Here's a breakdown of the major milestones:
* Early Earth (4.5 billion years ago): The early atmosphere was primarily composed of hydrogen and helium, similar to the Sun.
* Volcanic Activity (4.0 - 3.5 billion years ago): Volcanic eruptions released gases like carbon dioxide, nitrogen, methane, and water vapor, gradually changing the atmosphere.
* First Life (3.5 billion years ago): Early microorganisms, likely anaerobic, began to alter the atmosphere by releasing methane and other gases.
* Photosynthesis (2.5 billion years ago): The development of photosynthesis by cyanobacteria led to the release of oxygen into the atmosphere, a process that took billions of years to significantly change the composition.
* Oxygenation (2.4 billion years ago): The Great Oxidation Event occurred when the amount of oxygen in the atmosphere reached a critical level, leading to the formation of an ozone layer and the development of aerobic life.
* Present-day composition (past 100 million years): While the atmosphere is relatively stable now, it's still evolving due to human activity.
Therefore, while we can't pinpoint a precise date, we can say that the Earth's atmosphere has been steadily evolving for billions of years, reaching a relatively stable composition within the past 100 million years.
It's important to note that even within this stable period, the composition is constantly fluctuating, primarily due to human activities like burning fossil fuels and deforestation.
