Evidence of past or present life elsewhere in the universe will undoubtedly be a turning point in human history. There are many issues at hand when considering the probability that life exists beyond Earth. Over 3500 planets, many of them Neptune-sized or larger, have been identified outside of our Solar System. Until recently, there has been no evidence indicating that any of these planets are habitable. However, with the success of the Kepler satellite, hundreds of Earth-sized have been discovered and a significant fraction of the these are expected to be habitable. The census of habitable worlds projects to 10-20 billion across the Milky Way, a staggering number. Furthermore, experiments conducted in the laboratory as well as evidence from the early history of life on Earth have significant implications for the formation of life on other planets. It has been suggested that life will probably begin on another planet if energy, liquid water, and the right chemicals are simultaneously present. The four main elements that are necessary for life as we know it (hydrogen, oxygen, carbon, and nitrogen) are acknowledged to be everywhere throughout the universe. However, through no lack of trying, scientists so far have not been able to reproduce the progression from organic molecules to simple life in the laboratory. Despite our inabilities to recreate the origin of life on Earth, we do know that life originated very quickly, within the first 10% of the planet's entire history. Is it likely, then, that life exists elsewhere?
The beginning of life on Earth as single-celled organisms and its subsequent evolution to complex life forms was neither smooth nor predictable. Nor is it clear that the beginning of life on Earth was a single, isolated incident. Throughout the geologic history of Earth, life was likely to have been eliminated or severely set back by external events such as meteor or comet impacts. Consequently, life may have arisen several times independently, in a number of forms. In any case, biological evidence shows that life is highly adaptable and species can evolve to thrive in different environments, from the extreme temperatures and pressures of the ocean depths to highly acidic environments in sulfur hot springs. Once life was established permanently on the planet, it began its evolution toward greater complexity, even though complexity is not necessarily required for an organism to endure or adapt to environments. Although most scientists agree that the origin of single-celled organisms on planets and moons is likely to be a commonplace occurrence, there is a debate amongst astrobiologists about the probability for complex (and thus intelligent) life beyond Earth. Peter Ward and Donald Brownlee wrote what is known as "The Rare Earth Hypothesis", a thesis that argues against the ubiquity of complex life in the universe. However, many of their arguments have a counter-argument, and other astrobiologists argue that complex life could be common.
If we examine the existence of complex life by observing our own planet, it becomes evident that eventually life evolved intelligence in a few species and the ability to manipulate the global environment in one. However, intelligence may not be the inevitable consequence of evolution. On Earth it took nearly four billion years to evolve and is only evident in a handful of the millions of species on the planet. What if the development of intelligent life is not a probable end result for life in general?
Despite the enormous uncertainty about the existence of intelligent life in the galaxy, astronomers have nevertheless made tentative efforts to communicate across interstellar space. Currently, there is no evidence that extraterrestrial life forms have made a visit to our planet or otherwise tried to establish contact with us. Humanity can only surmise about the reasons why. Perhaps the development of life in the universe is not commonplace and we are indeed alone. Perhaps the closest intelligent civilization is just too far away. Perhaps it is in a civilization's natural progression to destroy itself before successfully navigating the universe. Perhaps we are the only civilization that finds merit in communication across space. Perhaps they do exist, but we have not been able to recognize them. After all, there is no scientific reason to believe that aliens would resemble us physically or culturally. Arthur C. Clarke has remarked that any technology much advanced beyond our own would look like magic. Physicist Enrico Fermi proposed what has now become known as the Fermi Paradox. Perhaps we are too limited by our own concept of civilization. According to Fermi, we have reason to believe that Earth is in no way special or unique, so other Earth-like planets and civilizations should be in existence. However, if this statement is true, then our galaxy should have already been colonized. Yet there is no evidence for a galactic or universal civilization. Many scientists and philosophers have proposed their own solutions to this paradoxical situation. Despite hundreds of years of speculation, no one has arrived at any concrete answers about life beyond Earth. As in all scientific debates, the only way forward is to gather data.