Photo: Alien "ID Chart" to Aid Search for Extraterrestrial Life

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An artist's rendering depicts the tumultuous environment of early Earth.

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 have modeled the history of Earth's atmosphere to come up with "fingerprints" of life to look for on alien worlds.

Illustration courtesy David A. Aguilar (CfA)

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