If the nineteenth century was dominated by the industrial revolution and the twentieth was driven by the rise of electronics, surely the twenty-first century seems poised to be an era of biology. From do-it-yourself genome sequencing to replacement organs grown in tanks, from stem cells to proteomics to the Encyclopedia of Life, our knowledge about life and ability to reshape it are expanding at an exhilarating pace. The roots of this biological revolution are many, but surely one of the greatest leads back a hundred and fifty years ago, to the July 1858 meeting in London of a group of scholars called the Linnean Society. That meeting, you see, was when Charles Darwin first presented to the world his ideas of evolution based on natural selection. Darwin was an incredibly thorough and cautious scholar: by 1858, he had spent decades refining his ideas, exhaustively conducting experiments in pigeon breeding, studying specimens from the world over and interviewing cattle ranchers and fishermen and biological experts of all sort. His On the Origin of Species, published in 1959, is chock full of examples, fact after fact and story after story, clues all together building to an rock-solid case for the power of natural selection. (And it's a great read, too! Not exactly a summer beach page-turner, no, but of all the great foundational documents of science from Newton's Principia to Einstein's Electrodynamics of Moving Bodies and beyond, surely On the Origin of Species is the most accessible and enjoyable for a general reader. I'm not kidding; go get a copy and see for yourself!)

In the century and a half since then, our understanding of evolution through natural selection has been refined and improved in minor ways (for instance, through the concept of punctuated equilibrium introduced by the late Stephen Jay Gould—himself a grandmaster of science writing and my inspiration when it comes time to pen these columns!), but the general argument of Origin still stands strong. Darwin's great idea has passed every test mankind has thought of, and is now indisputably established as the central organizing principle of all biology. (Yes, biologists still debate some of the finer points, or debate how best to apply evolutionary theory to some given situation—but that no more implies that evolution itself is uncertain than physicists' having not yet measured the Higgs boson implies that we should be skeptical of the notion of atoms.)

So I have no doubt that my ancient ancestors—and yours—were once single-celled organisms eking out survival on a harsh hot young Earth. "From so humble a beginning," as Darwin famously wrote, life has gloriously diversified through countless generations of variation, competition, and survival of the fittest to produce all the creatures we share this world with today. And if it happened here, then surely it can happen elsewhere. Here too, Darwin can be our guide: evolution has proven so stunningly successful at coherently explaining all aspects of life on Earth, that scientists today don't hesitate to invoke it to predict and constrain the nature of potential life all across the galaxy.

Science fiction is too full of favorite alien species to list here even a tiny fraction of them all. The galaxy must be a crowded place, full of Vulcans and Kzinti, Wraiths and Leviathans, Hutts and Heechee, or at the very least Daleks and Drakhs, right? Well, maybe not. As I've written about previously, searches for extraterrestrial intelligence have thus far found only a great celestial silence. Perhaps we should instead be searching for non-technological life? It seems increasingly likely that our nearest neighbors will be the alien equivalent of celery rather than Centauri. But in contrast to the clear goal of listening for radio signals, how can one figure out where to look for lower alien lifeforms? Especially when such life could be like—well, who knows what it could possibly be like?

Astrobiologists, that's who. Over the past decade or so, spurred in part by the biological revolution and in part by our increasing confidence that earth-mass planets are potentially common, astrobiology has started to come of age. It's still a bit of a weird science, sure, one which is trying to study something which is known only in our collective hopes and imaginations. The universe is sometimes a stunningly surprising place; certainly no one claims that we can simply reason out all the answers in the absence of data. Yet guided by Darwin's legacy, astrobiologists are capable of drawing lines and making predictions that guide our experiments and searches for life—but not as we know it.

Extremes on Earth

Lacking alien lifeforms to study yet, much of astrobiology thus far is based on our understanding of how life on Earth can adapt to diverse and harsh conditions. So-called extremophile bacteria and archaea can live and thrive in places that would instantly kill most life on this planet: the inside of a boiling hot spring, for instance, or hidden inside pores in bone-dry desert rocks, or in extraordinarily acid or alkali waters. The extremophile archaeon "Strain 121" earned its name by happily breeding away and doubling in population while being cooked in an autoclave at 121 degrees Celsius. Yes, that's right, hospital sterilization procedures just make Strain 121 smile and think "gee, this is cozy just like back home in my undersea vent." Even stranger, researchers at Indiana University recently found that vast colonies of bacteria happily live inside rocks some 3 kilometers beneath the Earth's crust, where they've been completely cut off from the surface for tens of millions of years. You've probably heard it said that the sun is the ultimate source of energy for all life on Earth—but that's not true. These underground bacteria get their energy from a most unusual source: uranium minerals in those rocks release radiation that breaks water molecules into hydrogen and oxygen. The bacteria then consume the hydrogen, and react it with sulfates to get energy. Yes, that's right, these are nuclear-powered bacteria.

