What if Civilisations Act Like Parasites? Implications for SETI


The parasitic alien civilisation in Independence Day
The parasitic alien civilisation in Independence Day

We have no way of knowing how an alien civilisation will act.  This is one of the biggest stumbling blocks of the Search for Extraterrestrial Intelligence.  After all, it’s very easy to explain away the lack of contact with alien life (Fermi’s Paradox) by simply saying “well, they don’t want to talk to us”, or “they’re not allowed to because of the Prime Directive“.  These sorts of arguments are the weakest solutions to Fermi’s Paradox, because they rely on knowledge we don’t have.

So how do we solve this conundrum? Sadly, we can’t – at least until we make first contact, that is.  So in the meantime, we are forced to play let’s pretend, and speculate on how alien civilisations will behave.  But we can still be sensible, and rein in our wilder ideas.  Ideally, our educated guesses should have a basis in something biological – not too Earth-centric, but in processes that we think must occur regardless of where life arises.

So Jonathan Starling and I turned to the concept of symbiosis.  Symbiosis describes the relationship between two different species.  These relationships can be broadly categorised as

  1. Parasitical – one species (the parasite) uses the other (the host) to its own advantage, having a negative effect on the host.
  2. Mutualist – both species benefit from the interaction.
  3. Commensalist – both species interact, and one benefits, but the host is not affected positively or negatively

We decided to model the interaction between a civilisation and its host planet in the same way, using my computer models of civilisation growth in the Milky Way, to see how this would affect the number of communicating species in the Galaxy.  Our simulated civilisations can either feast on their planets, destroying it as a deadly virus kills its host, or they can work with their environment, in a slower but more rewarding growth.

But what happens when the virus can jump from one host to the next? If a deadly virus has plenty of new hosts within close distance, it can kill its host and jump to the next without killing itself.  But if there are not enough new hosts, then the virus will die.

We added interplanetary and interstellar colonisation into the model to see how different species behaviours are transmitted into the Milky Way at large.  Does it pay to be a parasite, sucking your planet dry and moving on? Or do better behaved, less aggressive civilisations win the day?

The number of hosts/planets available to a civilisation will depend on how “near” they seem, which depends on how quickly the civilisation can travel.  A fast ship will be able to cross larger distances and “infect” planets more easily.  We ran the model several times, increasing the maximum colonisation speed from Voyager’s velocity, 10x Voyager’s velocity to 100x Voyager’s velocity.

These top speeds we are setting for these colonising species aren’t particularly high: Voyager is currently travelling at about a hundred thousandth of the speed of light.  A clever probe might be able to use gravitational slingshots to boost up to nearer a hundredth of the speed of light.

We found that if the civilisations can only colonise slowly, then it doesn’t pay to be a parasite.  As you can see from the graph, mutualists (blue) tend to do better than the parasites (red).

The number of civilisations in the Galaxy with time.  The red line shows the parasites, the blue the mutualists, and the black line is the total.
The number of civilisations in the Galaxy with time, in our model. The time parameter is scaled so that 1=the present, and 0=the Big Bang.  The red line shows the parasites, the blue the mutualists, and the black line is the total.

As we increase the velocity, we increase the number of available hosts, making it less costly to be a deadly parasite, until at 100x Voyager’s velocity, parasites dominate the Galaxy, colonising and destroying planets that would have hosted benevolent civilisations, before the good guys could even pick up tools.  This is analogous to invasions of species on Earth – sometimes, a species will enter a local ecosystem from outside and simply out-eat and out-breed its competitors.  In the UK, the grey squirrel’s dominance of the native red squirrel is a classic example.

Colonising at high speed allows the parasites to leave the mutualists behind and conquer the Galaxy.
Colonising at high speed allows the parasites to colonise mutualist planets before they develop civilisations, and conquer the Galaxy!

