Respiration requires `O_(2)` . How did the first cells on the earth manage to survive in an atmosphere that lacked `O_(2)` ?

Explanation

`O_(2)` is not always required for respiration.
Some living organisms survive in absence of `O_(2)` by doing anaerobic respiration.
The first cell on the earth, like photosynthetic bacteria are prokaryotic type organisms on the earth they obtained energy by break down of inorganic substances.
e.g. In chemosynthetic bacteria (Sulphur bacteria) chemosynthesis occurs in following way
`12 H_(12) S + 6CO_(2) to C_(6) H_(12) O_(6) + 6H_(2) O + 12S darr`