I'm looking for some ideas for ways to kill off my brand of precursors. Ideally, the cataclysm is predictable well in advance and direct survival is impossible or at least highly improbable.

The precursors are decently spread out (somewhere up to 1000 colonized star systems), and you can assume they're generic carbon-based humanoids. The cataclysm can take as long as it needs - I'm thinking up to half a trillion or so years. It should be something that gives them reason to believe that seeded planets have a reasonable chance of surviving the event, but they aren't 100% sure so they also research bailing to "another dimension" for a while.

All of the precursors still around when it happens should be wiped out definitively. Wiping out some or all of their tech as well is fine, but it should leave planets and stars mostly unscathed.

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    $\begingroup$ How about genetic erosion (something akin to y-chromosome degredation )? That would leave everything intact, take a long time, and render the entire population extinct over a few thousand generations. $\endgroup$
    – Josiah
    Commented Feb 6, 2015 at 23:36
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    $\begingroup$ Universe is about 14 billions years old. In 500 billion years, universe itself might not exist if Big Rip theory is correct and dark energy will keep expanding Universe at accelerated rate (as it seems so). $\endgroup$ Commented Feb 7, 2015 at 0:52
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    $\begingroup$ In about 150BY observable universe will be limited to local supercluster. See Heat death of the universe and Future of an expanding universe for interesting timelines. I mean, in 500BY there might be deeper problems in our Galaxy than some plague to fight. $\endgroup$ Commented Feb 7, 2015 at 0:56
  • $\begingroup$ Bring on the Reapers. $\endgroup$ Commented Feb 7, 2015 at 14:45
  • $\begingroup$ Do you need 500 billion years? 5 million allows ample time for soul searching and technological progress, while making natural phenomena much more likely to be fatal (in 500 billion years, you would probably have survived anything the universe can throw at you - you've survived star death, supernovas, galaxy burnout...) $\endgroup$
    – Josiah
    Commented Feb 7, 2015 at 15:58

11 Answers 11


Yes there are cataclysms that may wipe out life in a large portion of a galaxy. I assume the 1000 colonized star systems are within

There's about 1 star for every 280 cubic light years. So there should be about... 1875 stars within 50 light years 15000 stars within 100 light years 1875000 stars within 500 light years

If we assume that only F,G type stars are colonized (similar to the sun's life span not cataclysmic and high metallicity) 100 solar systems similar to our own for every 1000 stars your colonization of star systems will extend to approximately 80 cubic light years.

For such an small radius of stars it is very plausible that a very powerful gamma ray burst occurring in the neighborhood and focused to that region will eliminate the ozone layer of the planets within this radius and cause large extinctions due to genetic mutations.

As an example:

GRB 080916C is a gamma-ray burst (GRB) that was recorded on September 16, 2008 in the Carina constellation and detected by NASA's Fermi Gamma-ray Space Telescope. It is the most powerful gamma-ray burst ever recorded. The explosion had the energy of approximately 5900 type Ia supernovae, and the gas jets emitting the initial gamma rays moved at a minimum velocity of approximately 299,792,158 m/s (0.999999c), making this blast the most extreme recorded to date.

The energy comparison with a supernova ignores that most of the energy of a supernova is carried away in the neutrino burst. The total isotropic energy of GRB 080916C is estimated at 8.8 × 1047 joules (8.8 × 1054 erg) (the oft quoted 4.9 times the sun’s mass turned to energy) and should be jet-corrected to a much lower actual energy output due to the narrow angular width of the actual bursting jet. Thus it would be significantly less than the energy of a supernova neutrino burst, but is about equal to the energy in a supernova’s material explosion. Also, the peak energy flux of GRB 080916C is significantly less than a number of other GRB’s, such as GRB 080319B which peaked at nearly 1044 watts (1051 erg/s) in visible light alone. However, the total energy flux of the very long duration GRB 080916C is higher than any other measured GRB to date.

"If the event that caused this blew out in every direction instead of being a focused beam, it would be equivalent to 4.9 times the mass of the Sun being converted to gamma rays in a matter of minutes.

