Fifty years ago next month, a young American astronomer named Frank Drake turned a radio telescope on a nearby star and began a systematic search for messages from an alien civilisation. It was an extraordinarily daring experiment. In those days, looking for aliens was regarded as part way between pseudoscience and lunacy.
Today the mood has dramatically shifted. Over the past few months, Seti — the Search for Extraterrestrial Intelligence — has featured prominently at scientific meetings in both the Vatican and the Royal Society, and around the world scientists are celebrating the half-century. Astronomers now estimate there could be billions of Earth-like planets in our galaxy alone. But the key factor that determines whether or not we are alone in the universe remains stubbornly mysterious. That factor concerns the origin of life. Without life, there will be no alien intelligence.
Drake had assumed that if a planet resembled Earth, then life of some sort was pretty much bound to arise on it eventually. That assessment was echoed by Carl Sagan, Seti's charismatic champion, who pointed out that no sooner was Earth ready for life than “up it popped.” If Drake and Sagan are correct, then it's easy to imagine thousands of technological civilisations in the galaxy.
But Drake and Sagan were swimming against a huge tide of opinion from molecular biologists. Francis Crick, for example, remarked that the origin of life seemed “almost a miracle, so many are the conditions which would have had to have been satisfied to get it going.” The problem is that even the simplest living thing is already so stupendously complex that if such an entity were to be thrown together by chance, it would be a fluke of such magnitude as to be unlikely to happen twice in the observable universe, vast though that may be.
A cosmic imperative
However, we don't know that life's origin was purely a chemical accident. Scientists are aware of all manner of self-organising processes that might have fast-tracked mindless molecules down a path of complexification leading to life. Indeed, that is the fashionable view. The biologist Christian de Duve expresses it splendidly with the evocative slogan that “life is a cosmic imperative.”
Unfortunately, there are few grounds for this new-found optimism. Scientists have no agreed theory of the origin of life — plenty of scenarios, conjectures and just-so stories, but nothing with solid experimental support. Life may emerge from unremarkable chemical sludge with a high degree of probability; but then again, it may not. We haven't a clue either way. And while we are completely in the dark about precisely what it takes for life to start up, putting an estimate on the numbers of alien civilisations is pointless.
There might be a way to solve this problem at a stroke. No planet is more Earth-like than Earth itself, so if life really does pop up readily in Earth-like conditions, then surely it should have arisen many times right here on our home planet? And how do we know it didn't? The truth is, nobody has looked.
Biologists think all familiar forms of life on Earth are inter-related and descended from a common ancestor. Evidence comes from the universal nature of biochemistry, and also from gene sequencing, which enables organisms to be positioned on a single tree of life. If we found a life form with a seriously weird biochemical makeup, it could point to a second genesis.
The vast majority of terrestrial species are in fact microbes, and scientists have only begun scratching the surface of the microbial realm. It is entirely possible that examples of life as we don't know it have so far been overlooked.
If there is a second sample of life on Earth, it could constitute a sort of shadow biosphere, perhaps restricted to obscure pockets, or possibly spread all around us, interpenetrating the familiar biosphere. In the latter case, “alien” microbes might be intermingled with our own microbial relatives. They could be literally under our noses. Identifying the aliens presents a challenge, but a few scientists are finally starting to look.
If we do discover more than one type of life on Earth, we can be fairly certain that the universe is teeming with it, for it would be inconceivable that life started twice here but never on all the other Earth-like planets. And once life gets going, there is least a chance that intelligence will evolve. Who knows, beings far across the galaxy may even now be wondering whether or not they are alone in the vastness of the cosmos, and trying in some way to attract our attention. — © Guardian Newspapers Limited, 2010
(Paul Davies is author of The Eerie Silence and director of the Beyond Centre at Arizona State University.)