Where did the Earth's atmospheric oxygen come from 3.5 billion years ago? Did it replace another gas or just increase the atmosphere's mass? What was the role of phytoplankton in this process, and how did oxygen exist before photosynthesis?
Context The Great Oxidation Event, approximately 3.5 billion years ago, marked a significant increase in Earth's atmospheric oxygen content. While we understand that photosynthesis by phytoplankton is responsible for maintaining oxygen levels today, the origins of this initial oxygen surge and its impact on the early atmosphere remain a fascinating topic. Simple Answer Imagine Earth billions of years ago, with an atmosphere very different from today's. There was hardly any oxygen, but lots of other gases. Tiny, single-celled organisms called cyanobacteria, like tiny plant factories, started making oxygen as a byproduct of their food-making process. This oxygen, which was initially absorbed by rocks and oceans, gradually started building up in the atmosphere, changing its composition. The increase in oxygen didn't just replace other gases; it added to the total atmosphere mass, like adding more air to a balloon. Think of it as a slow process of adding more oxygen to the air...