Final answer:
The fact that oxygen is a major constituent of the Earth's present-day atmosphere is due to the appearance of life on Earth, specifically the process of photosynthesis which began about 2.4 billion years ago, resulting in the Great Oxygenation Event and the formation of the protective ozone layer.
Step-by-step explanation:
The fact that oxygen is a major constituent of the Earth's present-day atmosphere is a direct consequence of the appearance of life on Earth. The emergence of photosynthesis had far-reaching evolutionary consequences, including the accumulation of free oxygen in the atmosphere.
Around 2.4 billion years ago, the process known as the Great Oxygenation Event began, thanks to photosynthesizing organisms. This increase in atmospheric oxygen eventually led to the formation of the ozone layer, which protects life from the Sun's harmful ultraviolet radiation, enabling life to thrive on land.
Initially, Earth's primitive atmosphere was very different from what it is today, lacking free oxygen and primarily consisting of carbon dioxide, water vapor, and other gases. This early atmosphere was formed through volcanic outgassing and was inhospitable to modern forms of life.
However, as photosynthetic life forms evolved and flourished, they altered the chemical composition of the atmosphere, making it more suitable for other life forms that depend on oxygen for respiration.
The planet's geology, including plate tectonics and volcanic activity, also played a role in shaping the current atmosphere. However, the presence of oxygen as a major component of the atmosphere is largely attributed to biological processes, particularly photosynthesis.