Amongst the different kinds of GRBs, long ones are most dangerous. There is a very good chance (but no certainty) that at least one lethal GRB took place during the past 5 gigayears close enough to Earth as to significantly damage life. There is a 50% chance that such a lethal GRB took place during the last 500×106  years, causing one of the major mass extinction events. Assuming that a similar level of radiation would be lethal to life on other exoplanets hosting life. We find that the probability of a lethal GRB is much larger in the inner Milky Way (95% within a radius of 4 kpc from the galactic center), making it inhospitable to life. Only at the outskirts of the Milky Way, at more than 10 kpc from the galactic center, does this probability drop below 50%. When considering the Universe as a whole, the safest environments for life (similar to the one on Earth) are the lowest density regions in the outskirts of large galaxies, and life can exist in only ≈10% of galaxies

  • $\begingroup$ These are some really nice numbers. I was thinking about this as a possibility, but I didn't think it was plausible on this scale. $\endgroup$ Commented Feb 7, 2015 at 18:03
  • $\begingroup$ Added some more info (last paragraph) which you may find interesting as it relates to our own milky way and the plausability to such an event ocurring $\endgroup$
    – Barnaby
    Commented Feb 7, 2015 at 21:19
  • $\begingroup$ Gamma rays. Plus one. $\endgroup$
    – Mazura
    Commented Sep 11, 2016 at 2:48
  • $\begingroup$ If a civilization has managed to colonize 1000 planets they would have ships capable of radiation shielding to protect them. $\endgroup$
    – Cbm.cbm
    Commented Feb 19, 2018 at 9:39
  • $\begingroup$ @Cbm.cbm putting radiation shielding around a whole atmosphere would be tricky and expensive. Also, ships would only shield themselves enough for your average interstellar radiation. $\endgroup$
    – Muuski
    Commented Feb 19, 2018 at 13:17

Interstellar Grey Goo.

Nanobots capable of traveling across the interstellar void that begin self-replication upon coming in contact with biological matter or artificial materials, would be a fairly complete way to wipe out all life in a galaxy.

There are a lot of reasons such a thing might happen, radical-ultra-hippies wanting to return the galaxy to "like, a pre-life state maaannn...", deconstruction/cleanup bots gone haywire, or as an attack from another galaxy.

Your race (the precursors) might be able to create seeds that will be invisible to the nanobots, but it's difficult to be sure if it will work. No one can get close enough to the bot swarms without being infected and melted into grey goo.

If you want them contained to the galaxy then give them a life cycle. While the nanobots are active, replicating, and seeking targets they continue spreading all directions in space. If they have no detections for a few thousand years they enter a sleep state. If the sleep state endures for a few million years (or thereabouts) they self-destruct. This means the galaxy will eventually become clear and they won't likely remain intact for an intergalactic journey.

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    $\begingroup$ Clever way of making a seed that can survive. I hadn't considered making it invisible. And a gray goo invasion would certainly be impetus to move to another dimension. $\endgroup$
    – Josiah
    Commented Feb 7, 2015 at 1:28
  • $\begingroup$ Maybe after extinguishing the Precursors, the goo nanobots run into something akin to the Y2k bug, and all break at the same time. $\endgroup$ Commented Feb 8, 2015 at 8:35
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    $\begingroup$ @EmilioMBumachar As is often the case on this stack, there is a relevant XKCD. $\endgroup$
    – KSmarts
    Commented Feb 9, 2015 at 17:31
  • $\begingroup$ Clever but I have to disagree that this would remove everyone - surely it can be seen from afar, and, if this species has FTL technology, outrun $\endgroup$
    – Zxyrra
    Commented Dec 10, 2016 at 16:32
  • $\begingroup$ @Zxyrra This question is science based. There is no such thing as FTL and robots will always be able to endure a higher acceleration than humans or any other humanoid living thing. $\endgroup$
    – Samuel
    Commented Dec 11, 2016 at 18:44

Because you're beings are spread out over such a large area, external causes are really not going to be feasible on that scale, unless you somehow localize them (all within a galaxy, with no method of long-distance travel etc). The likely means of wreaking havoc on galactic or even cluster scale would probably destroy everything on seeded planets as well.

So you're looking for an inexorable internal killer. You have a couple of options:


Your population unleashes a disease that is passed from mother to child, infects the population before it is identified, and is incurable. Maybe it's a side effect of a longevity drug that is taken by the entire race in hopes of attaining longer lives. It grows in potency with each generation, so that it is maybe 100 generations before the disease is identified. By that time everyone is infected, and they can determine the exact date of extinction.

