Earth's early atmosphere of nitrogen, methane and carbon dioxide was hostile to life as we know it, but friendly to the first methane-loving bacteria. Astronomers modeled the history of Earth's atmosphere to learn what fingerprints to seek on alien worlds. This artist's rendering shows Earth at 4 billion years ago, before continents had formed and while our planet still suffered bombardment from asteroids and comets left over from the solar system's formation. Credit: David A. Aguilar (CfA)
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