So what does this mean for the real world? Sadly, not as much as we would like.  This work is a speculative exercise in civilisation behaviour, a somewhat contrived numerical gedankenexperiment in a sandbox.   It doesn’t solve the problem of our ignorance of other civilisations’ behaviour.  It describes a (most likely fictional) Milky Way where civilisations develop one behaviour type very early in their existence, and don’t learn from their experiences.  The survival of the human race has depended (and will depend) on our ability to understand and move on from our mistakes!

But, the message of this experiment is important.  Once a parasitical civilisation passes the technological barriers to interstellar colonisation – at a speed that is relatively modest – then it will make its presence felt very quickly.  In this scenario, we must solve Fermi’s Paradox with one of the following possibilities:

i) Interstellar travel is impossible or very difficult;

ii) Parasitical civilisations have a very short lifetime – they either destroy their host and themselves, or they change their behaviour and stop being parasites

iii) Parasitical civilisations are not that common in the first place

Ultimately, Jonathan and I wanted to use this experiment to reflect on humanity’s relationship with the Earth.  Are we parasites, commensalists or mutualists? I’d love to know your thoughts in the comments below.

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5 thoughts on “What if Civilisations Act Like Parasites? Implications for SETI

  1. I think solving Fermi’s paradox is harder than even that, Duncan! Not only must parasitical civs have a short lifetime, but they -all- must have a short lifetime. And/or not only must they not be that common, they must be -exceedingly- rare.

    Your conclusions are a bit like my favourite objections to intelligent life based on the absence of some simple contact via von Neumann machines (which behave rather like your parasitic civ, moving from planet to planet in a self propagating way). It’s not hard to make one of these (we’re getting close without any direct goal to do so). And all you need is -one- intelligent species to make one, and -everyone- else will know about it. Finding an excuse for this absence is hard – you can come up with specific explanations, but you need a -general- rule about all advanced civilisations that stops this from happening. Hard, if life or intelligence is not exceedingly rare.

  2. Agreed! Uniformity of motive and action across species is crucial to maintaining the Paradox. The last few papers I’ve written have shown that we are technologically close to polluting the Milky Way with probes at very little energy cost (if we choose to do so). All you need is one thoughtless civ…

  3. I’d like to compare it to Chris Boyce’s analysis in his guest chapter for my “Man and the Stars” (1974). He assumed the galaxy’s civilisations were divided into paranoid, suspicious (like us) and trusting. The trusting can always form alliances, the suspicious do it 50% of the time, the paranoid never. The paranoids can destroy any single civilisation they meet, but can’t beat an alliance. When you model it on a galactic scale, after three recursions 60% of the trusting survive, 40% of the suspicious, and none of the paranoids. And on a galactic scale, it always turns out much the same even if you alter the proportions to load the dice.
    Best wishes, Duncan L.

  4. As I see it from my non-scientific perspective I think the major flaw in all of these analyses, no matter how well constructed analytically or rationally, is that we ascribe to whoever or whatever is “out there” our human behavior characteristics. We are a warlike people; therefore we assume all other beings are equally warlike. We explore for what we consider riches; we assume that all other beings are equally avaricious.We are omnivorous; we assume all other beings are seeking food. We believe (well, some of us do) that there is a “higher power some choose to call God”; so we assume any being that comes from “out there” is a manifestation of that god, the list goes on.
    Isn’t this a manifestation of our supreme egotism that said for thousands of years that the Earth was the center of the universe and that humankind is the epitome of evolution? Since we may be the most violent, dangerous, and rapacious beings in the known universe, should we also assume that we must defend ourselves from other beings that act just like us?
    And why should we assume that out of the billions of possible habitats out there we are the only ones who matter? Well, yes, we matter to us, but to anyone else? As one of the top writers, (Asimov, Clarke, or someone like that) said, “Which do I find more frightening–that we are not alone–or that we are?”

  5. Interesting thought-experiment, but perhaps the wider physics context would limit any expansion strategy? Everything must conform to thermodynamics. Life works because it ultimately dissipates more energy than it locks up. And in every system there are kick-backs that prevent one process from subsuming everything else. The Borg assimilation process, say, would surely have been overcome by chaos on a larger scale that would have caused changes to occur in the ‘species’? Are we, ultimately, all mathematics?!

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