What are one-celled organisms?
One-celled organisms are typically referred to as unicellular. Most of them are prokaryotes (Bacteria/Archaea) or protists.
What are characteristics that differentiate organisms into unicellular organisms and multicellular organisms?
It would be impossible for oxygen requiring organisms to exist before photosynthetic organisms because there was not oxygen in the atmosphere for these organisms to breathe. Photosynthetic organisms take carbon dioxide and water in and create oxygen as waste. Before these photosynthetic organisms existed there was not enough or any oxygen in the atmosphere for organisms requiring oxygen to survive.