What is life? At the most basic level, we’re not entirely sure. We’ve got a lot of good theories that accurately explain most of life’s inner workings, but there are quite a few loose ends. We don’t, for example, truly know how life began, nor do we know if it exists on other worlds. (Not that we don’t have a few candidates elsewhere in our solar system: Mars, Europa, Enceladus, Titan…)
All life on Earth works in about the same way, however, and we’re pretty sure we know how. It’s all based on the same fundamental building blocks, the same chemical and biological processes—processes that are not the only way to do things. When you get right down to it, we have more in common with the smallest bacteria than we probably would with any extraterrestrial, unless you invoke some sort of higher principle. As of yet, that’s uncalled for, considering we have a total biosphere sample of 1, but who knows?
The way it is
Earthly life is based on carbon. There’s no question about that. In fact, carbon is so important to life that it’s a requirement for a chemical compound to be called organic. (Think about that the next time you see “organic salt” for sale. Table salt is mostly sodium chloride, NaCl. No carbon there.)
Carbon is a great element for life, as countless astrobiologists and writers of science fiction have discovered over the years. It’s very stable, and it’s good for forming long “chain” molecules, or polymers. It readily bonds with lots of other molecules, and the compounds made from that are…interesting, to say the least. Things like carbon dioxide, sodium bicarbonate (baking soda), hydrogen cyanide, and, of course, ethanol.
But life is more than carbon. We also need a solvent, and nothing works better than plain old water. It’s perfect as a medium for the biochemical reactions necessary for life. Water has a relatively large liquid region (0°C to 100°C, or 32°F to 212°F), and it’s a fairly simple compound (H2O). It’s essentially neutral, so it doesn’t affect the reactions as much as other liquids might. Oh, and it covers about 70% of our planet’s surface, so there’s that.
Other elements find their way into Earth life besides carbon, hydrogen, and oxygen. Nitrogen is a big one; percentage-wise, there’s more of it in the air than there is water on the surface, so life would be silly not to do something with it. Phosphorus, despite its volatile nature, is relatively stable once it’s bonded to a bunch of other atoms, and it’s the backbone of adenosine triphosphate (ATP), one of the most important complex molecules at the cellular level. Calcium is needed for our bones. Iron is what makes our red blood cells, well, red. Sulfur, potassium, sodium, magnesium, and a dozen or so other elements are all vital for us and most other life as we know it.
On Earth, as we know, life is made from DNA. We all know the double helix that is its form, and some might recall the scene in Jurassic Park explaining DNA and the genes made from it. From another point of view, DNA is like a computer program: a series of “instructions” that make a blueprint for life. But from a biochemical standpoint, it’s nothing more than four moderately complex molecules organized on a pair of polymer chains. These four nucleic acids bond in pairs across the chains; strictly speaking, only one chain is necessary, as in the similar (but single-stranded) RNA.
One of DNA’s functions is to encode which higher molecules are needed where. These large polymers are proteins, and they’re made up of smaller parts called amino acids. In the genetic code (on our planet, at least), three nucleic acids in the DNA chain represent an “instruction” for an amino acid. We use about twenty of those. So do dogs. And trees, mushrooms, and any other “complex” life you can think of. Bacteria use a slightly different set, but even their genetic code is built around the same twenty.
To a chemist, that’s life: a bunch of molecules acting and reacting, bonding with each other, splitting apart. What makes it unique is the fact that it can do this in such a way that it perpetuates itself. Life, once it gains a foothold, will reproduce as long as it can. That’s why the “life on Mars” debate is so polarizing: if life did exist on Mars for any serious length of time, then it would have spread all over, and some evidence should be relatively easy to find even after a billion years.
True, there’s a difference between any old kind of life and the sentient, sapient species we expect when we think of aliens. But life like that didn’t spring fully-formed. By all that we know, it had to come from somewhere, and it likely came from the same place that every other living thing on its planet did. (One hypothesis, however, argues for a shadow biosphere, a whole set of lifeforms unrelated to anything we know, yet still living all around us.)
The ways it could be
The four basic elements of organic life—carbon, oxygen, nitrogen, and hydrogen—are everywhere. They’re common here, and they would be in just about any hospitable world you could imagine. Life would almost be expected to use them. Carbon, because of its polymerization qualities, is the best backbone of the four. Hydrogen and oxygen make good compounds, notably water, but also—when bonded to carbon chains—sugars and carbohydrates. Those may sound like dirty words to a health nut, but we can’t live without them (in moderation). Nitrogen isn’t of much use on its own, but throw it into molecular compounds, and it’s suddenly great.
For “life as we know it”, those four are the big ones. It’s the trace elements that will be in different concentrations. Phosphorus and calcium are important for us because of their chemical properties; it’s not entirely unreasonable to imagine alien life using them for the same purposes. The rest, though, are fair game. It’s possible that alien organisms could find ways to use elements and compounds that are toxic for us, such as heavy metals (lead, mercury, etc.).
All life on Earth is based on DNA (or its precursor, RNA), but that’s not a given. Rather, the form isn’t a given. Complex life does need some way of replicating, something resilient, resistant to random mutations, yet easily formed from common materials. Chemists have created a chain of peptides that can hold nucleic acids. And those nucleic acids don’t have to be our familiar four. Xanthine, for instance, is chemically related to adenine and guanine, and it’s found throughout the body.
