Just how many forms of life are there? We often speculate here about life on other worlds, but Paul Davies (Arizona State) is currently exploring the question from a different perspective entirely. Davies would like to know whether a ‘second genesis’ might have occurred, producing a fundamentally different form of life that would have evolved right here on Earth and might still occupy our planet. Life may, in other words, have started many times, perhaps with significantly different results we just haven’t uncovered yet.

Call it a ‘shadow biosphere,’ a concept the physicist calls for exploring:

“…[It] is still just a theory. If someone discovers shadow life or weird life it will be the biggest sensation in biology since Darwin. We are simply saying, ‘Why not let’s take a look for it?’ It doesn’t cost much (compared to looking for weird life on Mars, say), and, it might be right under our noses.”

Finding these alternate life forms, if they exist, may be tricky, as they could be lurking in places where conditions are extreme, such as deep sea hydrothermal vents, salt lakes or areas high in ultraviolet radiation. And as Davies told a symposium at the AAAS annual meeting in Chicago, our assumptions about biochemistry may be getting in the way of our finding life in the shadow biosphere. “We don’t quite know how weird life would look. It’s as wide as the imagination and that’s why it’s really hard to look for.”

That last quote comes from a BBC story on the possibilities of alien life forms on Earth. The question Davies is asking is profound, for if life did get its start multiple times on our planet, we can assume it’s far more likely to have appeared elsewhere in the universe. The case for life on those trillions of habitable planets Alan Boss talks about in yesterday’s entry becomes more and more formidable. The BBC story cited above also discusses the work of Steven Benner (University of Florida), whose team has created what Benner calls “…an artificial synthetic chemical system capable of Darwinian evolution.”

Benner says the molecule — a modified version of the DNA double helix but with a six-letter genetic alphabet instead of four — is not self-sustaining (“You have to have a graduate student stand there and feed it from time to time”), but it is evolving. He wants to apply natural selection to it to watch it evolve under selective pressure. So is Benner’s creation alive, or are we basing too much on our Earth-based assumptions? “Remember – just because you are a chemical system which is self-sustaining and capable of Darwinian evolution, that doesn’t mean that is the universal definition of life,” the scientist adds.

A useful caveat. The question that hovers over our first encounter with truly alien life, assuming it is out there, is not what we’ll say to it as much as whether or not we’ll even recognize it as life. Last April I quoted Jacob Bronowski on this in a statement from The Ascent of Man that bears repeating:

“Were the chemicals here on Earth at the time when life began unique to us? We used to think so. But the most recent evidence is different. Within the last few years there have been found in the interstellar spaces the spectral traces of molecules which we never thought could be formed out in those frigid regions: hydrogen cyanide, cyano acetylene, formaldehyde. These are molecules which we had not supposed to exist elsewhere than on Earth. It may turn out that life had more varied beginnings and has more varied forms. And it does not at all follow that the evolutionary path which life (if we discover it) took elsewhere must resemble ours. It does not even follow that we shall recognise it as life — or that it will recognise us.”

As to Davies, I’m looking forward to this prolific author’s next book. The Eerie Silence, scheduled for publication next year, is to be a new look at SETI and its dilemmas. Davies wrote up the ‘weird life’ concept in “Are Aliens Among Us?” Scientific American Vol. 297; No. 6, pp. 36-43 (available online). Thanks to Dave Moore for the tip on this story.