How did Earth's atmosphere become oxygen-rich?A.Biological activityB.Volcanic outgassingC.Ozone depletionD.Tectonic movementSUBMITarrow_backPREVIOUS

The Earth's atmosphere became oxygen-rich primarily due to biological activity, specifically photosynthesis. Here's a step-by-step explanation:

1. Around 2.5 billion years ago, the first photosynthetic organisms, likely cyanobacteria, evolved. These organisms used sunlight to convert water and carbon dioxide into glucose (a type of sugar) and oxygen.

2. The oxygen produced by these organisms was initially absorbed by the oceans and the Earth's crust. However, as these sinks became saturated, excess oxygen began to be released into the atmosphere.

3. This process, known as oxygenation, gradually increased the levels of oxygen in the Earth's atmosphere over hundreds of millions of years.

4. The increase in atmospheric oxygen allowed for the evolution of more complex life forms that use oxygen for respiration.

So, the correct answer to your question is A. Biological activity.