The mere fact that such things can exist shows how adaptable life is, in a much wider range of conditions than typically thought. If boiling hot springs and nuclear decay can sustain life deep within the Earth, it seems entirely plausible that life could be similarly supported in the dark distant ocean under the ice cap of Europa, one of Jupiter's moons. Europa remains today one of the most promising candidate homes for extraterrestrial life, just as it was 50 years ago when Arthur C. Clarke and Stanley Kubrick crafted 2001. But without any helpful alien monoliths around, any hypothetical Europan life is hidden and inaccessible until and unless we send a very ambitious probe there to drill into the ice and explore, as has been suggested more than once. In fact, NASA does have a "planned" Europa Astrobiology Lander penciled in—but for launch no earlier than 2035, so don't hold your breath!

In the mean time, there's plenty of other robotic exploration going on. The Mars Rovers Spirit and Opportunity continue rolling on, nearly five years into a three-month mission (is Gilligan working at Mission Control these days?). But lately the spotlight has shifted to their stationary sibling: the Phoenix polar lander. While Spirit and Opportunity found ample forensic evidence of water in Mars' ancient past, Phoenix has for the first time touched where at least some of that water is right now: locked up in permafrost a few centimeters below the Martian polar surface. Phoenix's robot arm has already delivered samples of this Martian dirty ice to its chemical analysis suite, the most advanced chemistry lab we've ever sent to another world. This research is still ongoing and the final results remain yet unknown. (As I write this, NASA's just held a press conference saying essentially "be patient, science takes time!") Still, it seems fair to say that life on Mars remains only a remote possibility.

But extremophiles on Earth suggest that life can adapt to cold, dry semi-Mars-like conditions, too. So-called psychrophiles thrive in extraordinarily cold conditions, in permafrost, sea ice, and the midst of snow fields. One Earthly analog for Mars can be found in the far north of Canada, on Devon Island. There in the midst of an ancient impact crater, the Mars Institute, SETI Institute, and NASA jointly run the Haughton-Mars Project Research Station where sciencists simultaneously study life in some of the world's harshest conditions while also practicing manned exploration techniques that may someday be used by future astronauts. Their ongoing studies have already suggested some promising avenues for the hunt for Martian life, for instance by showing that sulfate materials found in the impact crater can preserve the evidence of microbes for very long periods. Studies on Devon Island have also shown that 95% of a random sample of polar rocks were colonized by photosynthetic bacteria hiding under the rocks for protection from damaging ultraviolet light. This suggests that the harmful UV flux on Mars is unlikely to pose a prohibitive barrier to life there, at least life of the small and well-hidden sort.

The other famed terrestrial analog for Mars is the high Chilean desert, the Atacama, which is not quite as cold as polar islands like Devon but is much dryer and has far thinner air. While half the point of the Devon Island research station is perfecting techniques for future human exploration, the Atacama is where robots go to practice. NASA has for some time used that desert as a testing ground for its electronic explorers. The latest, developed jointly by NASA and Carnegie Mellon (home to the robotics geniuses behind this past year's champion self-driving car), is a robot called Zoe. If Spirit and Opportunity are robotic geologists, then Zoe is an autonomous robotic astrobiologist bearing special sensors to detect microorganisms and chlorophyll. Zoe is in the midst of a three-year study of the Atacama, carrying out for the first time a full transect (crossing) of the desert to measure how the properties of desert life change from edge to center to edge. Zoe is far smarter and more self-directed than her elder sibling rovers; she's capable of driving literally kilometers across the desert entirely without human intervention and automatically designing and executing her own science observations along the way. Techniques developed this way in the Atacama, and the knowledge of extremophile bacteria gathered through observations there, will play key roles in the ongoing planning of our next Mars missions.