You're going to need a slow moving disease (unlike bubonic plague) so that they have time to contemplate really complex cures. But the slower it is, the more likely it is that a work around can be developed. Half a trillion years is on a stellar time scale - if something is impossible, throw enough time at it, and it can become a matter of probability.

Genetic Factors

It's also possible that extinction is implied in the very DNA of the species. This is not really that far fetched - for example, Y-chromosome degradation could feasibly have destroyed humanity if it had continued. Imagine a mitochondrial disorder that can't be checked, or something makes more and more anencephalic (warning:graphic) members.

In either genetic or disease causes, you're going to be looking for something that affects a system that can't be easily replaced by machines, like:

  • reproductive system
  • nervous system
  • mitochondria (fuel for the body's systems)

The circulatory and even muscular systems could potentially be replaced given the technology. But there is some question whether consciousness can be replaced or emulated, and without the ability to reproduce it's just a matter of time.

In any case, it will be overwhelming depressing. But what mass extinction isn't?

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    $\begingroup$ Y-chromosome degradation could not have destroyed the human species. Even if the Y-chromosome were to disappear, the SRY gene, the sex-determining gene, would have just moved to another chromosome or been replaced. It simply doesn't make sense from a natural selection perspective. Harmful mutations that result in death or infertility cannot be passed on because they cause death and infertility. The entire concept of a 'doomed' genome fails for the same reason. The only way genetic mutations could destroy a species is if every member of that species became mutated independently. $\endgroup$ Commented Aug 7, 2015 at 20:31
  • $\begingroup$ Good comment @MikeNichols - thanks for the feedback! $\endgroup$
    – Josiah
    Commented Aug 7, 2015 at 21:50

So they can't avoid it, but have a reasonable expectation that species similar to themselves in the same area will survive? I think we can safely exclude natural causes. Including disease natural or artificial. Either the disease is aggressive and adaptive enough to be certain death to other species or an interstellar civilization would find it trivial to avoid given warning.

The extinction should thus be caused by a sentient agent deliberately targeting the precursors. The agent needs also be impractical to avoid or defeat. I think there are few options.

Magic aliens

Magic here can be taken to mean technology so advanced it is indistinguishable from magic. I think H.P. Lovecraft would be a good starting point. The precursors would have been unable to even comprehend what the monsters attacking them are and how they move from planet to planet. The only real downside is that unless the reason for extinction is left unknowable, there will be an effect on the tone of the setting.

Voluntary extinction

The precursor might have noticed that they are occupying the ecological niche that the species the were seeding would need to "grow up". The logical solutions would be either to stop wasting resources on seeding new sentients or to remove yourselves from the equation. There would be some uncertainty whether the seeded species would be able to survive without support, which I understood was desirable. The process would also take whatever time was required to gain optimal balance of survival and interference for the local new sentients. So the time table would be highly varied in different locations.

Typically in such scenarios, the precursors would simply leave. This can be done either in space as a vast exodus to a remote part of the galaxy or even a neighbouring galaxy. Alternately it can be done in time, by placing the precursors in stasis in remote locations. Asteroid sized bodies in interstellar space would be unlikely to be stumbled upon accidentally. And if you have stasis technology, you might be able to hide inside stars or gas giants. Both of these options have been done in fiction.

Precursors could also devolve themselves either culturally or biologically, more likely in both ways, in effect becoming one of the new species, although oddly present all over the local area. Precursors could devolve to different species on different planets to avoid that oddness. One option is to devolve to be compatible with the seeded species and be assimilated into them biologically. Maybe the new species are all actually precursors starting again. Evolving to some more advanced exotic form is also a common variation.

It is also possible for the species to decide to really kill themselves. Or at least majority of the population while others take one of the other options. This might happen as a result of a "civil war" with survivors deciding not to rebuild. Precursors might also have a society where the "lower castes" making up vast majority of the population would be considered expendable to begin with. Or the precursors might simply not be bothered by dying, either because of religious belief in afterlife or because they no longer value living.

