Yeah O2 is an input of cellular respiration. The presence of O2 is more likely an indicator of plant-life than it is an indicator of animal life. But plant life

Yeah O2 is an input of cellular respiration. The presence of O2 is more likely an indicator of plant-life than it is an indicator of animal life. But plant life is still life. You do not need animals to have plants. Plants came before animals here on earth. But I’m not sure of a way for animals to exist without plants. So yeah a stronger indicator of photosynthesis than cellular respiration.