The last time I asked a worldbuilding extinction question, I asked on what sorts of factors would create an extinction event in which 96% of all terrestrial species and only 70% of marine species died out in a short span of time. In there, I got only two answers--a rogue ice age (not interesting, personally) and a gamma ray burst from a nearby supernova (worth exploring, but that raises another question--how long would it take for the atmosphere re-establish itself?)

But now we investigate a different kind of catastrophe--an extinction equilibrium, in which the death rates of terrestrial and marine species are the same. There are countless inexhaustible variables on this, so let's narrow it down to one extinction event with three different options.

In this episode, we investigate the extinction of 50% of all terrestrial species and 50% of all marine species, the minimum requirement for an extinction event to be "mass".

What combination of factors would create any of the specified damage?

Just a reminder, this question does not ask how long it would take for life to recover, who would fill in the vacant niches, etc. The question is what would cause an extinction event as described in the paragraph above. It doesn't matter when it happened. But no man-made!

Also, a similar-looking question has been deleted, so this is not a duplicate.

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    $\begingroup$ Just a tip about the Sandbox: it's recommended to wait at least 24 hours before graduating a post and to edit the draft there. Obviously that's up to you, but this time it just basically slowed you down half an hour and there wasn't much feedback apart from a couple comments from me ... Nearly nobody, not even the very active Sandbox users, have had the chance to see your draft in the Sandbox, let alone help you with whatever it was that you expected the Sandbox to help you with. $\endgroup$ – Secespitus Aug 13 '18 at 14:30
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    $\begingroup$ Hard science? This? Really? Well, it's your question... $\endgroup$ – user Aug 13 '18 at 17:20
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    $\begingroup$ I also have no idea from where you got the idea that it takes the extinction of "50% of all terrestrial species and 50% of all marine species" to be a "mass" extinction event, let alone how that would be an "equilibrium". A state of equilibrium is normally taken to be a state that is naturally stable in some way, and no matter how you slice it, I really doubt there will be a stable anything if you've just wiped out half of the species of life on the planet, no matter how you do it. $\endgroup$ – user Aug 13 '18 at 17:23
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    $\begingroup$ Hard science questions require a great deal more effort to answer properly (and I think hard science is wrong for this question). $\endgroup$ – StephenG Aug 13 '18 at 18:40
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    $\begingroup$ Do you mean 50% of the species (for instance all the giraffas die, but all the ants survive), or 50% of the individuals of each species (50% of the giraffas and 50% of the ants)? Also do you mean all reigns, or only animals? $\endgroup$ – Legisey Aug 16 '18 at 11:20

What you need here is a form of Phage that attacks specific forms of plants.

Bacteriophages are naturally occurring forms of genetic disease, although there has been quite a bit of research into these as a form of genetic manipulation but that is out of scope of this question.

Why plants? Because a significant reduction in plants means a significant reduction in oxygen production. That in turn means a significant reduction in animals consuming that oxygen, starting with the ones that are less tolerant and working down. Eventually, the animal population and the new oxygen production levels reach equilibrium, and you achieve your even reduction in population.

But wait (I hear you say), there are a LOT more terrestrial plants than there are marine plants; something else has to be at work, surely?

Actually, no. It's true that terrestrial vegetative biomass outnumbers marine vegetative biomass by around 200 to 1, but that marine biomass is actually incredibly effective at outputting oxygen. This is in part because they don't need the structural reinforcement mass that their terrestrial counterparts do, like tree trunks. It turns out that their oxygenation levels for the oceans are relatively similar to those of terrestrial plants oxygenating the atmosphere.

The same article shows that different types of species are going to react differently to significantly lower levels of oxygen as well, meaning that the losses will be mostly aligned to species, reducing the biodiversity in the process.

Ultimately your phage needs to target plants, and needs to be effective against a cross section of them that is evenly distributed between marine and terrestrial species. I'm not sure what vector that would be genetically speaking as I just don't have access to that level of detail, but this is certainly a good place to start in terms of planning a partial extinction level event that hits both marine and terrestrial species equally.

  • $\begingroup$ A phage that can target such a large variety of species and is virulent enough to wipe out said species is impossible. The biochemistry just varies too much. $\endgroup$ – John Aug 18 '18 at 4:54

When mass extinction is not very massive: The Hadean.

hadean earth https://en.wikipedia.org/wiki/Hadean

Life had to start sometime. It would have been sometime in the past and that is when this scenario is set. When life did first start, there was probably not much of it. It used to be thought the Hadean earth of 4 billion years ago was too inhospitable for any life to exist - lots of extraterrestrial strikes, volcanism, and so on. But it may be that life got its start during this period around deep sea vents in the early ocean.


In the study, Bell and her colleagues examined zircons from the Jack Hills in Western Australia, a site that has yielded more Hadean samples than anywhere else on Earth, searching for inclusions of carbon minerals like diamonds and graphite...

