offspring. These quality may be - better digestion, immediately, sensation, etc. Due to those genetic mutation prokaryotic cells finally becomes eukaryotic cells which have a well defined organelle.
Eukaryotic to multicellular
About 1.2 billion years ago some eukaryotic cells decided to join themselves in a ordered manners and become a complex unit and this event gave birth to the multicellular organisms on the earth. Then after 600 million years the amount of oxygen in the atmosphere raised. As result of it a sheet of ozone gas formed over the atmosphere of the earth. And now aquatic animals can come on land because their is no ultraviolet rays are falling on the earth surfaces and also their is a huge amount of oxygen on earth surfaces in comparison to in water. Thereafter 100 million years, the first plant, animal and fungi appeared. Then the formation of much complicated organisms were started. And finally the whole earth filled with small fishes, sharks, beetles, etc. 270 million years later the dinosaurs becomes the most dominant species on the earth at that time. With a monster sized body and full of strength they lead whole earth. But 65 million years ago, an event occurred that changed everything. A huge meteorite of radius 10 km striked the earth in today's mexico that cause the ultimate downfall of dinasour.
To be continued.... In another post.
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