Concept: For some twisted reason, a xenocidal space-race living somewhere in our vicinity decides that nothing deserves to exist.

So they make a species. Any feasible size, any kingdom.

  • It eats everything that contains organic tissue
  • It can survive any environment that we know life exists in on earth.
  • It itself is 100% guaranteed organic (not a virus)
  • It follows the rules of known physics and organics
  • The makers don't have any unobtanium
  • They made the trip from Europa safe and sound and hungry
  • These creatures won't leave the planet
  • We can't eliminate them. Sorry.

How do you make this happen?

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    $\begingroup$ something with the same kind of anatomy as a water bear maybe? but faster and carnivorous? they are extremely durable. $\endgroup$
    – XenoDwarf
    Commented Mar 10, 2016 at 4:53
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    $\begingroup$ If it is 100% organic, does it not eat itself? It could be a short lived organism. $\endgroup$
    – Cort Ammon
    Commented Mar 10, 2016 at 5:21
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    $\begingroup$ Detail: These aliens are genocidal but not terrorists. Terrorist use fear of violence to influence the political process in the target population. These people don't. $\endgroup$
    – o.m.
    Commented Mar 10, 2016 at 6:07
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    $\begingroup$ Humans seem to be doing a pretty good job going everywhere on Earth and killing everything. $\endgroup$
    – Schwern
    Commented Mar 16, 2016 at 6:19
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    $\begingroup$ Nitpick: viruses are organic, they just aren't alive. They're composed completely of biological molecules (nucleic acids, proteins, sometimes lipids and carbohydrates), the same as cells, they just lack a metabolism and can't reproduce without infesting a cell. $\endgroup$ Commented Jul 8, 2016 at 5:36

5 Answers 5


I agree with the previous answer that a possible Tardigrade type micro animal combined with bacteria could be very effective at surviving and spreading throughout the ecosystem.

A useful addition to the life destroying beast would be altered Chirality: this is how chemicals can have alternate right or left handed arrangements. For all Earth life most amino acids are Left handed and sugars are Right handed. If we assume this is a universal trait of life (we won't know until we find actual xeno biology to study) then having the destroyer employing altered Chirality would have several advantages. The bacteria portion of our life-form could devour current life it encounters and process it into the alternate type of amino acids and sugars. These alternate handed bio components would be inedible to all native life, but would be a feast to the invading organism allowing them to spread.

A common problem on Earth is when a organism is introduced to a new ecosystem with no existing predators, with altered chirality the new organism could have no predators. Anything trying to eat them would starve as their bodies would provide no nutrients.


Your answer might lie in an angrier/carnivorous version of the Tardigrade

This microscopic creature is arguably the toughest organism to live on Earth.

Some extremes it can handle:

It can...

  • Survive in ocean trenches where pressure is vast
  • Survive ionizing radiation doses 100s of times greater than a human
  • Survive without food for 30 years
  • Survive in a vacuum ie in Space, with no oxygen and no air pressure, for 10 days
  • Survive temperatures as low as a fraction of a degree above absolute zero

That seems to tick off the "Can't get rid of it" thing on your list.

Maybe a genetic hybrid between this and a flesh eating bacteria (This one is evil.) could be your best bet.

EDIT: To try and cover all bases:

  • It eats everything that contains organic tissue

A kind of genetic hybrid (some degree of creative freedom needed because I don't think it exists/anyone wants to make it, but as my old Biochemistry lecturer said, the right equipment and enough time are all that stands between possible and impossible) of a Tardigrade and Necrotising Fasciitis would destroy everything that contains living tissue, not eat. Direct quote from the wiki:

"Flesh-eating bacteria" is a misnomer, as in truth, the bacteria do not "eat" the tissue. They destroy the tissue that makes up the skin and muscle by releasing toxins (virulence factors), which include streptococcal pyogenic exotoxins.

While this might not cover the whole 'eating' idea you have, it will lead to the destruction of all living tissue.

  • It can survive any environment that we know life exists in on earth Covered this bit in original answer.

  • It itself is 100% guaranteed organic (not a virus) Its an organic life, just look at his wittle face.

  • It follows the rules of known physics and organics This one I'm not so sure of, because of the aforementioned 'creative freedom' needed to get the thing to exist, but I'm pretty certain that only 2 things would differ from the normal Tardigrate and that would be

  • It would be flesh eating for a start

  • Possibly up reproduction rates? I'm not a biologist, so this is just from reading articles, but I'm really certain it would match these rules you need.

  • The makers don't have any unobtanium No unobtainium needed as genetic manipulation of organisms goes on a lot. This is probably just a higher level (maybe not even at that) of that area of science. Another wiki quote incoming from the Reproduction section:

Research by the University of North Carolina on the genome of one species, Hypsibius dujardini, revealed that approximately one-sixth (17.5%) of the species’ genome is foreign DNA. These 6,000 genes are of primarily bacterial origin, as well as DNA from fungi, plants, and Archaea.

So it does seem to be amenable to sharing its cells house (nucleus) with visitors (other things DNA)

  • They made the trip from Europa safe and sound and hungry This is where it might get a little hurt. It can survive in a vacuum for 10 days, so if its on the outside of the ship, it probably wont make it if your travel speeds are conventional(>10 days from Europa), BUT following THIS XKCD What-If article a large enough initial population would allow for some to reach earth and start another colony there.

  • These creatures won't leave the planet They are tiny organisms. The only way they'll leave the planet is if you take them with you.

