In this context of accepted scientific procedures, single occurrences present a knotty problem. Their "truth" cannot be denied, but how can we use their existence to assert any generality rather than an explanation for a singular circumstance? For specific events of history--the rise, domination and extinction of dinosaurs, for example--we seek no such generality, and specific narrations for bounded events supply the explanations we seek. Thus a particular asteroid, striking the earth 65 million years ago and leaving evidence of its impact off the Yucatan Peninsula, probably triggered a global extinction that sealed the fate of dinosaurs and many other creatures. In developing such evidence, we have explained a unique historical event, but we have not discovered a general law of nature.
But when we ask questions about the nature of life--when we wonder, for example, how common life may be in the universe or inquire whether any potential life on other worlds will look like the life we already know on Earth--then we are seeking to understand general principles about the essential character of the natural universe and not simply to explicate a particular set of historical events. To formulate such general principles, as I argued above, we need replicates, either made in our laboratories or found elsewhere in the universe.
The life that we know, however wondrous in extent and variety, all proceeds--or so our best inferences tell us--from one single experiment. The biochemical features underlying this amazing variety and the coherent fossil record of 3.5 billion years (implying a single branching tree of earthly life with a common trunk) indicate that every living thing on Earth, from the tiniest bacterium on the ocean floor to the highest albatross that ever flew in the sky, arose as the magnificently diversified evolutionary outcome of one single experiment performed by nature, one origin of life in the early history of one particular planet.
Thus we can define the life we know by specifying its common features and properties. But if we wish to move from this knowledge to statements about the general nature of any potential life in the universe, we remain stymied in two key ways.
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.