The authors list several nonbiological processes that could explain their findings, but they favor the idea that the graphite started out as organic matter in sediments that got dragged into the Earth’s mantle during the collision of tectonic plates. As the sediments melted to form magma, the elevated temperatures and pressures transformed the carbon into graphite, which eventually found its way into a zircon crystal.

If this story is true, and life existed 4.1 billion years ago, Bell says that the new results would corroborate growing evidence of a more hospitable early Earth than scientists once imagined. “The traditional view of the Earth’s first few hundred million years was that this was a sterile, lifeless, hot planet that was constantly being bombarded by meteorites,” she says. But partly thanks to the wealth of information revealed by the Jack Hills zircons in recent years, scientists have come to see the early Earth as much milder and more amenable to life.

In this scenario of the very early earth, the deep sea vents host the first life forms - probably something like chemoautotrophic bacteria. During this period, extinction of 50% of all terrestrial life is easy because there is no terrestrial life and 50% of 0 is 0. As regards the first life forms huddled around their vents, a localized event would be enough to extinguish nascent life around these vents. Today, vents go out and their biota die. Vents erupt and their biota cooks. With such a small and localized population, a minor change in local volcanism would be enough to wipe out 2 of the 4 extant species because their habitats disappear.

Given how rapidly life did appear on Earth, it is very plausible that early life suffered through many false starts of this kind because of the instability of conditions on the early earth. Only once things got more stable could bacterial life persist and spread.

  • $\begingroup$ Usually you explain why, Dailey. Why the down vote this time? "Just felt like it" is a fine reason too, but if I did not answer the question that helps me. $\endgroup$ – Willk Aug 20 '18 at 2:10
  • $\begingroup$ Why just bacterial? $\endgroup$ – JohnWDailey Aug 20 '18 at 3:08
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    $\begingroup$ @JohnWDailey I rolled my eyes at this answer; it is a bit "cheaty", especially "50% of 0 is 0", even the rest is not very usable for average world building. However it does appear to answer the question. You asked for half land and half water species to go extinct, and that is what Willik did. Even though I sigh at this answer, it was amusing in that it was outside the box. +1 for ingenuity. Even if you don't like the example, it is easily tweaked: in the pond with first life (4 water types and 2 land), a bit of magma enters killing the frogs, gators, and coral. 50% land and animal types gone. $\endgroup$ – Loduwijk Aug 21 '18 at 23:13
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    $\begingroup$ And, even though I suggest it may not be useful to your average world-building, that does not mean it is useless. It very well could be incorporated into a story. "And our interstellar explorer was called before the galactic court for the felony of 'causing the mass extinction of at least 50% of land and water creatures on a pre-space-capable planet' after having accidentally dropped a flare into a puddle containing the first few life forms" $\endgroup$ – Loduwijk Aug 21 '18 at 23:22
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    $\begingroup$ Not having terrestrial life kinda makes it hard to fulfill the criteria, Also it really doesn't answer the question, not having much life is still not a cause. $\endgroup$ – John Aug 22 '18 at 2:36

Mercury containing meteor shower.

A high amount of elemental Mercury is introduced into earth's atmosphere through the burning up of these meteors in the atmosphere. The Mercury is eventually incorporated into the water supply. Since the Mercury is bio-accumulative, the level of dose any individual gets will depend on what they eat. At some level of mercury dosing a 50% extinction can be achieved.

  • $\begingroup$ This sounds as extreme as a gamma-ray burst. $\endgroup$ – JohnWDailey Aug 21 '18 at 22:36
  • $\begingroup$ Extinction events are, by their nature, extreme. $\endgroup$ – Mathaddict Aug 21 '18 at 22:40
  • $\begingroup$ No, I mean, how does mercury poisoning ensure a global casualty of no more than 50% of all species? $\endgroup$ – JohnWDailey Aug 22 '18 at 0:37
  • $\begingroup$ Only those higher up the food chain actually die. $\endgroup$ – Mathaddict Aug 22 '18 at 14:45

Nearly all known global mass extinctions (one or maybe two exceptions out of dozens) were caused by impact events or highly correlated with impact events and their aftermath, which are likely to include flood basalts. The severity varies wildly and is controlled by too many poorly understood factors to be very predictable at the level you want. But things like impact angle, impact site geology, how recent other stress factors are all play a role, it is not something we can predict down to percentages.

enter image description here

source 2

source 3

enter image description here comparison of crater formation and mass extinctions.

Basically you can say a large asteroid hits and X number of species go extinct and no one can really argue. An asteroid hits triggering a massive volcanic event on the opposite side of the planet or at its own impact site (it would also trigger earthquakes all of fault lines at once, tsunami, fires, ect. all the large ones do) and the combined effects so drastically changes so many environments in so many ways that most species just cannot adapt to them fast enough. Who survives is mostly about being fecund generalist and a large degree of luck. This is basically the story for most of the mass extinctions in earth's history. You need to change many environments in many ways drastically to get mass extinctions, and there just are not that many ways to do it. Large bolides are by far the most reliable and the extinctions they cause are unpredictable enough you can just say it killed X number of species and no one can really argue with you.