  • $\begingroup$ Voluntary extinction is a great point. I had considered some kind of nuclear auto-annihilation or maybe a terrorist attack by a radical group that believed self destruction was a good thing, but the self-examination and the slow progression seemed incongruous with the idea. But voluntary extinction sounds plausible. $\endgroup$
    – Josiah
    Commented Feb 7, 2015 at 15:50
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    $\begingroup$ A common concept in science fiction is "ascension", where a race convert themselves into energy or travel to another plane of reality, or similar. $\endgroup$
    – Tim B
    Commented Feb 8, 2015 at 18:20
  • $\begingroup$ @TimB "Evolving to some more advanced exotic form is also a common variation." $\endgroup$ Commented Feb 8, 2015 at 18:54
  • $\begingroup$ I don't think voluntary extinction is likely for the whole population, I mean, some people will want to remain how they are. But a majority could definitely choose that option. $\endgroup$ Commented Feb 1, 2016 at 3:20

How about something that doesn't wipe them out?

Imagine a society that for some reason eschews the genetic manipulation they would be capable of--it's so anathema that the technology is not ever developed.

Along comes a disease--something with an airborne vector. The victims don't get noticeably sick so it doesn't draw anyone's attention. The disease attacks the genes for the reproductive system, though--while the victims are unharmed their children will be born without a functional reproductive system.

Everyone lives out a normal lifespan, it's just the next generation is the last.

The delayed nature of the harm means it's not going to be noticed for many years and thus has time to infect the whole population before it's discovered.

To save the species you need to cure and isolate women and implant them with embryos made with recombinant DNA to have the correct genes instead of the flawed ones.

Lets see how this timeline would work on humans:

9 months to make a baby. Figure 15 more before any sign of this can be noted and even then it's going to be subtle--the teen birth rate crashed. I don't think they will immediately attribute this to infertility as few such births are intended in the first place. I figure at least another 5 before it becomes something of note to anyone but a sociologist. The clock has been ticking (the ovaries of the last cohort without the damage), it's only got about 30 years left.

Now you have to find the bug. This isn't going to be easy because there are plenty of harmless viruses around, everything is going to have to be checked to see which is causing the mutation. Since you have very little in the way of uninfected samples to work with this is not going to be easy.

Furthermore you have to find some way to purge the virus from your hosts once you have identified it--something we are nowhere near accomplishing.

I'm figuring they can read the DNA code (think human genome project) but the recombinant technology has to be developed from scratch. Our current progress on recombinant DNA isn't to the point of using it on humans--the clock runs out. Of course a crash priority program could do it faster--but science doesn't do the 9-women-and-a-month formula well at all.

Note that "success" produces a generation that can never meet the old population, even BSL-4 protocols would not be adequate.


For some really out of the box thinking, perhaps we should consider that they have developed the ability to create a "pocket universe". By carefully setting up the starting point and all the variables of the physics they want, the pocket universe will be much more attractive for them and their sort of life that a "wild" universe. We can also stipulate the energy required to initiate a pocket universe would be similar to sending a large scale expedition of relativistic starships to the next habitable star.

Given the ability to create a new, much more user friendly universe, I suspect the race of aliens would come to the conclusion that this is the preferred course of action and instantiate a building program, "inflate" a pocket universe and disappear down the wormhole, taking everyone and everything they wanted and leaving the rest behind. They may or may not take their planets, depending on factors like how large the wormhole throat can be made and if it would be easier to reach a "better" planet on the other end of the wormhole. The last crew of aliens carefully turn off all the lights and vanish down the throat of the wormhole, closing it behind them.

A billion years later, Human expeditions reach uninhabited star systems with evidence of massive gravitational perturbations sometime in the distant past, perhaps missing planets and possibly ancient ruins preserved on the airless moons of various planets, but no real indication of where they have disappeared to.

  • plague. (engineered or otherwise)
  • computer virus. (disabling life support and other vital systems)
  • A collective of AI's that persistently create computer viruses, sometimes distributing them under the guise of anti-virus software.
  • self-replicating nano-bots.
  • computer viruses that hijack 3d printers, replicators, or other industrial machines to output nan-bots
  • A desperate war that annulled both forces, leaving the few survivors stranded in unsustainable environments, without enough industry to escape.
  • A drastic drop in reproduction due to porn addiction (leading to complete economic disaster.)
  • computer viruses that psychologically profile people and infect their pc with the most effective kind of porn possible
  • religious or idealogical fanaticism (endorsing mass murder suicide)
  • religious or idealogical fanatics that persistently create AI's that create computer viruses, that psycologically profile people and then persuade them to their idealogical point of view.
  • A computer virus that hijacks lab equipment to output biological viruses.
  • Nanobots that assemble biological viruses
  • some other combination of these things.

