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How does Earth’s atmosphere differ from the original atmosphere of the planet?

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Wispy cirrus clouds stretch across a bright blue sky above trees and a river at Pinawa Dam Provincial Park in Manitoba, Canada, with sunlight illuminating the scene. — Atmosphere The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.

Earth’s current atmosphere differs significantly from its original state, as evidence suggests that a primordial atmosphere—if it ever existed—was completely lost due to intense solar radiation or early planetary conditions.12

The current atmosphere is a secondary development, formed through the gradual release of gases from the planet's interior and the metabolic activities of life-forms. Key differences include:2

Oxygen Content: The early atmosphere lacked free oxygen, which was absent because of abundant "sinks" like ferrous iron and sulfides that consumed any oxygen produced. Today, oxygen makes up approximately 20.95 percent of the atmosphere, a result of photosynthesis by primitive organisms.342
Carbon Dioxide: Early levels of carbon dioxide were likely much higher—perhaps 100 times current levels—to provide a greenhouse effect that kept the oceans from freezing under a dimmer early Sun.5
Composition: While the modern atmosphere is dominated by nitrogen (78.08 percent) and oxygen, the early atmosphere was shaped by volcanic outgassing, consisting of water vapour, carbon dioxide, and other gases, but notably lacking the free oxygen found today.25