The most basic possible definition of life is this: life is a self-replicating pattern.
The simplest forms of life on Earth today are bacteria, who exist for the sole purpose of reproducing more of themselves. Over time, copying errors + natural selection cause versions of the pattern that are better and better at surviving to arise. The patterns that are best at surviving and reproducing themselves eventually drive the less effective patterns to extinction by outcompeting them for limited resources.
Scientists today believe that, although bacteria and archaebacteria (an even older form of living cell) are the simplest surviving forms of life today, they weren’t the first. Instead, it’s thought that life on Earth might have begun with a genetic material called RNA copying itself, without help from the complex machinery used by cells today.
RNA is made of molecules called nucleotides. These molecules can be created by natural processes, and can even be found in deep space. But when they’re suspended in a solution of water and given an energy source, such as sunlight or lightning, they start to form long chains. It turns out, the right combination of nucleotides strung together into a chain might be able to reproduce itself.
Scientists have shown in the laboratory that some RNA strands can actually make copies of themselves. They can interact with free-floating nucleotides produced by natural processes, and attract those nucleotides into making copies of themselves. These copies can then self-replicate, and so on.
The important chemical property that allows RNA and DNA to make copies of themselves – copies of instructions to build living things – is base pairing.
Remember that hydrogen bonding we talked about in the section about water? That’s what makes nucleotides special, too. Nucleotides can hydrogen bond with each other – but the molecules’ shape means that only one type of nucleotide will hydrogen bond to any other. Other nucleotides simply won’t fit.
RNA and DNA both have four base pairs . For RNA it’s U, A, G, and C (uracil, adenosine, guanine, and cytosine). DNA is an improvement upon RNA – it’s harder to make in the natural world, but it’s less prone to errors, so over time it won the battle to become our primary genetic material. Its nucleotides are T (thymine), A, G, and C. The “T.”
The important thing to remember here is that when nucleotides form a long chain, that chain will attract the nucleotides that will hydrogen bond with the nucleotides in the chain. Only A nucleotides will bond with the U nucleotide in the existing chain, only G nucleotides will bond with Cs, etc.. This allows one chain of nucleotides to copy themselves by determining the nucleotide sequence of the other chain.
This means that the language of the genome is similar to binary code. Instead of 1s and 0s, the nucleic acid code used by life on Earth to pass along hereditary instructions has As, T/Us, Cs, and Gs. Through the hydrogen bonding and base pairing of nucleotides, the information obtained in one strand of RNA and DNA can be copied, in theory, an infinite number of times. With a small copying error/mutation rate, of course.
In other words, this pattern can now reproduce itself. Any useful traits this pattern develops can be duplicated countless times, and the “fittest” versions of that pattern can continue to be duplicated and spread.
This may be how life began on Earth. Although some computer models show that protein could have begun the self-replicating process of life in just the same way, other scientists believe in an “RNA World” theory – that the very first genetic material from which we evolved were simple, self-replicating strands of RNA, which gradually learned to use other chemical ingredients to become even more efficient at surviving and replicating themselves.
Over the course of hundreds of millions of years of evolution, these RNA sequences learned to make many improvements to their original form. Through hundreds of millions of years of trial, error, and survival of the fittest, they learned to use, and eventually even build, other chemicals like the ones we’re about to discuss below.
This definition of life as “any self-replicating pattern” (which can evolve to be progressively more complex, even eventually evolving intelligence) opens the door for many hypothetical speculations about “life not as we know it.” In the loosest sense, anywhere you can have a self-replicating pattern arise, you can end up with intelligent life after a few billion years of evolution have passed.
What if you want to mix it up? What if you want your alien life’s genetic material to be recognizably and unquestionably different from that of Earth life on biochemical analysis, or you want the two forms of life to be incompatible? Then you might use something like…
Different or Extra Base Pairs
On Earth, we have two possible base pair combinations: A-T/U and C-G. But nucleic acids are a family of compounds that contain more than just those four structures. You might remember that in the X-Files, at one point alien DNA was identified based on the presence of additional base pairs – base pairs not used for genetic material by Earth life.
Some scientists have speculated that forms of life on other planets could use an additional set of base pairs, drawn from the family of nucleic acids that aren’t used by Earth life. They might use these in addition to A-T and C-G, or instead of one of those pairs. Scientists have even synthesized artificial DNA using eight base pairs, which they’ve called Hachimoji DNA. The unusual base pairs incorporated by Hachimoji DNA are pictured below, alongside regular Earth DNA.
This is a surefire way to identify life that is either alien in origin, or artificially created, since no natural life forms on Earth use more than those two base pairs.
Scientists are split on just how likely it is that life would evolve to use additional or different base pairs. Early laboratory research does suggest that the base pairs we use are the best chemicals for the job, meaning that these same base pairs might be favored by natural selection on other worlds.
This gives you the convenient option of being able to choose either life that uses the same base pairs as life on Earth, or different ones, depending on the needs of your plot.
One additional way in which you can make alien DNA recognizably alien, or incompatible with Earth DNA is chirality.
Chirality refers to the “handedness” of a molecule. Look at your hands. They’re exactly the same shape and they’re able to perform exactly the same functions, but they’re not identical. Why? They’re mirror-flipped. You can’t put a left-handed glove on a right-hand – in just the same way, you can’t hydrogen bond a left-handed base pair to a right-handed one for purposes of DNA or RNA replication.
All Earth life that we know of uses “right-handed” DNA and RNA. Scientists aren’t sure if that’s because right-handed DNA and RNA is more likely to survive damaging stellar radiation, or works better for replication. But because they aren’t sure, we have wiggle room to speculate. Could life elsewhere use left-handed DNA and RNA?
If so, scientists would have an interesting time. The alien DNA would show up as being made of the same stuff as Earth DNA or RNA on some chemical tests, but Earth life wouldn’t be able to use or digest the Earth DNA or RNA. This means that alien and Earth-origin viruses and other infectious organisms likely wouldn’t be able to infect each other, because they wouldn’t be able to hijack each other’s cellular machinery or break down each other’s component parts.
A detailed chemical analysis would show that this DNA or RNA couldn’t possibly have come from an earthly organism, unless it was artificially engineered.
The Genetic Code
It’s important to note that any culture which has genetic analysis technology will be able to tell the difference between alien life and Earth life if they try to sequence it.
This is because our genes all come from a common ancestor – so they have certain similarities that go way beyond function. Scientists have actually been able to develop more accurate understandings of how living cells on Earth are related to each other, called phylogenetic maps, since the advent of genetic engineering.
When we read an organism’s genetic code or sequence its genome, we’re essentially reading its source code. That means we’re seeing exactly how the organism is produced, right down to the different “choices” evolution made about how to create this end product, and the weird little programmer’s notes that might not have a direct function, but are preserved in future iterations of the program.
If we open up a genome, or a “file” from an alien organism, we’re likely to immediately realize that it’s not using any kind of code we’ve seen before. We may be unable to read its genetic code until we do a lot of research to learn how. Assuming this life isn’t related to Earth life through a common ancestor, its genetic language would be so different from our own as to mark it immediately as unrelated to us. Imagine being the scientist to read that first alien genome!
These differences would appear everywhere in the genome, from the “codons” used to translate genetic information into protein structure, to the actual amino acid sequences of proteins and the weird little structural artifacts left over by evolution.
This means that scientists with a gene sequencer would quickly learn that alien life was alien – though it would likely take them years of study to understand how exactly is biochemistry and genome work.