Asteroids and comets pummeling Earth delayed rise of oxygen

Remnants of ancient asteroids revealed Earth was bombarded by massive space rocks more often than previously thought, vastly altering oxygen levels in the planet’s early atmosphere.

When Earth formed 4.6 billion years ago, it had almost no atmosphere. As the planet cooled, an atmosphere started to form, though it was mainly of carbon dioxide and nitrogen at first, which was inhospitable for life as we know it today. Eventually, Earth experienced a major shift in surface chemistry triggered by the rise of oxygen levels, also known as the Great Oxidation Event (GOE).

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