Initially, the earth was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium. But now it is well and beautifully organised planet full of life.
We will understand the evolution of the earth in two heads viz. Evolution of Lithosphere and Evolution of Atmosphere & Hydrosphere.
Evolution of Lithosphere
The earth was mostly in a volatile state during its primordial stage. Due to the gradual increase in density the temperature inside has increased. As a result, the material inside started getting separated depending on their densities. This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter once to move towards the surface.
With the passage of time, it cooled further and solidified and condensed into a smaller size. This later led to the development of the outer surface in the form of a crust.
During the formation of the moon, due to the giant impact (the big splat), the earth was further heated up. It is through the process of differentiation that the earth forming material got separated into different layers. From the crust to the core, the density of the material increases.
The Giant impact (the big splat)
A body of the size of one to three times that of mars collided into the earth sometime shortly after the earth was formed. It blasted a large part of the earth into space. This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.4 billion years ago.
Evolution of Atmosphere and Hydrosphere
There are three stages in the evolution of the present atmosphere. The first stage is marked by the loss of the primordial atmosphere. In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere. Finally, the composition of the atmosphere was modified by the living world through the process of photosynthesis.
The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds. this happened not only in case of the earth but also in all the terrestrial planets, which were supposed to have lost their primordial atmosphere through the impact of solar winds.
During the cooling of the earth, gases and water vapour were released from the interior solid earth. This started the evolution of the present atmosphere. The early atmosphere largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen. The process through which the gases were outpoured from the interior is called degassing.
Continuous volcanic eruptions contributed water vapour and gases to the atmosphere. As the earth cooled, the water vapour released started getting condensed. The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains. The rainwater falling onto the surface got collected in the depressions to give rise to oceans.
Every line of this post can be 100% potential prelims question. So, read it again and again.