An artist’s impression of the rocky exoplanet Kepler-186f, which is one of the most promising candidates for a planet could potentially be habitable, but how similar or different does it have to be compared to Earth to be able to support life? Image credit: NASA/Ames/SETI Institute/JPL–Caltech.

Three billion years ago, Earth was a very different place. The sun that shone on its oceans and continents was not as bright as it is today, and rather than the oxygen-rich atmosphere humans need to survive, methane played a much bigger role in the gas layer that encased our young planet. Despite their differences, this early Earth and our current one have something important in common: they could both support life.

For much of its existence, Earth has been inhabited. But if researchers remotely analyzed the atmosphere of that young Earth, they might have missed the evidence for life ...

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