Final answer:
Oxygen first appeared in Earth's atmosphere around 2.4 billion years ago due to photosynthesis by cyanobacteria. Evidence for this event includes changes in the chemistry of ancient rocks and minerals that indicate varying oxygen levels through time.
Step-by-step explanation:
Early Earth's Atmosphere and the Appearance of Oxygen
The early atmosphere of Earth significantly differed from today's atmosphere, primarily due to the absence of oxygen. Oxygen first appeared in Earth's atmosphere approximately 2.4 billion years ago during a period known as the Great Oxidation Event.
The driving factor behind the appearance of oxygen was photosynthesis by cyanobacteria which started producing oxygen as a waste product. Although this process likely started earlier, the free oxygen was initially absorbed by oceans and seabed rocks. Eventually, the rate of oxygen production exceeded the rate of absorption, leading to significant accumulation in the atmosphere.
One crucial form of evidence supporting this theory is found in the chemistry of ancient rocks and minerals, which indicate oxygen levels through their oxidation states. The lack of oxidized minerals before 2.4 billion years and their increase after this period supports the presence of increasing atmospheric oxygen.