(2 of 3)
First, we can make no reasoned conjecture about the frequency, or even the existence, of life elsewhere in the universe. As an optimist by temperament and as a betting man, I allow that certain features of the natural world would lead me to place my chips on yes if someone forced me to wager. But I also know the difference between a pure flutter based on hope and a smart play based on genuine probabilities.
The fact that fossilized life of the simplest bacterial grade appears in some of the most ancient rocks on Earth suggests that an origin of life in these conditions may be nearly inevitable, since incredibly improbable events should not occur so quickly. But my skeptical side retorts that good luck in one try proves nothing. I may win the lottery the first time I buy a ticket, and I might flip 10 heads in a row on my first sequence of tosses.
I might also argue that since our immense universe contains gazillions of galaxies filled with appropriate stars and planets, and since life did emerge on the one and only planet we really know, how can we deny that a sizable proportion of these other planets must also contain life? Yet a logical fallacy dooms this common argument because either alternative can be reconciled with the positive result that I must obtain for the only place I can sample--our Earth. For if all appropriate planets generate some form of life, then I should not be surprised that I have found living things on my own world. But if life really exists on my planet alone, then I must still record a positive result from this only possible sample. After all, I knew the answer for the earth before I ever formulated my scheme for sampling.
Unfortunately, we are stymied by the fact that our knowledge about life must, at least for now, be limited to studies of a single experiment on Earth. All earthly life shares a remarkably complex set of biochemical features, but does this commonality record the only conceivable building blocks for any entity that we would call "alive"? Or do all earthly creatures share these features only because we have inherited these properties from a common ancestor that used one configuration among a million alternatives unknown to us but quite conceivable and workable? Indeed, would we, in our carbon-based parochialism, even recognize otherworldly forms of life--pulsating sheets of silica, perhaps--well beyond our ken?
The architect of this conceptual prison built only two doors leading to a solution, with the path to each door marked by the same sign: FIND A REPLICATE! On one path, we make the replicates ourselves by gaining such an improved understanding of the nature of things that we can define the set of all conceivable living forms and then test their properties by chemical synthesis in our laboratory.