which cell
In order for the principles of mutation and natural selection in the theory of evolution to work, there have to be living things for them to work on. Life must exist before it can to start diversifying. Life had to come from somewhere, and the theory of evolution proposes that it arose spontaneously out of the inert chemicals of planet Earth perhaps 4 billion years ago.


The most probable route from where where the cell emerged is the small self replicating RNA molecules which migrated into phopholipid vesicles which also grow and split. 

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RNA world hypothesis is generally accepted regarding formation of first cell and reproduction of cells.

It is possible that after Earth was formed there were chemical elements and compounds present in the Earths crust, oceans, in the atmosphere.  In time due to the cosmic radiation (UV too)  and sunlight there were many chemical reactions.

1. Simple reactions could have taken place to form organic compounds and resulting in a RNA molecule, with carbon, nitrogen, phosphorous, hydrogen, oxygen etc (forming nucleotides).  This process could have happened in atmosphere.

2.  The RNA molecules had evolved into chains and self-replicable chains.

3. Proteins were formed from elements and we joined with RNA.  A cell membrane eventually formed around them.  This process may have happened in the oceans or on land.