Final answer:
The Earth's initial atmosphere was formed by the outgassing of volcanoes, as the planet cooled and gases like ammonia, methane, water vapor, and carbon dioxide were released, without oxygen being a significant component initially.
Step-by-step explanation:
The Earth's initial atmosphere was primarily formed by outgassing of volcanoes. This process occurred as the Earth cooled after its formation, leading to the release of various gases from volcanic activity, which included ammonia, methane, water vapor, and carbon dioxide, but notably lacking in oxygen. It took a significant amount of time and biological evolution, particularly the emergence of cyanobacteria and the advent of photosynthesis, before oxygen became a major component of the atmosphere.
It is also worth mentioning that while photosynthesis played a crucial role in the transition to an oxygen-rich atmosphere, it was not responsible for the early atmosphere's formation. Instead, photosynthesis and the proliferation of phototrophic organisms, such as blue-green algae, increased the levels of oxygen over billions of years.
Current evidence suggests that the origin of Earth's atmosphere and oceans is likely due to a combination of volcanic outgassing and impacts from comets and asteroids. These events contributed to the chemical composition of the early atmosphere, which set the stage for the development of life on our planet.