Beyond the Extreme

"But hold on a sec," I hear readers cry in protest, "All those projects assume that life on other worlds will be pretty similar to life on Earth. What if instead it's something entirely strange and different instead? Could it be something entirely unrecognizable?" That's a good question, and one that many scientists have pondered. Here we must leave behind the our ability to point directly at Earthly examples, and proceed by reason alone. What is life, anyway? "Life" is one of those words that's so basic it resists being pinned down by any firm definition, though many have tried. Some scientists say that life is any system with a metabolism capable of taking advantage of gradients in energy or entropy. Others argue that the real essence of life is in fact its ability to reproduce and undergo natural selection. Evolution has become part of the very definition of life! Well, one definition, anyway.

It is certainly clear that there are other possible chemistries which could give rise to life. Life on Earth is based on the rich chemistry of carbon, which along with handfuls of hydrogen, oxygen, and nitrogen and traces of other elements gives rise to DNA, amino acids, lipids and esters and ketones and all their brethren that so bedevil college chemistry students. DNA is famously made up of four bases which come in matched pairs: A and T, C and G. But scientists have proven that other letters are possible in this alphabet, by creating two completely new artificial bases which are nonetheless accurately copied by the same enzymes that replicate regular DNA. Artificial amino acids, too, have be hacked into heavily rewritten bacteria, along with complete metabolic pathways for synthezising them and modified ribosomes for making proteins from the new amino acids. Stop for a moment and think about all this: the transcription cycle of DNA to RNA to proteins is the core mechanism of all Earthly metabolism, and now it's something we can reshape for our own purposes. We can rewrite not just the words spelled out in DNA, but change the very alphabet itself. Wow. That chemical wizardry unequivocally demonstrates that alien life could arise using an entirely different chemical language than our own. It might even use some molecule entirely other than DNA.

Well, so how weird could the chemistry get? What about ditching carbon and oxygen entirely for something else? It's a common trope of televised science fiction for aliens to all happily breath the same kind of air. (And sometimes, the rare ones who claim to need their own special air turn out to be faking it.) Should we instead expect in real life for each species to need its own kind of encounter suit? Spared from the budgetary realities of Hollywood, written SF more prominently features alien life based on silicon instead of carbon, or ammonia instead of water, and other such possibilities. Should we take any of these seriously?

The two key requirements for biochemistry are a structural backbone capable of building complex molecules, and a solvent for lubricating all the flows of biochemistry. Carbon is a superb backbone-builder due to its ability to bond with up to four other atoms at once and the ease with which those bonds can be broken and re-forged as needed to build new compounds. The element with the closest chemistry to carbon is silicon, one row lower in the periodic table. Silicon is theoretically capable of forming compounds every bit as complicated as carbon can. Yet silicon isn't anything like a perfect replacement atom. Compared with carbon bonds, the ties between silicon atoms are about half as strong. Worse yet, while carbon has an approximately equal affinity for oxygen, nitrogen, or other carbon atoms, silicon bonds especially strongly to oxygen, forming inert quartz, sands, or glasses. When mixed with water, chains of silicon atoms simply fall apart, as both silicon-oxygen and silicon-hydrogen bonds are stronger than silicon-silicon! The most complex reasonably stable silicon molecules are the silicones, formed of interlocking chains of silicon and oxygen—but in fact silicones are so boringly inert that we make non-stick cookware and heat-resistant spatulas out of them. Not exactly the stuff of dreams for biochemistry! Under conditions even remotely Earthlike, the chemistry of carbon is simply much more capable than silicon. I doubt Darwin ever dreamed of silicon aliens, but I can tell you what he would have predicted if he had: anywhere silicon life arises, if any carbon life also arises there, then creatures of carbon will inevitably outcompete and drive out all silicon life. Survival of the fittest applies to entire biochemistries, too. Perhaps the only plausible way for silicon-based biochemistry to not lose out to carbon would be if conditions utterly prohibited carbon bonds—for instance, at temperatures of many hundreds of degrees.

Even then, silicon biochemistry would face a more fundamental problem: atomic abundances. The four most common elements in the entire universe are hydrogen, helium, carbon, and oxygen—and helium is nonreactive under most conditions. So the elements out of which we are built are in fact the most common elements we possible could be built of! For every silicon atom in the universe, there are twenty oxygen atoms. Hence even if carbon plays no role, it's overwhelmingly likely that all the silicon will get locked up in rocks and sands, not life. Carbon atoms happily jump back and forth between CO2 and other molecules, but once silicon bonds to form silica (SiO2), that's pretty much the end of the line. This expectation is borne out by telescopic observations of the distant universe: while carbon compounds up to and including amino acids have been observed by the hundreds in comets, distant molecular clouds, and the atmospheres of some stars, we see no silicones, and only tiny amounts of silane—and no more complex silicon molecules at all. As on Earth, the vast majority of silicon we see in space is locked up tight in silicates, which we see in copious amounts.

