What are the odds that Earth could sustain life?
You must have at least heard this question asked, if not asked it yourself. "What are the odds...? What are the chances...?" It's one of those questions that creationists tend to ask, especially since it's a bit of a loaded question. It makes Earth, as we know it, sound so improbable that it could not have happened without a conscious effort. Well, since we're asking the question, we might as well answer it:
One. Or, as a ratio,1:1. As a percent, 100%. In a word, "Yes."
That is to say, Earth was destined to become what it is today, and life as we know it could only have occurred as we know it. Given that Earth contained all the preconditions for carbon-based life, it's silly to call Earth's biosphere anything less than probable. All it took was our ozone layer (which only requires oxygen in the atmosphere [check] and solar radiation [check]) to shield us, and the rest is practically a matter of time. All the chemical components and atmospheric conditions were present. There really is nothing that stood in Earth's way from creating life.
The above link summarizes the Miller-Urey Experiment, wherein abiogenesis (that is, life forming from non-life) was essentially proven. It's really not a miracle. It's basic chemical compounds from primordial Earth--water, methane, ammonia and hydrogen--an electrical charge, and time. As observed in the write-up:
At the end of one week ... as much as 10-15% of the carbon within the system was now in the form of organic compounds. Two percent of the carbon had formed amino acids, including 13 of the 22 that are used to make proteins in living cells...The molecules produced were simple organic molecules, far from a complete living biochemical system, but the experiment established that the hypothetical processes could produce some building blocks of life without requiring life to synthesize them first.
So there you have it. Given our planet's proximity to the sun, its inherent elements and compounds, and enough time, it shouldn't come as a surprise that life sprouted. If the universe were a casino, Earth would be the table to bet on.
But maybe that, itself, is the question. Perhaps when people ask "What are the odds...?" they mean to say, "What are the odds that our planet, amidst countless others in the universe, is one with life?" This question seems like a better argument for creationism, as the odds seem outrageously low. After all, what ARE the odds that any random planet will be satisfactorily close to a star, contain the right combination of chemicals, and undergo the proper atmospheric changes to create life? Unfortunately, there's no objective and accurate equation for such odds, but we can safely say that the percentage of planets capable of creating and sustaining life is extremely low. By many accounts, it is a fraction of a fraction of a percent.
However, the question is illogical. We are not actually asking "What are the odds of life on any one planet?" because Earth is not any one planet. Earth is Earth. If we were not Earth, we would not be here to ask the question. The question only exists because we exist, and therefore what we are really asking is, "What are the odds that I exist?" Do you see the circular logic at work, here? It's difficult to explain just how silly this question is, but I'll try to sum it up in a couple of sentences:
We cannot objectively question the odds of our own existence, for we already exist and can only question in retrospect. For every person who questions the odds, consider the theoretically infinite number of creatures who do not exist and therefore do not question. The only reason we wonder about our odds of existing is because we do, in fact, exist.
And that brings us back to the original answer:
The odds of us existing as we are today are a certainty, just as they would be for any other species questioning its existence. And the odds of Mars having no life, along with innumerable other lifeless planets, is a certainty. There are many more planets which are lifeless, to be sure, but by that very definition, there is no consciousness on those planets to question in the first place.
So before you cite Earth's intricate and seemingly miraculous biosphere as proof of a god, consider this: if every other planet had equally impressive and complex life, our odds of life wouldn't have changed at all. We'd still be us, and we'd still have formed life the same way. The only reason people view Earth as a miracle is because we compare ourselves to planets that are incomparable. Earth's (thus far) unique biosphere isn't evidence for creationism any more than a uniquely shaped rock is evidence for a rock-maker. Every planet and rock in the universe is different in its own way - our way just happens to be lively.
