The early atmosphere in the earth was different from what we are experiencing right now. Before, it is hard for any organisms to thrive in. The age of the earth is theorized to be approximately 4.6 billion years old and self-replication RNA molecules started from complex organic molecules present in the earth.
After the theory of self-reproduction of RNA molecules, the first life was believed to have started 3.5 billion years ago. They are the simple cells or prokaryotes. They are chemoautotrophs which feed on CO2 as source of carbon and oxidize inorganic molecules for energy.