So carbon will keep its central role, but what about replacing water? Ammonia and methyl alcohol have both been suggested as replacement solvents. Such possibilities were once extolled by Isaac Asimov himself (recall he was a research biochemist before he was a writer!) Recently, the Cassini spacecraft has now definitively shown that Saturn's moon Titan has huge lakes of ethane. Could they support life? Once again, the abundance argument suggests that alien life won't be so different from us: water is the third most common molecule in the universe, after molecular hydrogen and carbon monoxide. Water also has many special properties, such as its high heat capacity and heat of vaporization, which help regulate temperatures across the biosphere, its extraordinarily high surface tension, which helps bind together droplets and may have played a key role in the origin of cells, and its famous and surprising property of expanding upon freezing so that ice floats, which keeps our lakes and seas from freezing solid. Even more essential is that water is an outstanding solvent and is capable of dissolving a far wider range of compounds than any of the competing liquids. This is particularly a problem for ethane and methane, which are nonpolar solvents and thus would have trouble dissolving DNA and proteins and many other carbon compounts. Yet some researchers think those issues are not insurmountable, and that ethane- or methane- based life may indeed exist on Titan, evolving in the slow-motion chemistry of the ultra cold.

A good rule of thumb for explorers is that the universe is usually far more creative than we imagine. It certainly would do us well to keep in mind all these diverse possibilities as we hunt for life around the universe. Yet the evidence suggests that life on Earth is based on carbon compounds and water not by accident, but because those are both the most common and most capable molecules available. It seems fairly likely that, at temperatures close to those of Earth, the carbon/water combo will overwhelmingly be the choice of discerning life forms everywhere (though tastes may differ widely in such details as which base pairs to use for DNA, or indeed whether to use DNA at all.) Only at high temperatures (several hundred degrees C) does silicon life become the least bit vaguely plausible, and even that is being generous. Only at cold temperatures (-50 or -100 degrees C below) do ammonia or methane become options. There are still other options I've not discussed here, chlorine or sulfur or hydrogen fluoride based life, but those seem even less compelling. And one can speculate about beings of pure energy, or sentient molecular clouds in deep space, or nanoaliens living on the surface of neutron stars, but such notions are so strange that it's often hard to imagine in detail how they could possibly work in real life.

Perhaps in the end, silicon will have the last laugh after all. Our growing knowledge and control of biochemistry is coming fast on the heels of our ability to shape silicon into countless micro-miniaturized electronics. As these two trends merge, we're already starting to become part machines ourselves. Whether or not it's a good idea, we seem poised to soon take control of our own evolution as a species, replacing natural selection with DNA retroviruses, brain patch hacking, and all that cyberpunk jazz. Will our descendants ultimately choose to become superhuman cyborgs or perhaps uploaded minds joined in a world-wide virtual civilization? Will the biosphere itself give way to the technosphere? And has this already happened to any alien civilization(s) out there somewhere? Such post-biological life would play by entirely different rules: free of all biological needs and drives, potentially immortal, and able to clone and merge minds at will. A civilization of such beings would likely have very different motivations than any conceivable biological civilization, argues Milan Cirkovic of the Belgrade Observatory. Would they be robot berzerkers or benevolent superpowerful Minds? It's hard to say, and these ideas still remain controversial, to say the least. Darwin's great guiding principle of evolution has time and again achieved success at explaining life on Earth, and gives us confidence in many predictions about alien lifeforms. But paradoxically evolution's greatest product, intelligent minds, may open the way to a post-evolutionary, and very alien, future of our own.

For more reading:

• Olivia Judson argues in favor of Darwin but against the term 'Darwinism': http://judson.blogs.nytimes.com/2008/07/15/lets-get-rid-of-darwinism/

• An article on Extremophiles and astrobiology from Ad Astra: http://www.astrobiology.com/adastra/extremophiles.html

• Milan Cirkovic's provocative articles on astrobiology, advanced civilizations, and the future of the universe: http://www.aob.bg.ac.yu/~mcirkovic/