Why must aerobic organisms such as yourself breath oxygen and exhale co2?

Aerobic organisms like humans are dependent on oxygen for their survival because in us without oxygen formation of ATP(energy) can not be possible.

In aerobes, oxygen acts as the final electron acceptor in the electron transport chain after which oxygen converts into H2O and formation of ATP occurs. Therefore we have to breathe oxygen continuously so that our cell can form ATP.  

CO2 is a byproduct of aerobic respiration in which is generated when glucose gets oxidized into CO2, therefore, we exhale this CO2 through our lungs.  

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".