Interactions between terrestrial and alien biology

Can this space virus make my character sick? Can my captain sleep with the alien Princess?

This article is going to talk about the implications of mixing alien and earth biology.

The science here is going to start off fairly strict, but this is a subject in which a hard science framework is going to seriously limit what you can do, so I will progress to talking about ways in which you can justify softening things up a bit.


The important bit – chemistry and biochemistry

This is where the problems start. As you will probably know, biochemistry involves the subset of chemical reactions associated with living organisms. Whilst you can expect alien life to be made from the same basic elemental building blocks and react in somewhat predictable way to straightforward chemistry, biochemical reactions are often very very specific.

An acid for example, is always going to be an acid. If it is corrosive, this is usually the result of a very straightforward hydrogen exchange reaction, and you can reasonably justify any alien with even the remotest biological similarity to us being burned by it.

Many substances which have a major effect on earth biology do so because they are inherently reactive molecules. Hydrogen cyanide poisons humans because of its ability to interrupt an incredibly specific set of reactions that our cells use to produce energy, and although this is unlikely to happen with alien life, it would be plausible for it to have some other effect instead. It is a simplistic molecule, which is relevant to the organic (carbon based) chemistry that alien life would probably be based upon.

Why Carbon?

It’s versatile and relatively common.

Carbon is one of the most common elements in the universe and forms strong, stable, bonds with a wide variety of other elements. This allows it to form the extremely wide range of different molecules critical to life’s diversity and complexity.

Silicon has similar, if more limited properties, and there are a handful of other chemical systems that it is speculated might produce life within specific niche environments, but the chances that alien life would share a common carbon heritage is very high.

Silicon is actually more abundant than carbon within a terrestrial environment, but it’s probably less available for the formation of life, as it is mostly tied up in none water-soluble silicate materials such as sand, whilst analogous carbon molecules dissolve readily. However, carbon’s real advantage comes from its incredible ability to form compounds, and it is this versatility that renders the compatibility of different biological systems problematic.

It’s complicated

Biology is built on complex reactions built on complex reactions. The proteins that your biochemistry is based around are exceedingly complicated molecules that are reliant on, and which evolved in association with, other complicated systems.

Whilst it is certainly probable that alien life will involve similar levels of complexity, it becomes completely implausible that they will evolve identical complexity, which is what would often be required for such systems to interact.

There are certainly grey areas here. There may be some complicated systems that are necessarily similar just because there are only so many ways for them to arise within nature. It could be that the metabolic reactions that an alien uses to create energy within their cells could be disconcertingly similar to our own, for example. But this can only go so far.


So what does this mean?

The more straightforward the chemistry, the more likely systems will interact. Highly complicated molecules such as DNA and other proteins will simply be incapable of making complex interactions with alien biology. Some particular sticking points-


Alien viruses are straight out. I’m going to talk a lot more about viruses and bacteria and DNA elsewhere in this blog, but the problem is as follows.

Viruses have arisen as an artifact of the way DNA works; they are not so much living organisms as much as rogue instruction manuals for our cellular machinery.

While it’s possible that something similar could arise in alien environments, they would not be viruses as we define them, and they could not harm us in the same way as terrestrial viruses. It is utterly implausible for alien life to interact with terrestrial viruses or vice versa.

To make an analogy, it would be like trying to get a specific book published by posting random strings of gibberish text to someone at the building containing the printing press.

Other bugs

Other micro-organisms are more complex. They aren’t going to be able to interact with us in all of the ways that a terrestrial bacteria might, by releasing complex toxins, or forming symbiotic interactions with our biochemistry. However, they could be similar enough to us that they could try to eat us, and some of the chemicals that they might release in the attempt could be simplistic enough to damage our tissues.

Likewise, our immune system could plausibly provide some degree of protection to such attack. Antibodies released by our cells would have problems with alien biochemistry, but immune cells that envelop and digest foreign molecules through crude oxidative reactions might have more success although it’s possible that they would not be able to fully digest the alien molecules, which could lead to more complications.

Poisons and toxins

As I already mentioned, complicated biological toxins are not going to influence alien biology. But, if anything, it may even be more likely for alien life to contain simpler chemical hazards.

Take the aforementioned Hydrogen Cyanide. The extreme sensitivity of terrestrial life to this substance severely limits the extent to which it is incorporated into earth biology (excepting those parts of it that are attempting to produce it to kill other creatures), an alien life form could use it far more ubiquitously in their own biological processes and therefore be inherently toxic to terrestrial life.

DNA and mutations

Yeah, sorry, you’ve probably figured out by now, but mutation by alien DNA is definitely off the table, as is pretty much anything else cinematic involved with reproduction.

So the question a lot of people probably want to ask, at this point is…


How do I get away with ignoring this?

Honestly, this is one of those subjects where ignoring the facts is sometimes going to be your best option, but there are a few possible get out clauses.

Chariots of the Gods

First option is to try and explain why alien and terrestrial life is so implausibly similar. Random chance isn’t a great option here, because of the magnitude of unlikeliness we are talking about, but the idea of life in different parts of space sharing common progenitors has a long science fiction tradition (even if a lot of it was been published as non-fiction). This can come across as pretty weak if introduced as a random plot point, but it’s obviously much easier to work into other types of story.

We’ve seen your type here before

Biology evolves; an alien ecosystem that has been exposed to terrestrial biology at some time in the past could eventually evolve to interact with it at a more complex level. The more complicated the interaction, the longer this is likely to take.

Alien life from a planet that was contaminated with terrestrial bacteria from a crashed space probe could plausibly evolve to produce terrestrial specific toxins within a relatively short period of time. Incorporating DNA into alien reproduction however would be a much bigger deal (I can think of a few ways it could happen, but it’s a massive shift in complexity, plausibility, and likely time frame).

An alien overlord did it

These are obviously issues that can be overcome by intelligent beings. There is nothing stopping Alien invaders deliberately creating viral weapons that can infect us, or creating organisms that are compatible with our biochemistry and ecosystem.


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  1. » The writers guide to poison, poisoning, and antidotes part I — April 16, 2012 @ 4:22 pm

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