For how long have the proportions of the different gases in the atmosphere remained relatively constant?

The correct answer is that the proportions of the different gases in the atmosphere have remained relatively constant for around 200 million years. This stability is significant because it indicates that the composition of Earth's atmosphere has not experienced major changes over that period, allowing for the development and diversification of life as we know it.

This relative constancy primarily involves gases such as nitrogen, oxygen, argon, and carbon dioxide. Over geological time, the atmosphere has undergone changes due to processes such as volcanic activity, plant life, and the formation of oceans, yet the balance of these primary gases has remained stable since the Mesozoic era.

Understanding this timeframe is crucial not just in terms of geological history, but also for comprehending the conditions necessary for life, including the role of photosynthesis in increasing oxygen levels and how carbon dioxide levels have varied in comparison. By acknowledging this significant period of stability in atmospheric composition, one can appreciate how it has allowed ecosystems to evolve and adapt without drastic fluctuations in air quality or composition.