Life on earth is impossible without -

(i) Oxygen

(ii) Carbon dioxide

(iii) Helium

(iv) Dust

For the survival of life on earth, the two important gases are oxygen and carbon dioxide. 

 The oxygen that is needed by humans is given out by plants and the carbon dioxide that is needed by plants is given out by humans  (animals). Therefore, it can be said that plants and animals are interdependent on each other. And both oxygen and carbon dioxide is needed for the survival of animals and plants respectively.

Therefore, life on earth is impossible without both carbon dioxide and oxygen.