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So.. I've decided to kill all the humans on Earth. Just a routine fumigation prior to a terraforming job.

Although I don't mind a bit of collateral damage I do want to keep most of the plant life and the ecosystem that's necessary to support that.

I plan to get this done over 200 to 1000 years. My preferred method is slow-acting poison. Slow enough that nobody knows I'm doing this. I'm not going to fly space ships in and start shooting everything up like in the movies because I'm at a huge metabolic disadvantage and humans would, once they figured out I was malevolent, run rings around me and my kind.

What kind of poison's good for a job like that? It's eventually going to draw attention, no matter how it works, so it's probably best if it doesn't cause too much obvious drama while there's still any hope for treatment.

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    $\begingroup$ Do you need it to vanish from the environment on its own? Or are you willing to undertake planet-wide cleanup later? $\endgroup$ – SRM Mar 9 '17 at 7:01
  • $\begingroup$ @SRM, the attacking species has a physiology so drastically unlike humans that the poison almost certainly won't affect them. If we can arrange for it to be moderately benign for plant life and the supporting ecosystem we can just leave it in place. If it puts a ceiling on the viable size/lifespan of animal life then all the better, as it may help keep upstart species from evolving in the future. $\endgroup$ – sh1 Mar 9 '17 at 7:05
  • $\begingroup$ Does it have to be chemical poison or would a global scale pandemic only affecting humans also do the job for you? $\endgroup$ – Youstay Igo Mar 9 '17 at 8:33
  • $\begingroup$ What is your assumption on the level of technology? If you are thinking about present technology and what might be developed over the next 200 -1000 it's difficult to imagine how a chemical based cause of the population crash would not be identified by an organisation such as CSIRO or CDC. $\endgroup$ – pHred Mar 9 '17 at 8:52
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    $\begingroup$ When you say that it takes 200 to 1000 years, but nobody knows you are doing it, how do you visualise that actually happening? That's 8 to 40 generations. People tend to notice each other dying! They particularly notice their own children dying. And they REALLY notice their kids dying of something new/strange or lots of kids dying of something rare. Grandpa dies of a heart attack - sad, but might not be noticed. You baby dies of a heart attack - autopsy, inquest, public health scare. $\endgroup$ – DrBob Mar 10 '17 at 16:02

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You have various options to get rid of the civilized part of humanity.

Chemical poison

The roman empire was poisoned over centuries from using lead to build their water pipes and cups. They also used to boil wine into lead pots, since it gave their wine a sweet taste.

Lead was used extensively in Roman aqueducts from about 500 BCE to 300 CE Julius Caesar's engineer, Vitruvius, reported, "water is much more wholesome from earthenware pipes than from lead pipes. For it seems to be made injurious by lead, because white lead is produced by it, and this is said to be harmful to the human body." Gout, prevalent in affluent Rome, is thought to be the result of lead, or leaded eating and drinking vessels. Sugar of lead (lead(II) acetate) was used to sweeten wine, and the gout that resulted from this was known as "saturnine" gout. It is even hypothesized that lead poisoning may have contributed to the decline of the Roman Empire, a hypothesis thoroughly disputed.

Also Arsenicum is a slow acting poison, and there are reports of the first copper mining population being poisoned by the Arsenicum rich wastes of their mining activity.

"Pleasure" poison

Start a company which sell junk food, make it cheap and make it cool by using a lot of advertising and branding. Make it an habit and socially accepted. In parallel demote healthy activities and induce mankind to be more sedentary. Use your lobbist to reduce health services. Hearth diseases and obesity will make the job for you, while you earn nice moneys out of it.

For the few thousands living far from civilization, deeply hidden in forests all around the world, you have to decide. You can keep them as "no nuisance" since you got rid of 99.999999% of them and they always refused civilization. Or you can send hunters to target them.

