What was the biggest difference between the Earth's second atmosphere and third (present) atmosphere?

Question

Carbon dioxide from volcanic activity became a major part of the atmosphere.

Oxygen from plant photosynthesis became a significant part of Earth's atmosphere.

High percentages of Carbon dioxide caused a sustained warming cycle 500 million years ago.

Oxygen from water vapor in the atmosphere became a large part of Earth's atmosphere.

Answer 1

The biggest difference between Earth's second atmosphere and its third (present) atmosphere is that oxygen from plant photosynthesis became a significant part of Earth's atmosphere.

The second atmosphere, primarily composed of gases released by volcanic activity (including carbon dioxide, water vapor, and other gases), did not contain free oxygen. The emergence of photosynthetic organisms, particularly cyanobacteria, introduced oxygen into the atmosphere, leading to the Great Oxygenation Event around 2.4 billion years ago. This dramatic increase in oxygen levels transformed the atmosphere, making it more conducive to the development of aerobic life forms and significantly changed the planet's environmental conditions.