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.
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.
Carol Cleland on the Shadow Biosphere, from Astrobiology Magazine.