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    $\begingroup$ While they are both indeed good choices for slow poisoning, they would both be immediately detected in water processing plants and then again, when heavy metal poisoning cases spike within a few years. $\endgroup$ – Youstay Igo Mar 9 '17 at 8:34
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    $\begingroup$ TIL That the GOP is probably an alien race trying to kill of the human race $\endgroup$ – Marshall Tigerus Mar 9 '17 at 18:09
  • $\begingroup$ The lead pipes is not a problem, since they are glazed over from carrying water after a while. The problem was specifically pots used to prepare an acidic condiment (grape sweetener) that was used widely in cooking. The chemistry prevented a protective later from forming and instead added lead to every batch. In modern equivilent: poison the ketchup. $\endgroup$ – JDługosz Mar 13 '17 at 6:28
  • $\begingroup$ A hidden-in-plain-sight solution. Since humans are already actively poisoning themselves with lead, even in the modern world, you merely help them along and foil their efforts to curb it when (if) they realise what they've done. Lead poisoning may reduce fertility, reduce intelligence, and increase violent crime and other behavioural problems. Even if you remove it from the environment temporarily effects may be heritable, as it's stored in bones and released during pregnancy to be passed on to the foetus. The effect on intelligence may eventually make people too stupid to notice. $\endgroup$ – sh1 Mar 26 '17 at 16:45
  • $\begingroup$ And unlike anthropogenic contamination, it's not self-limiting when society collapses, because an external actor will keep on supplying it. $\endgroup$ – sh1 Mar 26 '17 at 16:47
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Feed the people memes that encourage them to have below-replacement fertility rates. Preferably get the militarily strongest groups to believe the memes enough that they "convince" all groups to accept the memes.

A total fertility rate of about 1.6 children per woman results in a population decline of about one percent per year. After 1,000 years, that is enough to reduce the population from 7,000,000,000 to 300,000.

A total fertility rate of about 1.4 children per woman results in a population decline of about 1.5 percent per year. After 1,000 years, that is enough to reduce the population from 7,000,000,000 to 2,000.

If the memes are based on ideas like "overcrowding" or "living space is too expensive", they might need to be replaced when the population gets low enough that there is obviously lots of room. For example, a population of 20,000,000 is less than one person per square mile, even if nobody lives in deserts or mountains or glaciers. With population decline rates of 1 - 1.5 percent per year, that population would be reached after about 400 - 600 years.

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    $\begingroup$ Treating ideas as poison is quite possibly an underappreciated concept. If you can embed your memes in RNA, I suspect you could even push their acceptance beyond rational limits. $\endgroup$ – papidave Mar 11 '17 at 21:19
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L.Dutch has made an important point about the people living in far flung, uncivilized corners of the world. Unless you use a global airborne poison, these folks will not be affected. But a global airborne poison would be immediately detected and your agents will be exposed.

So, we have either of two options:

  • use a genetically engineered pathogen which only targets humans

  • use chemical poisons to first poison the mainline population of humans and then target those in far flung communities

Genetically Engineered Pathogen

First you collect all the viruses and bacteria which only target humans (smallpox, typhoid, whooping cough etc). Now you enhance their capabilities and make them more deadly. For example, you could create a flu virus which makes infected cells produce and secrete ricin or abrin or shigatoxin into the bloodstream. This would make flu a killer. For the worst hit, you could work on the HIV virus and enhance its functioning so that it doesn't have to be injected into the bloodstream to function and would start destroying the immune system when inhaled. The possibilities are endless.

Once you create your super pathogen, you release it in the atmosphere through aerosols. This would target all the human population and would make it extinct within a few generations.

Chemical Poisoning

Here again, you have endless possibilities. You could go the painful route, by spreading carcinogens into the environment and create a gigantic spike in cancer rates. This would quickly overwhelm all national health budgets and situation would soon be out of control. You would easily kill 80% of the population within a few decades or so.

Or if you want to go the classical poisoner's way, you could use a slow poison such as copper sulfate, sodium selenate, corrosive sublimate etc, grind them very finely and spread them in the atmosphere through aerosols.

The downside of this method is that these mineral toxins affect all complex life alike, so all large animals in the ecosystem would be getting affected alike. You would have a lot of dead animals to dispose off, at the end of the day.

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A virus that damages the ovaries of baby girls so they don't produce eggs. By the time the girls hit menarche and anyone notices the problem, the virus is long gone, damage done. You have essentially sterilized the population without giving CDC and WHO any clues about the source. Make it something that easily lingers in the populace, like a light version of common cold.

(I heard about this plot somewhere but I do not know where, so I cannot cite sources. If anyone knows origin of this idea, please edit this answer to add attribution.)