Even if you have DNA, even if it functions the same way as ours, that still doesn’t mean you’re Earthly life. A simple bit of multiplication shows that there are 64 possible ways to code amino acids. But we barely use a third of that space. Most amino acids have multiple, redundant encodings, probably for added security against mutations. Mix up some of those encodings or add different options for amino acids, and you’ve got a whole new way of life.
Another option to look into is chirality. Amino acids are peculiar molecules; they can appear in one of two forms that are identical in composition, but slightly altered in shape. Chemically speaking, they’re called isomers, and life on Earth overwhelming prefers a specific shape for them. But “opposite” amino acids could be the basis for life on a different planet. That life might be otherwise terrestrial, but utterly incompatible with our proteins, enzymes, and so on. (Mass Effect used this idea in a couple of places.)
As much as any kind of life can be considered common, most alien aficionados expect ours to be so. Carbon, water, and DNA/RNA work. We’re living proof of that. Anything that uses those will be “like us”, at least at the most basic level. But can we change even that?
The problem with designing an alternate biochemistry is that it’s entirely hypothetical, and it will remain so for the foreseeable future. That hasn’t stopped some from trying, and they’ve come up with a few ideas that are scientifically possible, if not necessarily plausible.
Silicon is the favorite of sci-fi, and it also has its proponents in “serious” scientific work. Looking at the periodic table and the very ground beneath your feet, you’d think it was ideal. It’s a heavier analog of carbon, able to form polymer chains by bonding with itself, with hydrogen and oxygen and a few other molecules attached to the sides. But its added mass renders it less common, less stable, and less…free. Silicon doesn’t have the same breadth of possibilities as carbon, and many of its more interesting compounds aren’t suitable as the basis for life. Still, it does have some potential, though it would take an entirely new subfield of chemistry to explore that potential.
Sulfur, as a core element, is another possibility. It can form long chains like carbon, and it’s a bit more common than silicon. But its downfall is again in the “organic” chemistry. Those long chains are all linear. They don’t branch, and branching carbon chains are responsible for the vast array of organic molecules we know today. That doesn’t mean sulfur-based life is impossible, but it doesn’t look like it could become complex. On the other hand, sulfur can be—and is—used instead of oxygen in some lifeforms. (Bacteria that do this on Earth are used to create hydrogen sulfide, H2S, the sulfuric analog of water.)
Metallic life could use any of the more common metals as a basis. We haven’t explored much of this sort of chemistry, but titanium and a few other metals are potential candidates, particularly in high-temperature, high-pressure environments. Apart from the practicalities of using a heavier, rarer element, the lack of knowledge on this subject is what keeps us from positing “metallo-organic” life.
Arsenic is mostly poisonous for Earthly life. The very reason why it’s toxic is the same reason why it could be a potential alternative for life: it’s in the same atomic class as phosphorus. It reacts with many of the same molecules as phosphorus, “competing” with it. That’s what makes it harmful to us, but other life could use it in something like DNA. That was actually the working hypothesis for a strain of bacteria found a few years ago in California. (That claim has since been discredited.)
For solvents, authors both serious and fictional have devised a host of possibilities beyond “just add water”. Ammonia could work, but only at much lower temperatures (or higher pressures) than on Earth. And ammonia is flammable, so oxygen-breathing organisms would find it problematic, to say the least.
Another option, one that has a few scientists awfully excited, is methane. It’s a very simple hydrocarbon (CH4), it’s fairly abundant in the universe, and—under the right conditions—it’s pound-for-pound as effective as water. Methane also has the bonus of being related to bigger hydrocarbons like ethane, some of which could also be used by life. Imagining a “sea” of liquid hydrocarbons isn’t even that hard. We’ve got three of them in our solar system: Titan’s Kraken, Ligeia, and Punga. And there are indications from that moon of something that could be life-related.
The list of also-rans is long, and getting longer all the time. In addition to ammonia and methane, people have imagined life using as solvents everything from hydrogen sulfide to peroxide to silicon dioxide, AKA glass. All of them have their drawbacks, usually coming from the range of temperatures where they are liquid. But don’t let that stop you from trying.
Finally, there are a few ideas that are even more “out there”. Clouds of Venus? Sure. Charged dust inside a plasma sitting in space? Yeah, that could work. Life on a neutron star? A bit harder, but we can work something out. None of those would resemble life as we know it, though. Indeed, we might not even recognize them as living, although they would fit every definition.
So…life is complex, you see. This was only a basic overview of one aspect of it, and it’s one of the longest posts I’ve written. And truth be told, most writers won’t need any of it. Unless you want to be wild and weird, you’ll probably stick with something Earth-like: carbon, water, DNA, etc. And that’s okay. You can still make great aliens while staying inside those borders. Such life is far easier to see as alive, too, although it will almost certainly resemble nothing on our planet. (And forget about eating it. The proteins and enzymes and such will probably be completely different.)
In the next part, we’ll switch from biochemistry to actual biology, as we look at evolution and how it makes possible the variety of life in a biosphere. Aliens, like us, will evolve. They may not have genes like we do, they may not reproduce in the same way, but they’ll evolve. Next month, we’ll see how and why.