Are We a Biological Miracle?

According to a Roper poll taken in 2002, two-thirds of the American populace believes that intelligent, extraterrestrial life exists. But that means that one-third is skeptical: A generous slice of the citizenry thinks we might be the cleverest creatures in the Milky Way, or even the entire cosmos.

When I lecture about our hunt for life elsewhere, there are always folks in the audience who share this pessimistic point of view (which surprises me, given that they've voluntarily come to my talk). When I ask what motivates their disbelief, the response usually boils down to this: Extraterrestrial intelligence is too improbable.

Too improbable? Given the recent tide of planet discoveries -- hinting that tens of billions of Earth-size worlds might infest the Milky Way -- you'd think that the case for cosmic company was hardly "improbable."

But there's always the biology card. Sure, habitable worlds might be plentiful, but just because you find a home for ET doesn't mean the lights are on. In Europe, some academics have recently weighed in on the side of the skeptics, claiming to find biological roadblocks that would stall the easy evolution of thinking beings.

Two years ago, Andrew Watson, at the University of East Anglia, made a mathematical model of what he thought were the four transformative steps in the evolution of Homo sapiens: the emergence of bacteria, complex cells, specialized cells (permitting multicellular life), and eventually intelligent creatures with language. This concatenation of biological development is reminiscent of the Drake Equation, and Watson used it for estimating the probability of sentient beings.

What he notes is that we've arrived on the planet almost five billion years after the Sun began to shine. Since our star is no longer a spring chicken, Watson argues that evolution almost missed its opportunity to produce us. That's because the gradual warming of the Sun will soon (within a billion years or so) make Earth too toasty for habitation by sophisticated animals. Ergo, Homo sapiens just made it under the wire, and we're lucky to be here; we won the lottery. Watson figures that the probability of a jackpot is roughly one in ten thousand for any Earth-like world. That's pretty low, and he guesses we'll have a hard time finding ET.

More recently, Nick Lane of University College London and Bill Martin of the University of Dusseldorf have argued that complex life -- including the reader of this blog -- are only in existence because of a "one-off" event billions of years ago, when simple, single-celled organisms swallowed some other bacteria that became their engines of energy -- mitochondria. Lane and Martin argue that without this combination of cellular capabilities, the type of specialized cells that could eventually form complex plants and animals would never have arisen (this is one of the four steps in Watson's chain of development, you'll note). Bottom line? You're the consequence of a biological accident, and an improbable one.

These researchers are not the first to opine that our presence on Earth is unlikely, of course. Evolutionary biologist Stephen Jay Gould made that argument, too. But the question is, are they right?

The answer should be obvious to anyone who's machete'ed their way through a statistics course: We don't know. We have only one example of a world with life. And when you have but a single data point, you don't know whether a phenomenon is commonplace or breathlessly rare.

Frankly, I'm not terribly discouraged by the arguments. Even if Watson is correct, and only 0.01 percent of Earth-like worlds produce intelligence, millions of such species will have arisen in the Milky Way. If so, it's likely that thousands of them are out there right now, enjoying their alien lives. As for the supposed improbability of complex cells, University of Colorado biologist Norman Pace put it to me this way:

My guess is that the origin of life is a common consequence of the origin of planetary systems, and the real question is whether that life survives for a few billion years of evolution. There is nothing special about us so far as I know, and no reason to predict such.

It once again comes down to our inability to make a statistical assessment of just how "lucky" we are. So clearly, we need to search for another example of life.

But the suggestion that we're a very, very special case makes me uneasy. It flies in the face of the so-called Copernican principle and implies that our existence is... well... a miracle.

Apparently one-third of the population likes this idea. But I don't. After all, miracles are science's last resort.