What is the significance of oxygen in early Earth's atmosphere?

Oxygen's significance in early Earth's atmosphere primarily lies in its role in facilitating the development of animals. Initially, Earth's atmosphere had very little free oxygen, which was primarily produced by photosynthetic organisms, such as cyanobacteria. As these organisms proliferated and photosynthesis intensified, oxygen levels began to rise, an event known as the Great Oxygenation Event, which occurred around 2.4 billion years ago.

This increase in oxygen was crucial for the evolutionary trajectory of life on Earth. Oxygen is essential for aerobic respiration, a process that generates much more energy per glucose molecule compared to anaerobic processes. As oxygen levels rose, it enabled organisms to develop more complex cellular functions and ultimately led to the evolution of multicellular organisms. This transition marked the first steps towards the emergence of animals.

The other options do not accurately reflect the direct impact of rising oxygen levels. For example, while volcanic activity and anaerobic life existed independently of atmospheric oxygen, and changes in carbon dioxide levels have a different causal relationship with atmospheric conditions, the rise in oxygen levels directly set the stage for animal evolution.