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Earth's atmosphere began forming approximately 5 billion years ago, starting from Earth's formation around 4.5-4.6 billion years ago. The primordial atmosphere came from volcanic outgassing and was very different from today's oxygen-rich atmosphere. The Great Oxygenation Event occurred much later (~2.4 billion years ago), but the atmospheric formation process started with Earth's origin. Options A and B (2-5 million) are far too recent, while 2 billion years is when significant changes occurred, not when it started.