A strong argument against the likelihood of extraterrestrial life that I rarely see is this (I'm sure someone has thought of it before): If life will always arise if conditions are fine, how come all earthly life has the same source? In other words, why did life only arise ONCE on Earth? Why are plants, and humans and rodents and amoeba (as I understand it) cousins related to a common ancestor? Surely, if life can arise anywhere with decent conditions as the assumption goes, then it would have formed over and over again over the 4.5 billion years of Earth history, and we would have lots of species (indeed kingdoms and phylae and higher) originating from " a fresh start"? Surely no one can deny that if there is one planet hospitable to life, it is Earth? And yet, it only happened once here, and never again. To me this seems to indicate that life is a lot (like, A LOT) less common than most seem to assume.