paul_davies's picture
Theoretical physicist; cosmologist; astro-biologist; co-Director of BEYOND, Arizona State University; principle investigator, Center for the Convergence of Physical Sciences and Cancer Biology; Author, The Eerie Silence and The Cosmic Jackpot

A hundred and fifty years ago, Charles Darwin gave us a convincing theory of how life has evolved, over billions of years, from primitive microbes, to the richness and diversity of the biosphere we see today. But he pointedly left out of account how life got started in the first place. "One might as well speculate about the origin of matter," he quipped in a letter to a friend. How, where and when life began remain some of the greatest unsolved problems of science. Even if we make life in the laboratory in the near future, it still won't tell us how Mother Nature did it without expensive equipment, trained biochemists and - the crucial point — a pre-conception of the goal to be achieved. However, we might be able to discover the answer to a more general question: did life originate once, or often?

The subject of astrobiology is predicated on the hope and expectation that life emerges readily in earthlike conditions, and is therefore likely to be widespread in the universe. The assumption that, given half a chance, life will out, is sometime called biological determinism. Unfortunately, nothing in the known laws of physics and chemistry single out the state of matter we call "living" as in any way favored. There is no known law that fast-tracks matter to life. If we do find life on another planet and we can be sure it has started there from scratch, completely independently of life on Earth, biological determinism will be vindicated. With NASA scaling back its activities, however, the search for extraterrestrial life has all but stalled.

Meanwhile, there is an easy way to test biological determinism right here and now. No planet is more earthlike than Earth itself, so biological determinism predicts that life should have started many times on our home planet. That raises the fascinating question of whether there might be more than one form of life inhabiting the terrestrial biosphere. Biologists are convinced that all known species belong to the same tree of life, and share a common origin. But almost all life on Earth is microbial, and only a tiny fraction of microbes have been characterized, let alone sequenced and positioned on the universal tree. You can't tell by looking what makes a microbe tick; you have to study its innards. Microbiologists do that using techniques carefully customized to life as we know it. Their methods wouldn't work for an alternative form of life. If you go looking for known life, you are unlikely to find unknown life.

I believe there is a strong likelihood that Earth possesses a shadow biosphere of alternative microbial life representing the evolutionary products of a second genesis. Maybe also a third, fourth... I also think we might very well discover this shadow biosphere soon. It could be ecologically separate, confined to niches beyond the reach of known life by virtue of extreme heat, cold, acidity or other variables. Or it could interpenetrate the known biosphere in both physical and parameter space. There could be, in effect, alien microbes right under our noses (or even in our noses). Chances are, we would not yet be aware of the fact, especially if the weird shadow life is present at relatively low abundance. But a targeted search for weird microbes, and the weird viruses that prey on them, could find shadow life any day soon.

Why would it change everything? Apart from the sweeping technological applications that having a second form of life would bring, the discovery of a shadow biosphere would prove biological determinism, and confirm that life is indeed widespread in the universe. To expect that life would start twice on Earth, but never on another planet like Earth, is too improbable. And to know that the universe is teeming with life would make it far more likely that there is also intelligent life elsewhere in the universe. We might then have greater confidence that the answer to the biggest of the big questions of existence — Are we alone in the universe? — is very probably, no.