- The planet earth initially was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium. This is far from the present day picture of the earth.
- Hence, there must have been some events– processes, which may have caused this change from rocky, barren and hot earth to a beautiful planet with ample amount of water and conducive atmosphere favouring the existence of life.
- The earth has a layered structure.
- From the outermost end of the atmosphere to the centre of the earth, the material that exists is not uniform.
- The atmospheric matter has the least density.
- From the surface to deeper depths, the earth’s interior has different zones and each of these contains materials with different characteristics.
Evolution of Lithosphere
- The earth was mostly in a volatile state during its primordial stage.
- Due to 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 ones to move towards the surface.
- With 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 earth was further heated up.
- It is through the process of differentiation that the earth forming material got separated into different layers.
- Starting from the surface to the central parts, we have layers like the crust, mantle, outer core and inner core.
- From the crust to the core, the density of the material increases.
Evolution of Atmosphere and Hydrosphere
- The present composition of earth’s atmosphere is chiefly contributed by nitrogen and oxygen.
- There are three stages in the evolution of the present atmosphere.
- The first stage is marked by the loss of 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.
- The earth’s oceans were formed within 500 million years from the formation of the earth. This tells us that the oceans are as old as 4,000 million years.
- Sometime around 3,800 million years ago, life began to evolve.
- However, around 2,500-3,000 million years before the present, the process of photosynthesis got evolved.
- Life was confined to the oceans for a long time.
- Oceans began to have the contribution of oxygen through the process of photosynthesis.
- Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to flood the atmosphere.
Origin and evolution of Life on Earth
- In The Origin of Species, Charles Darwin (1859) hypothesized that new species arise by the modification of existing ones—that the raw material of life is life. But somehow and somewhere, the tree of life had to take root from nonliving precursors. When, where, and in what form did life first appear? The origin of life is one of the most intriguing, difficult, and enduring questions in science.
- To begin with, conditions on earth were inhospitable for life.
- Gases of the primitive atmosphere were primarily methane, ammonia, carbon dioxide and hydrogen.
- Water vapour filled the atmosphere but there was no free oxygen.
- It was thus a reducing atmosphere on primitive earth and no life existed.
- Biological evolution– from the simple organisms to complex organisms
- As earth cooled, water vapour condensed to form liquid water.
- Rains poured to form water bodies on earth.
- The molecules of life were formed in the water.
- From the molecules of the life evolved bacteria, the earliest and simplest organisms.
- The oldest fossils of bacteria which were the first living organisms on earth have been found in rocks that are 3-5 billion years old.
- For almost two billion years, different kinds of bacteria lived on earth.
- One of these evolved a green pigment called chlorophyll.
- These chlorophyll-containing bacteria used carbon dioxide and water and released oxygen through photosynthesis and started accumulating in the atmosphere.
- Continued photosynthesis by such bacteria progressively accumulated oxygen in the atmosphere.
- Thus the atmosphere gradually transformed from reducing to oxidizing.
- At one point of time oxygen content in the atmosphere become 21%.
- Such changes served as a big trigger for biological evolution to begin and progress and this led to the invasion of land by living organism.
- As time passed, protists evolved from bacteria.
- Both bacteria and protists are unicellular.
- Then came multicellular organisms, the fungi followed by plants and animals.
- Today the diversity of living organisms is comprised of five kingdoms of life. Monera, Prototictista, Fungi, Plantae and Animalia.
