It is easy to understand why lay people as well as biologists would consider the detection of extraterrestrial life as possibly the most exciting scientific discovery ever made. Just imagine how it would affect our self-image to find that the universe holds other intelligent creatures, with complex societies, languages, and learned cultural traditions, and capable of communicating with us. Among those of us who believe in an afterlife and an ethically concerned deity, most would agree that an afterlife awaits humans but not beetles (or even chimpanzees). Creationists believe that our species had a separate origin through divine creation. Suppose, though, that we should detect on another planet a society of seven-legged creatures more intelligent and ethical than we are, and able to converse with us, but having a radio receiver and transmitter in place of eyes and a mouth. Shall we believe that those creatures (but still not chimps) share the afterlife with us, and that they too were divinely created? Many scientists have tried to calculate the odds of there being intelligent creatures out there, somewhere. Those calculations have spawned a whole new field of science termed exobiology—the sole scientific field whose subject matter has not yet been shown to exist. Let's now consider the numbers that encourage exobiologists to believe in their subject matter. Exobiologists calculate the number of advanced technical civilizations in the universe by an equation known as the Green Bank formula, which multiplies a string of estimated numbers. Some of those numbers can be estimated with considerable confidence. There are billions of galaxies, each with billions of stars. Astronomers conclude that many stars probably have one or more planets each, and that many of those planets probably have an environment suitable for life. Biologists conclude that, where conditions suitable for life exist, life will probably evolve eventually. Multiplying all of those probabilities or numbers together, we conclude that there are likely to be billions of billions of planets supporting living creatures. Now let's estimate what fraction of those planetary biotas include intelligent beings with an advanced technical civilization, which we will define operationally as a civilization capable of interstellar radio communication. (This is a less demanding definition than flying saucer capability, since our own development suggests that interstellar radio communication will precede interstellar probes.) Two arguments suggest that that fraction may be considerable. Firstly, the sole planet where we are certain that life evolved—our own—did evolve an advanced technical civilization. We have already launched interplanetary probes. We have made progress with techniques for freezing and thawing life apd for making life from DNA—techniques relevant to preserving life as we know it for the long duration of an interstellar trip. Technical progress in recent decades has been so rapid that manned interstellar probes surely will be feasible within a few centuries at the very most, since some of our unmanned interplanetary probes are already on their way out of our solar system. However, this first argument suggesting that many planetary biotas have evolved advanced technical civilizations is not a compelling one. To use the language of statisticians, it suffers from the obvious flaw of very small sample size (how can you generalize from one case?) and very high ascertainment bias (we picked out that one case precisely because it evolved our own advanced technical civilization).
A second, stronger argument is that life on Earth is characterized by what biologists term convergent evolution. That is, seemingly whatever ecological niche or physiological adaptation you consider, many groups of creatures have 'converged by evolving independently to exploit that niche, or to acquire that adaptation. An obvious example is the independent evolution of flight by birds, bats, pterodactyls, and insects. Other spectacular cases are the independent evolution of eyes, and even of devices for electrocuting prey, by many animals. Within the past two decades, biochemists have recognized convergent evolution at the molecular level, such as the repeated evolution of similar protein-splitting enzymes or membrane-spanning proteins. It is now difficult to pick up any issue of any journal in any field of biology without encountering further examples. So common is convergent evolution of anatomy, physiology, biochemistry, and behaviour that whenever biologists observe two species to be similar in some respect, one of the first questions they now ask is: did that similarity result from common ancestry or from convergence? There is nothing surprising about the seeming ubiquity of convergent evolution. If you expose millions of species for millions of years to similar selective forces, of course you can expect similar solutions to emerge time and time again. We know that there has been much convergence among species on Earth, but by the same reasoning there should also be much convergence between Earth's species and those elsewhere. Hence although radio communication is one of those things that happens to have evolved here only once so far, considerations of convergent evolution lead us to expect its evolution on some other planets as well. As the Encyclopaedia Britannica puts it, 'It is difficult to imagine life evolving on another planet without progressing towards intelligence.
That conclusion brings us back to the puzzle I mentioned earlier. If many or most stars have a planetary system, and if many of those systems include at least one planet with conditions suitable for life, and if life is likely eventually to evolve where suitable conditions exist, and if about one per cent of planets with life include an advanced technical civilization ~ then one estimates that our own galaxy alone contains about a million P^nets supporting advanced civilizations. But within only a few dozen light-years of us are several hundred stars, some (most?) of which surely have planets like ours, supporting life. Then where are all the flying saucers that we would expect? Where are the intelligent beings that should be visiting us, or at least directing radio signals at us? If intelligent beings from elsewhere had visited Earth after literate civilizations began to develop here several thousand years ago, those beings would probably have searched out the most interesting civilizations on Earth, and we would now have written records of the visit. If the visitors had arrived in the pre-literate or prehuman past, they might have colonized Earth, and we would know of it as an abrupt arrival of drastically different life forms in our fossil record. We are bombarded by Hollywood films depicting such visits, and by tabloids actually claiming them. You will see the headlines at any US supermarket checkout counter: 'Woman kidnapped by UFO', 'Flying saucer terrorizes family', and so on. But compare that pseudo-bombardment, or our expectations, with reality. The silence is deafening. Something must be wrong with the astronomers' calculations. They know what they are talking about when they estimate the number of planetary systems, and the fraction of those likely to be supporting life. I find these estimates plausible. Instead, the problem is likely to lie in the argument, based on convergent evolution, that a significant fraction of biotas will evolve advanced technical civilizations. Let's scrutinize more closely the inevitability of convergent evolution.