The Shadow Biosphere


Let's define alien.

Definition number one: unfamiliar. By that description alone, a good 99% of life on this planet is alien. Breathing water, living nestled in thermal vents, stalking prey on the veldt, growing out of the Earth and eating sunlight, without eyes, without legs, with extra legs, color-blind, carapaced, marsupial, with exoskeletons, with jelly for brains, microbial, in a test tube, growing from spores. Not to mention the extremophiles, those nutty organisms that thrive in hellish environments like boiling acid, liquid asphalt, radioactive waste, and under extreme pressure.

I've been thinking a lot about SETI recently, but the thing is that alien life exists among us, to the extent that this planet is a rich steaming pot of crawling flagellae, fur, and ooze. It's also possible, according to the very interesting Professor Paul Davies and a host of other scientists, that Earth plays host to an even more alien life. No, not visitors from another world -- Davies isn't one of those "very interesting professors."

Rather, Davies, physicist and famous SETI nerd, argues that it's entirely possible for life to have evolved more than once on Earth, and that the descendants of this so-called "second genesis" could have survived until today in a shadow biosphere within our own. Or, if not, then at least traces of their ancient existence could still be found in the fossil record. After all, why couldn't life have arisen many times? It's certainly had enough time and opportunities -- in the quiet periods between asteroid impacts in Earth's early history, hemmed into an isolated pocket of geography, underground, or even on Mars, before being transported to Earth on some loose rock or another over the eons. The point is that there's no reason to believe that life spontaneously occurred only once.


If started from scratch independently of normal life, this theoretical weird life would -- most likely -- use a different set of amino acids, have a different genetic code. Even more radically, weird life could be made of fundamentally different stuff, like silicon, or arsenic. Like the extremophiles, it could live in inhospitable environments it hasn't even occurred to us to search for.

We haven't found this life yet because we haven't thought to look for it, and because all our life-detecting equipment is designed to snoop out the familiar chemical composition of "normal" life. If a microbe of weird life were to turn up in a biochemist's petri dish, it would most likely be overlooked -- or tossed out. Besides, despite the fact that microbes easily constitute the majority of terrestrial life, the microbial world is still largely unexplored. Less than one percent of existing microbes have been cultured and described, and, because their morphology is limited, it can be hard to deduce much from even the ones we know. If weird life exists, it's probably among these unmapped throngs of microbes. In his new book, The Eerie Silence: Renewing Our Search for Alien Intelligence, Davies observes that "if you set out to study life as we know it, then what you will find will inevitably be life as we know it."

Davies asks, "does all life on Earth belong to this single [evolutionary] tree, or might there in fact be more than one tree? Might there even be a forest?" If, indeed, an entirely separate tree of life coexists with our own, we'd be forced to conclude that our genesis wasn't a unique incident. Perhaps, even, there is a cosmic imperative for life to develop, and thus the universe may be seething with it.

[Editorial aside: It's interesting to me how all the theories about life in the universe boil down to the potential two extremes of the Drake equation: "none" or "teeming." Could we even bear to live in a universe with, say, only one other instance of life, somewhere far away and unreachable?]

OK. The importance of "are we alone?" as a question is that the answer, regardless of what it is, will have a profound effect on our species. As SETI scientist Jill Tarter cited so elegantly in her winning 2009 TED Talk, the discovery of intelligent life elsewhere beyond our earth wouldn't just change everything -- it would change everything all at once. As a species we have a sense of privilege, Tarter says, that the universe doesn't particularly share. We are defined by our "loneliness and solipsism." To find that we are not, in fact, alone: it may motivate us to comport ourselves better, just as an audience gives an artist meaning, or a jury lends truth its gravitas.

Finding a communicating alien civilization in the void of space, finding living bacteria on Mars, or finding evidence of a second genesis on Earth: all these would simply be gradations of the same shocking discovery, that our particular variety of living is not the only solution, nor the unilateral peak of some evolutionary pyramid. Such a revelation would not only lay out our human chauvinism, but it would also lay bare the fact that life is an insane wonder, an unstoppable force of being in a universe of indifference and chaos.

Supplementary Reading:

The Eerie Silence: Renewing Our Search for Alien Intelligence by Paul Davies.

We Are Not Alone: Why We Have Already Found Extraterrestrial Life by Dirk Schulze-Makuch.

Extremophiles: Microbial Life in Extreme Environments, edited by Koki Horikoshi and William D. Grant.

Signatures of a Shadow Biosphere (PDF) by Davies, Benner, et al., from Astrobiology.

Carol Cleland on the Shadow Biosphere, from Astrobiology Magazine.

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I think that a good working definition of alien life has to be that it did not evolve from a common ancestor of the life we know about (Eucarya, Procarya, Archaea). As you say, identifying life like this, unless it parks it's spaceship on our front lawn, will be a difficult challenge. I'm not even sure how you would start (read a lot of SciFi, maybe)!

By Paul Orwin (not verified) on 22 Sep 2010 #permalink

the discovery of intelligent life elsewhere beyond our earth wouldn't just change everything -- it would change everything all at once.

I think this overestimates the imagination and willingness to change of the average person. After a few months of speeches, oohs-and-ahhs would give way to the grim realization that Planet Fooglebain is at least thousands of light-years away, and we're never going there, and they ain't never coming here, and what's-the-next-shiny-thing on TV?

Look at climate change, for example. We're told with a high probability that our actions are going to drastically change the environment, but until the water is ACTUALLY lapping at the door, a critical mass of people wave this off and won't lift a finger. How will those same people react to being told there's life on another world? "Interesting. Will they be here soon? No? That's neat to know, but why should anything change at all?"

