Have you ever imagined, how life originated on the Earth? What came first, plants or animals, birds or insects, snakes or crocodiles? How oxygen came in the Earth’s atmosphere?
There is a proper sequence as to what came first, what happened first, what came first. Different scientists have different beliefs and hypothesis on a big question as to how life originated on the Earth. Some believe that life originated due to some supernatural activities that took place while others believe that life originated in simple forms that complexed with the time.
Some scientists believe that life just originated spontaneously from non living forms while others believe that life originated due to some chain of chemical reactions.
Our Earth is 4.54 billion years old. Life originated on the Earth approximately 3.77 billion years ago in water (perhaps ocean) in the form of microorganisms (most probably bacteria), followed by evolution of Cyanobacteria (Blue Green Algae) approximately 2.4 billion years ago.
Before going into details of new life evolved on the Earth, it is important to know the molecules required for any form of life to occur. For origin of life, three things are necessary-
2. Water in liquid form
3. Source of Energy
After the birth of the Earth, the temperature was very high. This was due to the activities of comets and asteroids that continuously bombarded the Earth’s surface. It took approximately 1 billion years for the Earth’s environment to stabilize and life to originate.
In the beginning, the Earth’s atmosphere was rich in methane and it lacked oxygen. Thus, it wasn’t suitable to support life. Archaebacteria (a type of primitive bacteria) that can grow in methane rich environment, appeared first.
It was the evolution of blue green algae that transformed the whole scenario. Cyanobacteria are the microorganism that use sunlight and water to produce oxygen. Cyanobacteria were the first photo-synthesizers that changed the Earth’s atmosphere.
Due to rise in the concentration of oxygen in the environment, it became uncomfortable for certain microorganisms such as archaebacteria to survive. Also the oxygen produced by Cyanobacteria was not enough to support the growth and evolution of other life forms. It was because Cyanobacteria also died and decomposed reducing the oxygen level.
So, by now we understand that archaebacteria came first followed by Cyanobacteria. Now the question arises when did true form of bacteria and the complex organisms evolved. There are theories that suggest that due to mutation in the simple cells and natural selection when the true bacteria (eubacteria) emerged. The microorganisms that we’re lucky, survived the mutation and rapid evolution took place.
It is believed that the first multicellular organism emerged 1.2 billion years ago due to complexity, climate change, increase in the level of oxygen.
Great oxidation reactions event took place 800 million years ago that increased the level of oxygen in the Earth’s atmosphere
Later about 475 million years ago plants evolved from algae and moved on land.
The oldest animal fossil was recently found in Australia that dated 650 million years ago. The fossil looked similar to the sponges. in this way simpler animals came to life followed by complex animals. With the time and favorable atmospheric conditions more animals evolves, some of them got extinct while others are still there.
The figure above shows the timeline of events that took place on the Earth and what living organisms appeared when. The figure gives an idea about the whole topic. I hope it would help to understand how life originated on the Earth.