  • We can't eliminate them. Sorry. This is a bit difficult to imagine. We can eliminate EVERY organism with the right conditions, whether that's time or equipment. The problem shouldn't be eliminating them, but eliminating them without killing everything else

Hope this helps!

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    $\begingroup$ I like this answer for the mental image it's given me of a very aggravated tardigrade yelling at people on the street. $\endgroup$ Commented Mar 16, 2016 at 13:48
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    $\begingroup$ "Kids these days don't know what it's like. In my day we had <insert old thing> not <insert new thing> and WE WERE GRATEFUL!" $\endgroup$ Commented Mar 16, 2016 at 15:53

A large creature (sized like a human, tiger, or rat) would be unable to hunt down smaller lifeforms (protozoans, plankton). A smaller lifeform might be able to kill larger ones. So it has to be a small lifeform.

This lifeform would have to survive in extreme environments.

Perhaps the two could be combined. A lifeform which non-terraforms Earth similar to the oxygenation event.

  • $\begingroup$ Yes something like the nano-machine insects from the day the earth stood still movie, but organic instead. en.wikipedia.org/wiki/The_Day_the_Earth_Stood_Still_(2008_film) $\endgroup$
    – Javert
    Commented Mar 10, 2016 at 8:50
  • $\begingroup$ i like the idea of another oxygen catastrophe like event being caused by the biophages. but if that were so they wouldn't be 'biophages' by name, as biophage means 'to devour life' and there is no devouring in an environment change, just invasion. $\endgroup$
    – XenoDwarf
    Commented Mar 10, 2016 at 10:56
  • $\begingroup$ This isn't really an answer. $\endgroup$
    – user14789
    Commented Sep 21, 2016 at 14:50
  • $\begingroup$ @CalebWoodman, it seems that seven voters disagreed. $\endgroup$
    – o.m.
    Commented Sep 21, 2016 at 15:25
  • $\begingroup$ @o.m. True, but the scope of the question was at least in theory very clear about desiring a specific answer, more than just "A small animal that breaks the ecosystem." $\endgroup$
    – user14789
    Commented Sep 21, 2016 at 15:28

What's the best way to kill all life forms you ask? Destroying their natural habitat !

On Earth there is balance between the herbivore and carnivore life forms. This is observed in what we call an "Ecosystem". An ecosystem is a community of living organisms called producers, consumers, and decomposers. Consomers cannot live without producers, decomposers cannot do their job without consomers and finally, producers need decomposers to seal the deal. (It's basically what said Antoine Lavoisier, "Nothing is lost, nothing is created, everything is transformed.")

By destoying one of the group, your creature would disrupt the ecosystem. An exemple of this is happening right now with the Red king crab. To quote Wikipedia on the crab :

In the Barents Sea, it is an invasive species and its population is increasing tremendously. This is causing great concern to local environmentalists and local fishermen as the crab eats everything it comes across and is spreading very rapidly.

This species was introduce by men in an ecosystem where it didn't belong. In the new ecosystem, the Red king crab has no predators. It is why the crab has not been stopped.

Conclusion : Introducing a fast reproducing, omnivorous and resilient species would cause a lot of havoc on earth.

  • $\begingroup$ Surely this would be a great new food source for some organisms, even as it out-competed others. Damaging to the ecosystem, certainly, but it doesn't sound like an existential threat. $\endgroup$
    – user243
    Commented Mar 18, 2016 at 19:28
  • $\begingroup$ Humans destroyed a lot of ecosystems by causing the direct extinction of over 303 animals from 1760 to today... and many more animals died as consequence of a chain effect....and we still survived. $\endgroup$
    – user22398
    Commented Jul 7, 2016 at 22:04
  • $\begingroup$ If you can make them naturally produce waste that's harmful to the environment and/or completely non-biodegradable to terrestrial life, that'd help, too: imagine if it exhaled methane and sulphur dioxide, or defecated blobs of high-density polyethylene? $\endgroup$
    – Aesin
    Commented Oct 3, 2016 at 21:21

Your aliens invented humans. Who knows, we may be the result of their genetic experimentation.

It eats everything that contains organic tissue

People in Mexico eat crickets and drink cactus water.
People in Japan eat roaches and algae.
People in China eat spiders, scorpions, centipedes and dogs.
People in the Netherlands eat horses.
People in Spain eat squid ink.
People in Russia have even eaten a defrosted mammoth.

All those are typical foods, sometimes elevated to fine cuisine. The world's most expensive food is sturgeon roe for crying out loud.

It can survive any environment that we know life exists in on earth.

With our current technology we've been to the Marianas trench. With not so current tech we have McGyvered into the skies.

It itself is 100% guaranteed organic (not a virus)

Frame challenge. No life as we know comes even close to that.

We are mostly made of water which is inorganic, but dry a human into dust and you will find we are about 24% organic.

That said, if you meant 100% as not being a cyborg or a construct, we fit the bill.

It follows the rules of known physics and organics

Yeah we keep trying to break them, and so far it has been them that break us.

The makers don't have any unobtanium

It's cool, we're not made of that. Nor from that.

They made the trip from Europa safe and sound and hungry


These creatures won't leave the planet

Here and there you will find a human who would love if you could fly them to the moon and let them play among the stars. The majority is very emotionally attached to the specific piece of land they've been born on, though.

We can't eliminate them. Sorry.

Humans are a hard nut to crack. We have nukes, Jesus and Chuck Norris on our side.


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