Inertia is a powerful, powerful beast, and won't be turned by mere knowledge. Unless that new thing is really going to have direct impact on our day-to-day lives, most people will, after a short while, just ignore it.

first, thanks to steven andrew for directing me here from dailykos.

most biologists i know could legitimately say that they study life as we don't know it. that's the whole point, to learn new things. they make observations, and try to make sense of them. a claim that an observation is the result of alien life (as defined here) would be extraordinary, and as sagan said, "extraordinary claims require extraordinary evidence."

it would of course be foolish to discount the possibility of such alien life, either offworld in origin or as the result of parallel emergence. given how weirdly the protist species interact with each other, maybe life emerged independently multiple times during the past few billion years, and all those life forms interacted, traded parts, and adopted each other's biochemical processes to result in life as we know it now.

@astro- The question you raised has actually been studied. All indications (based on detailed DNA/RNA sequence similarities) are that all the life we've studied comes from a common ancestral pool. Especially in the early days, there was a lot of part-trading (called horizontal exchange).

As for whether there ever was some independent line, it's anybody's guess since the parameters in the equation for the likelihood of life getting started (Drake) are seriously uncertain. It's a nice idea to search locally, just in case.

Davies et al is a bit nutty, I believe already Darwin realized that his theory precludes many origins because already adapted life will effectively out-compete new attempts. Among other things life snacks on protobiotic chemicals and has nowadays changed atmospheric and geological chemistry so that current abiogenesis is realistic (oxygenating atmosphere). [Fun fact, IIRC ~ 45 % of Earth modern mineral types are biogenic. "Pollution", indeed.]

The main hypothesis is that life we see today "took" once. Of course it must be tested again and again, but the likelihood that it is wrong is minute.

All indications (based on detailed DNA/RNA sequence similarities)

Mostly based on the LUCA core of shared chemicals (amino acids, nucleic acids), metabolism (citric acid cycle) and genes, I believe.

Using a single gene phylogeny has turned out to be doubtful, I hear biologists suspect that even ribosomes can be horizontally transfered. Also, functionality is many times preserved as protein fold structures and not sequence, so deep time can pervert much sequence phylogeny.

Recently it was shown that using a combination of full genome and gene transition analysis recovers a common phylogenetic tree.

Especially in the early days, there was a lot of part-trading (called horizontal exchange).

This is an hypothesis that is based on earlier problems of single gene trees, when it was proposed that horizontal gene transfer (HGT) could overwhelm vertical gene transfer (VGT) and obliterate the tree at the root. However this is rejected by the above and similar successes.

The amount of gene transfer has been, and I suspect will be, banded about more. One recent figure I heard is that genes may experience HGT on average ~ 1.1 times, consistent with being one putative source of ribosome tree difficulties, but low enough that the VGT tree can be recovered with multiple gene methods.

But the hypothesis that early days traded genes to the extent that there was "a common gene pool" can be firmly rejected now, and as that was the only prediction of earlier higher rate I know of perhaps the hypothesis is dubious even. Most modern HGT methods are evolved (retro- and other viral insertions, plasmid gene exchange, HGT due to eukaryote endosymbiosis).

By Torbjörn Lars… (not verified) on 26 Sep 2010 #permalink

My hunch is that self-replicating, auto-catalytic protobiotic chemical systems evolved many times on the early Earth but all but one went extinct and all extant life is monophyletic. It may have been that the ancestors of all extant organisms out competed other systems but I would guess that the lineage that persisted was just lucky. There is probably a huge number of inner rocky planets with water existing in all three phases and on which life could have evolved, orbiting yellow stars scattered thruout the galaxy. Many of them may be covered with organic redox scums resembling Terran biofilms. This does not mean that the evolution of metazoan complexity and nervous systems is likely. After all, it only happened once, so far as we know, in four billion years of evolution here.

By darwinsdog (not verified) on 28 Sep 2010 #permalink

Hi. My name is Miguel Adrover Lausell. I'm living, writing, and working in a Caribbean Hyper-reality that would make Baudrillard, Borges and Dick blush. Yes, my home is San Juan, Puerto Rico. My English is a 'Spanish tainted simulacra' of the North American variety, spoken in the eastern shores of the mighty North.

I am kind of a bilingual, because of my colonial condition. Science and science fiction have been within my grasp because of the English language.

My country is in trouble, and I find myself studying at the University of Puerto Rico, RÃo Piedras Campus. I am a former science teacher, currently studying in the Graduate Science Curriculum Program of an endangered institution.

As a huge science fiction nerd that loves to teach, it is no wonder that, simply put, I loved your article. One of my current courses deals with creativity and scientific education, and a paper on the subject is required as a course requisite. It is my intention to integrate literary and cinematic science fiction discourses that would stimulate cognitive processes, capable enough, to deal with the challenges ahead. The reading of this article is part of my research endeavors (which, sometimes, can be interpreted as ludic).

I would greatly appreciate any help on the subject, any piece of information. Any hint of a suggestion or opinion, would be immensely valued.

I have never heard this idea put forth before, so perhaps it is original, or at least I have arrived at it independently. If life originated more than once on the Earth, competition and predation â as has been suggested in these posts â could easily have masked the multiple origins. But a truly independently evolved life form, i.e., evolved on another star system with no possibility of contamination from Earth forms, should have some differences in its fundamental chemistry. For example, there are many kinds of potential nucleic acids and amino acids beyond the five nucleic and 24 amino acids found in all life on Earth. Is there some fundamental law that limits polymers to this specific set of nucleic and amino acids, or could we expect an independently evolved life system to utilize some different building blocks? Are RNA and DNA the only possible polymers to act as genetic codes? I think a truly separate evolved life form will have at least some differences in the basic building blocks, as well as completely different sequences of genetic codes and protein configurations.