Question

The atmosphere of early Earth probably contained no O2 until the emergence of organisms that _____. A. had chloroplastsB. used hydrogen sulfide as an energy sourceC. were oxygen respiringD. were chemoautotrophicE. used water as an electron source for photosynthesis

Answer 1

The correct answer is C. were oxygen respiring.

Early Earth's atmosphere is believed to have been composed mainly of gases such as methane, ammonia, water vapor, and carbon dioxide. It was an anaerobic environment, meaning it lacked oxygen. The emergence of oxygen-producing organisms, such as cyanobacteria, which are believed to be the first organisms capable of oxygenic photosynthesis, eventually led to the accumulation of oxygen in the atmosphere. These oxygen-producing organisms released oxygen as a byproduct of their metabolic processes, and over time, oxygen levels increased in the atmosphere, leading to the oxygenation of Earth's atmosphere. This event, known as the Great Oxygenation Event or the Oxygen Catastrophe, is estimated to have occurred around 2.4 billion years ago and marks the emergence of oxygen-respiring organisms on early Earth.