LIFE IN EARTH?

Simple life appeared on Earth almost as soon as the planet cooled sufficiently to support water-based organisms. To be detectable from a distance, however, life has to evolve to the point where it dominates the planetary surface chemistry and has significantly changed the atmosphere, creating chemical “biosignatures” that can in principle be detected remotely. For instance, Earth itself would probably not have been detected as a life-bearing planet during the first two billion years of its existence. Concerning the evolution of intelligent life, the main open questions include: Physicist Enrico Fermi famously asked the question "Where are they?" to express his surprise over the absence of any signs for the existence of other intelligent civilizations in the Milky Way Galaxy. Although many potential resolutions to this so-called “Fermi paradox” have been suggested over the years, there is still no consensus on which one, if any, is correct. The question of whether we are alone in the Milky Way (or in the universe at large) remains, however, one of the most intriguing questions.