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    $\begingroup$ All girls in the world wouldn't reach menarche simultaneously. Let's say all girls conceived this year are affected; the ones born last year are not. By the time the ones born this year reach puberty, the ones born last year will be in prime child bearing age--and unaffected by the virus. One it appears there is a worldwide phenomenon of damaged ovaries, pregnant mothers WILL be tested and the virus isolated. It might cause a short term dip in population, but won't be significant in the long term. ...unless there are accusations of genocide and biological weapons testing leading to WWIII.... $\endgroup$ – nzaman Mar 9 '17 at 11:24
  • $\begingroup$ @SRM The Cynndor Technocracy? $\endgroup$ – Seraph Myrmidon Mar 9 '17 at 13:35
  • $\begingroup$ @nzaman You're assuming the virus only lasts one year. I think SRM wants his virus to last as long as babies are being born. Once no more babies are being born, then the virus peters out. $\endgroup$ – Visfarix Mar 9 '17 at 17:32
  • $\begingroup$ @Visfarix: I am assuming nothing of the sort $\endgroup$ – nzaman Mar 9 '17 at 18:13
  • $\begingroup$ @nzaman oops i misread your comment, but I think you're still assuming that girls who are older than 1 year are suddenly ineligible to being infected by SRM's virus. I think you're also assuming that this virus is easily treatable. What happens if it's resistant against most or all forms of antibiotics? $\endgroup$ – Visfarix Mar 9 '17 at 18:33
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The general theme I see with slow poisons like this is that: a) people don't realize they're consuming it; b) it is commonly used in various products, especially in products that hold food or drink

Do we have something like this currently in our society? Yes, plastic

More specifically, plastics made with BPA. They generally aren't biodegradable and instead breakdown in conditions of high heat or UV light exposure and even then it's just smaller pieces of the same plastic still polluting the environment. It isn't toxic in small amounts (and I'm not even sure how toxic/deadly it is in large amounts), but if you can create an environment that has a overwhelming build up of the component, to the point that it's impossible to avoid consuming it in significant amounts, this might work within your timeline of 200-1000 years.

You can even say your aliens are already doing this due to how commonly used they are and how often waste plastic is dumped in landfills and oceans. Yes, humans are very slowly moving away from using plastics as an easily disposable resource, but for your aliens, if they can slow or stop that process and keep humans using BPA-based plastics until pollution reaches a critical amount, this might create a toxic enough environment to kill off most complex life on earth. However this might leave a problem of cleanup for your aliens. Or use something similar that your aliens are immune to and is more deadly to humans.

Another route could be a biological attack, but that would probably occur on a much shorter timeline.

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    $\begingroup$ Problem is that BPA really isn't that toxic or harmful unless you are dumping it right into the water supply. The conclusion of more or less everyone was that it was safe, but the market dumped it for food products because enough people were convinced that it wasn't safe. $\endgroup$ – John Smith Mar 19 '17 at 17:56
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Introduce right handed microbes in the Earth's environment that will produce the mirror version of a chemical compound that is critical for some biological function and such that this mirror form will be lethal. The microbes will be released in some remote location where it will slowly multiply, releasing small quantities of the chemical that will not be noticed before it is too late. The microbe has very slow metabolism compared to most microbes we are used to.

It will divide only once every 5 years and you start with just 1000 microbes. It will then take 100 years for this to grow to a billion microbes, which only produce a tiny amount of the toxin and that at the remote location, so that won't be noticed. But after 400 years the microbe will have colonized the entire planet and vast quantities are produced.

When people start to die, chemical analysis will not yield immediate answers, as the chemical compound looks very much like its normal mirror version. Its spectrum is the same, MRI scans won't show a difference, and it won't show up as a different chemical quantity in a mass spectrometer. The microbe itself won't be easily detected either, because it doesn't grow on a petri dish. DNA analysis won't work either as its DNA has the wrong handedness.

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The problem with poison is that it can't be inherited so you would need to kill 3 generations in the span of 60 years to succeed. Otherwise they will notice.
Assuming that one generation is 20 years you want to kill 10 generations. Which I assume you want to do to not get attention from mass dying. So you go for he low hanging fruit which is autoimmune diseases. Lupus. You can't treat him and when he have vide range of organs it can attack.
Then when the developed countries are occupied with this you send good ol' black death, flu, HIV and ebola.
You could go with rabies but that would also take a toll among animals. And after let say 200 years you go after remote communities with smallpox, malaria.

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On purely technological means, you can't. First you have to degrade them, distribute pseudoscientific beliefs between them, undermine their education and promote a low-educated culture.

After that, you can help them to accept some longterm poisonous thing as ordinary one.

Best if you decrease the fertility rate of the females.

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  • $\begingroup$ That sounds like you borrowed the idea from real life. Do you suppose agencies that might have a duty to monitor the environment for such poisons should also be undermined? $\endgroup$ – sh1 Mar 13 '17 at 17:59
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Develop a new retrovirus that makes the humans barren.

Like in the book Inferno from Dawn Brown From this humans can reproduce in slower grades and slowly dies out.

Or Plan 2: Develop Viral vectors to induce Huntington’s disease.

