The question of the uniqueness of life on Earth, and thus the existence of extraterrestrial life, is easily answerable in logical terms. While science needs hard proof in order to validate an hypothesis, one can speculate on the likelihood of the outcome. If one thinks about the scientific arc on issues regarding the uniqueness of planet Earth in respect to everything else in the universe, one can see that the answers have always pointed in one direction, which is away from anthropocentrism.
Anthropocentrism is the belief that the human species and, by extension, anything relating to it (such as planet Earth or the solar system), is always at the center. Historically, this concept can be seen in the now debunked belief that the Earth is at the center of the universe or, on a more metaphysical level, on the idea of man is made in the image of God itself. The history of science has been a history of challenges to the anthropocentric belief, and if one start thinking in those terms, one can logically infer the answer to many of humanity’s unanswered questions.
In anthropocentric terms, life on Earth would have to be unique in the universe. A modified form of anthropocentrism may be able to accept that life could exist somewhere else in the universe as long as the human species continues to sit at the top of the evolutionary pyramid. The problem with this idea is that given the history of rebuttals to the anthropocentric idea, it is likely that neither of these assumptions will turn out to be true. The anthropocentric model, while possible, is one of the least probable.
What is more plausible, and thus probable, is that life is not unique to planet Earth and that there are life forms out there in the universe that are way ahead of humans on the evolutionary scale. Still along those anti-anthropocentric lines, one can easily say that life in the universe should actually turn out to be quite common, and that there is nothing special about it existing on planet Earth. Naturally, science will take some time to answer such questions, but it’s always good to be aware of what the most likely outcome will be as we may not be there to see it happen.