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.
Source:
https://phys.org/news/2020-05-alternative-earths-characterizing-earth-stages.html