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  • $\begingroup$ I’ve edited to use actual spoiler markup. But you have a problem: how can someone know whether the book is one they don’t want spoiled, without revealing the name of the book (which implies by context that it has something to do with driving humans extinct)? $\endgroup$ – JDługosz Mar 13 '17 at 6:22
  • $\begingroup$ Thank you with the spoiler markup, BUT virus not equal with viral vector. Viral vectors are (in short) viruses what insert modificated genes in the host cells. $\endgroup$ – Feralheart May 1 '17 at 20:00
  • $\begingroup$ A viral vector is a means of contagion spreading: e.g. airborn, contaminated water, etc. You're creating a new virus not a new way of distributing the particles. If you mean something different, you should link to it and consider using a less ambiguous synonym. $\endgroup$ – JDługosz May 2 '17 at 0:17
  • $\begingroup$ The expression “in slower grades” doesn’t make sense. I think you mean “will reproduce at a slower rate”. Note also will instead of can. “can” implies that it is allowed by optional, synonym for “may” in this context. $\endgroup$ – JDługosz May 2 '17 at 0:25
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Encourage burning fossil fuels. If you can access the communications networks, add conspiracy theories and junk science denying climate change.

The resultant natural disasters will kill off the majority of humans, and the breakdown in law and order will kill off most of the rest. You will have to search for and kill the survivors manually, but by then they won't have the infrastructure to be much of a threat.

The best part: not only is it undetectable (since you're not actually doing anything), but nature will reabsorb the $CO_2$ into biomass, given a few hundred years.

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    $\begingroup$ If mankind's society did not break down even in the horrors of the World Wars, what makes you think that a bit of climate change will do that? $\endgroup$ – MichaelK Mar 9 '17 at 9:52
  • $\begingroup$ @MichaelK: The World Wars, for all the hype, lasted for a combined decade; climate change effects will last for centuries. Think of the difference between a payday loan and a mortgage: one is painful to pay off, but you CAN pay it off in a month; the other is painful to pay off in a month, but then you have the next payment to worry about. $\endgroup$ – nzaman Mar 9 '17 at 10:00
  • $\begingroup$ I find you analogy to be nonsensical. The more time passes, the longer time we have to adapt and make new comforts for ourselves. If we were able to walk out of Africa with no civilization at all, and get to where we are today, then it is just silly to think that because of climate change, we will suddenly exterminate ourselves. No, that kind of reasoning completely illogical. Mankind may suffer hardships because of climate change but to assume it means our annihilation is just plain silly and unrealistic. $\endgroup$ – MichaelK Mar 9 '17 at 10:25
  • $\begingroup$ @MichaelK: And how many times did we nearly wipe ourselves out along the way, losing much of our knowledge and skills. Climate change won't kill off ALL humans, just most of them. What it WILL do, is destroy the infrastructure that could be used to beat off alien invaders. $\endgroup$ – nzaman Mar 9 '17 at 10:33
  • $\begingroup$ I wholeheartedly disagree with you. Mankind has survived through much worse than that. Assuming we are going Max Mad because of climate change is... well... nice fiction material by all means, but not at all that which OP is looking for. $\endgroup$ – MichaelK Mar 9 '17 at 10:35
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Induce an increasingly fast-acting prion disease. Per Johns Hopkins, prion disorders currently hit around age forty to sixty. Ideally, you'll be able to cause it to mutate to take a decade or two off this each generation of infected.

This will probably work best if you reveal yourselves to a select few elites who can manage society and sway public opinion during the last generations. Eventually there wouldn't be enough healthy people to take care of the increasing numbers of frail and senile. Your elites can step forward to "save the children"...whether for food, slavery, or mass execution.

Along with humans, you'll probably kill any mammal (maybe more, I'm not a scientist) that eats human remains but that might be acceptable as collateral damage to you.

http://www.hopkinsmedicine.org/healthlibrary/conditions/nervous_system_disorders/prion_diseases_134,56/

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You need multiple attack vectors.

If you use a single attack vector, humans will stand a chance of discovering that vector and finding a way to counter it before they are wiped out. Humans are annoyingly persistent that way.

As others in this thread have mentioned, reduce birth rates. This can be done via several simultaneous attacks that hit in reoccurring waves.

