Describe various theories of origin of life.

(N/A) Life on Earth appeared $500$ million years after its formation,about $4$ billion years ago.
In order to explain the origin of life on Earth,different theories were proposed by various thinkers and scientists:
$(i)$ Theory of Special Creation: It states that God created life through a divine act; i.e.,the Earth,light,plants,and animals were all created by a supernatural power.
This theory has three connotations: $(a)$ All living organisms or species that we see today were created as such. $(b)$ Diversity has remained the same since creation and will remain the same in the future. $(c)$ The Earth is about $4000$ years old.
$(ii)$ Theory of Panspermia: Proposed by early Greek thinkers,it states that life on Earth arose from spores or panspermia,which arrived from outer space and developed into living forms.
$(iii)$ Theory of Spontaneous Generation: It states that life originated from non-living matter automatically. This theory is also known as the theory of abiogenesis or autobiogenesis. For a long time,it was believed that life emerged from decaying and rotting matter like straw,mud,etc. Louis Pasteur (in $1860$) rejected this theory and demonstrated that life comes from pre-existing life. In his experiment,he kept killed yeast cells in a pre-sterilized flask and another flask exposed to air. Life did not evolve in the former,but new living organisms evolved in the latter flask.
$(iv)$ Theory of Chemical Evolution: This theory was proposed by Oparin and Haldane. According to them,the first form of life came from pre-existing,non-living organic molecules (e.g.,$RNA$,proteins,etc.),and chemical evolution was followed by the formation of life,i.e.,diverse organic molecules were formed from inorganic constituents. The conditions on Earth favoring chemical evolution were high temperature,volcanic storms,and a reducing atmosphere containing $CH_4$,$NH_3$,etc.