Alternative Earths: Characterizing the Earth during different stages of its 4.5-billion-year existence

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.