  1. Genetically modify various existing bacteria/viruses (common cold, flu, etc.) to attack the ovaries and testes. You won't have 100% infection rates with this. But if you infect 50% of the population and 50% of the infected are sterilized, you've reduced the global pool of child-bearing humans by 25%. And these viruses self-mutate and linger, so each successive generation will face re-infection over time.
  2. Genetically modify various existing bacteria/viruses (not the same specific strains in #1) to increase social anxiety disorders and other mental issues that reduce one's ability to interact socially. This will make people more prone to isolation from each other on an intimate level, again reducing humanity's birth rates.
  3. Targeted bacteria/viruses that attack various food sources. If you can find viruses that wipe out 50+% of existing corn, wheat, and rice varietals, you will devastate population. That devastation will likely lead to warfare and national isolationism, as people fight over the remaining food. That will further depopulate your planet. Yes, you might wipe out those crops entirely from the planet, but most of the non-human life will survive the loss.
  4. Pollute the oil wells. Engineer a bacteria that breaks down crude oil and can survive in the hostile environments of deep oil wells. Spiking one of their primary fuel sources will devastate economies, destabilize national relations, and lead to war. The death tolls will be catastrophic for those annoying humans.
  5. Pollute the coal mines. Same as #4, but for coal instead of oil.
  6. Improve existing bacteria and viruses' ability to resist human medicines. If their antibiotics fail, then they get sicker and die faster. And if this is the 2nd or 3rd wave of bio-attack, their medical research teams will be overwhelmed and incapable of facing the new attack with efficiency. Keep up the good work!
  7. Annoyances. These are minor, trivial things that will prove annoying to repair. The expense to do so will eventually outweigh the nations' resources. Destablize the orbit of communications and GPS satellites, cause electrical surges in various power grids, introduce microfractures into bridges and dams so they fail more often, introduce contaminants to major food storage depots, accelerate the oxidation of metals in trains or planes, etc. This is sort of a death by a thousand cuts. By themselves, none of these acts will kill enough people to matter. But keeping a background level of these events will rob resources from the other 6 items in the list. It will strain their ability to respond quickly or with enough person-power to mount effective responses.
  8. If you have the technical resources to do so, trigger tsunamis and earthquakes on an slowly-increasing frequency and severity scale. These have devastating impacts on human morale as they simply cannot defeat nature, and their religions make it difficult to understand why nature would be against them.
  9. Introduce lead and other slow-acting toxins to city water supplies. Sure, they test for such things, but it takes resources to clean up the mess. And if they keep getting poisoned, the experts capable of cleaning the mess will get fired, replaced by less experienced, less capable people.

Again, humanity is annoyingly clever, so they could probably overcome any one item off the list. Maybe even any 2 or 3. But not the entire list. Each will impact a different segment of the population and further weaken the rest until the entire thing collapses.

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Use a multi-step approach. First render the humans vulnerable in a way that they won't obviously notice, then expose them to a threat which is fatal to the modified humans.

The most promising long-term strategy is, I think, not a poison per se, but creating genetic or epigenetic changes in human DNA that would unexpectedly turn out to be harmful.

The timescale you mention (200-1000 years) is way too short to genetically-engineer humans to have specific gene in the natural way. The mechanisms of evolution - gene drift, gene flow, natural selection - with as large and diverse populations as humans work on the timescales of ten and hundred thousand years. Therefore the best strategy is to lure the humans into doing artificial genetic modification on themselves. Suppose that you can design a gene with particularly deadly hidden vulnerability, which at the same time brings extreme benefits to its carriers, for example, supernatural attractiveness, longevity or amazing resistance to common diseases. Which parent would not want to have this for his children? Indeed, in the future society doing this particular gene therapy or even (inheritable) germ-line modifications would be seen a moral imperative similar to vaccinating your kids nowadays.

As for the final threat, it could be something line a genetically engineered "superbug" exploiting some inherent weakness in the "supergene". Alternatively, it could even be a natural event! For example, with your supreme scientific ability you could predict that 500 years from now on the Earth will be hit by a very strong gamma ray burst, and genetically engineer humans so that they're extremely vulnerable to the aftereffects or gamma ray bursts. First, everyone genetically modifies themselves. Then, almost everyone dies. Then you can step in and clear the remains on the civilization easily.

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I'll go the genetic modification route but I'm not after deadly diseases. Rather, something very benign--one or more of the usual human gut bacteria. It continues to function as normal but produces an extra compound--one that mutates human DNA.

If your distribution is detected it will be identified as a harmless organism and not given much scrutiny. Meanwhile humanity is plagued by cancer and birth defects. Next generation release another modified gut bacteria, the problem goes up. Eventually the human race can't maintain reproduction, you succeed.

Another option would be a compound that causes asexuality. Humanity goes extinct due to a lack of reproduction.

If you don't mind taking out most of the animals also add another compound--one that causes paranoia. Someday humanity will take itself out.

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