Since the time frame is so large and the number of systems colonized is so vast, there are many ways that this species could become extinct.

I imagine a scenario where on one of the planets an indigenous population is came across that is compatible for reproduction. The offspring produced would be essentially different from either of their parents. A new race essentially. Or the original indigenous population was able to adapt to the new tech that came along with the new population and over time reconquer their planet and move out into the rest of the universe from there.

The new folks would be a more aggressive and capable people, and over some time would come to dominate the particular planet they were on. They would develop a Nazi like culture where they aggressively practiced genocide and conquest. Eventually taking this to the other systems from which there ancestors came. Over a long course of time they would conquer all.

Since there are a thousand planets the precursors on each planet may have divergent courses of evolution and over time such a dark society could evolve into some kind of thing that was in essence not anything that resembled the original precursors, and then come to dominate with a practice of conquest and genocide.

And lastly a species from another part of the cosmos that was bent on conquest and genocide could take over these worlds over the course of time.

Your original precursors while intelligent could have something in their culture that made them highly pacifists. So they see the aggressive species coming, but the ethos are such that they will not rise up to defeat the invaders. And they have never developed complex weapon systems so even if they do rise, it is a futile gesture. They just hope that the invaders will stop at some point, but that never happens until they are all destroyed.

The scenario does not have to be one of large fleets of starships invading planets, it could be a more peaceful scenario where over the course of time on each planet the new species just slowly dominates the precursors slowly overwhelming them until at one point it is decided the precursors are a drag on society and they need to be done away with completely. How they get there in the first place would be through trade and immigration or other fairly peaceful means. South Africa would be a good example, were the colonist dominated and essentially destroyed The cultures of the people whom occupied the land before they came. Take that another step to the dark side where apartheid never ends and slavery and genocide develop further, to a point were the whole country is only white. (Say Germany won the war so in western society genocide became more acceptable).


Darn, the ones I was going to suggest have already been suggested. Lett's try to think of some more.

Using up all of a resource. Perhaps they rely on some very complex chemical compound that they cannot reproduce (or for cultural reasons, are not willing to begin research into). The planet where the Spice is formed has a sun that'll go nova. They can't/wont transplant it.

They could all rely for their longevity on a single invention or device, controlled by a single person, company or organisation or religion, which loses the ability or desire to maintain those longevity devices.

Like Josiah's "genetic factors", they have a degenerative disease or population imbalance, which again they cannot or will not solve: perhaps they only noticed too late that their birth rate has become insufficient to restore their death rate.

Like the grey goo suggestion, and the voluntary suicide idea, they could hit the Singularity: their computers and robots could ultimately make their way of life completely pointless.

They could be working on a problem that would culturally destroy them: such as "is there a God", and finding definitive evidence that they were wrong would cause mass death.

Perhaps they discovered a method of immortality, so they just stopped reproducing as it was pointless: they didn't need to and it was wasteful of resources. Birthing in their hives, once done by queens, was no longer needed, and the queens, now being equal but having far higher needs for resources, were often rebelled against. And it was only some time after the last queen died, wen each planet asked around and discovered there were none to import, that they realized the immortality was ultimately their death warrant.

Or perhaps, as with vampires of lore, their immortality brought infertility. Economic considerations would mean that it's in a planet's best interests to grant immortality to all citizens, so's best to compete against other planets which have to spend the extra investment in nurturing the young and senescence of the elderly. And it was only once everyone was immortal that they realized they were all doomed.


what first popped into mind was a mix of the dust/dark matter (from golden compass et al) mixed with a virus or something malignant like that, basically a sentient disease. alt version of the disease is an extremist group doing a coordinated attack at key nexi (sp?)

an alternate option would be to go more "celastine prophesy" with it and have a whole culture tire of their mortal coil and mediate to another dimension or everyone uploads to the latest game and then the system crashes or somesuch convenient exit...


Genetic disease caused by inbreeding? Perhaps your precursors survived another disaster and their gene pool was so limited that they needed to resort to inbreeding to carry on the species.

Or maybe incest isn't taboo in this culture; perhaps it's even fashionable or encouraged by a growing religion or cultural zeitgeist that starts to dominate.

Sorry if this is too redundant; most everybody beat me to the punch of this one!


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