The Independent (UK) published excerpts from a lecture given by professor of environmental sciences Andrew Watson about the possibility of ever encountering alien life forms. Watson argues that for all intents and purposes, human beings are probably alone in the universe.
Watson notes that the best evidence is that life began about 3 billion years ago on this planet, and it took 2 billion years after that for complex life to arise. Life on Earth has only about 1 billion years left before changes in the sun and environment lead to a runaway greenhouse effect that will render life impossible. So, assuming other planets where life develops are similar to Earth, sentient life has a very narrow (by the universe’s standards, at least) window in which to evolve.
According to Watson,
Out beyond our own special planet, complex life is rare, and sentient life (aliens) rare still. That a large number of planets probably exist does not make it reasonable to assume that sentient life is inevitable on at least some planets if the chances of it arising are infinitesimally low. Our evolution at a late stage of our planet’s history is consistent with beings like us being so rare that we are very unlikely to contact any other. Whether we like it or not, therefore, we are probably, in effect, alone in the universe, and this planet the only place we will ever know where the universe has come into self-awareness.
Well, hopefully we will expand onto other planets, but don’t expect any visitors to come knocking anytime soon.
Source:
Forget about aliens: we’re all alone in the universe. Andrew Watson, The Independent (UK), March 28, 2002.