If you want to be more exact the dinosaur killer was 10-15km, you want to kill more, so go with 20-30km to minimize argument. if you need to pick an impact site go with a land target, ocean impacts cause massive amounts of acid rain which can change ocean PH, which has a big impact on marine life. Personally I'd go with a himalayan impact, because if your going to drop a mountain on the planet why not drop it on another mountain for symmetry.

  • $\begingroup$ We don't actually know that all mass extinctions are impact events. Impacts are common hypotheses for each, but there are alternate explanations as well. Some people think the dinosaurs were wiped out by rampant vulcanism causing climate change for instance. $\endgroup$ – Ryan_L Aug 13 '18 at 16:10
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    $\begingroup$ The end of the Permian was not caused by an impact. $\endgroup$ – JohnWDailey Aug 13 '18 at 16:14
  • $\begingroup$ Sure it was, Huge volcanic hotspot formation on the opposite side of the planet is one of the things large bolide cause. There are even two possible craters. $\endgroup$ – John Aug 13 '18 at 17:18
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    $\begingroup$ The Great Oxygenation Event pretty thoroughly counts as a mass extinction event, and it definitely wasn't caused by an impact event. $\endgroup$ – user Aug 13 '18 at 17:19
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    $\begingroup$ because not every meteorite is metal rich, Iridium is evidence for an in impactor but impactors o not require iridium. there is a possible crater onlinelibrary.wiley.com/doi/full/10.1111/ter.12269 and other markers of an impact lpi.usra.edu/meetings/lpsc2003/pdf/1490.pdf $\endgroup$ – John Aug 14 '18 at 14:46

Forgive my lack of references, but allow me to give you an answer.

Many mass extinctions are caused by a lack or solar energy getting to the earth. Photoplankton is the sea die, as well as plants on the earth. Killing off the lowest members on the food chain invariably kills animals higher on the food chain, as is shown by predator-prey cycles.

So whatever method you use, its just a matter of balancing sun blocking with animal populations. Now, I don't have any equations to calculate how fast animals die off in mass extinction scenarios, though that doesn't mean they don't exist, but blocking the sun for a period that kills approximately 50% of all life is the best way to kill 50% of all life. Volcanoes, Meteor, passing through a interstellar dust cloud, whatever it is just needs to kill about 50% of photosynthesizers to kill 50% of all life.

Although some life doesn't really on photosynthesis based food chains, they probably make up less than .1% of life on earth, so only the photosynthesis chain matters.

Good luck figuring out how many months/years that would take, cause I have very few planets to test the exact numbers on. Possibly mass extinction research can give you an estimate, but I'm just giving you a rough answer, so that you get at least something, even if you originally wanted a more detailed answer. You can always use this as a starting point for research.


It's Us.

It is theorised that we are currently in the Holoscene Extinction Level Event.

50% is a good number to reach from the perspective of a technologically dominant species that removes most natural habitats on land, replacing with mono-genetic herds or grasses, and fishing many species underwater to extinction, combined with altering atmospheric conditions to affect water temperature and underwater ecosystems, before humans may realise that they have done 50% damage and should rethink what they are doing.

Timescale wise this current Extinction Level Event is actually occurring very fast. Compared to glacial events, or the great Oxygenation event, the last 100 years (and the next 100 years) is a blink of an eye.

  • $\begingroup$ I said, "no man-made". $\endgroup$ – JohnWDailey Aug 13 '18 at 16:15

An extremely lethal virus or other pathogen that acts on core cellular functions of all life.

Almost all living plants and animals have a core of basic genes that encode the basic cellular structure and biochemistry common to all life in Earth (i.e. they all share a last universal common ancestor).

For example, humans share 60% of their genes with bananas, fruit flies and chickens, respectively.

This last universal common ancestor and the shared ancestry that flows from it is one of the vulnerabilities of life on Earth to catastrophic failure.

If a lethal pathogen evolves naturally that implicates this shared ancestry and perhaps also had a long latency period to make its effects pass unnoticed until it is too late, every species could be put more or less equally at risk. The evolution of a pathogen fitting these criteria could have an impact as profound as the singular event in which life arose on Earth that spawned that last universal common ancestor, a once in a multi-billion year phenomena that nonetheless happened at some point purely through random chance.

This would be basically a Grey Goo scenario, modified to reflect that fact that mutations to resist it would eventually materialize.

A pathogen that targets the basic core of the genetic foundation of almost all life would affect almost all species of plants and animals on Earth and in the sea almost equally if a suitable transmission method were found.

For example, perhaps it would start by infecting Plankton, sea grass and algae in the sea, and grass and other foods at the base of the food pyramid on land. Then, the infection would be of a type that could be carried up the food chain.

enter image description here

A Marine Food Pyramid

enter image description here

A Terrestrial Food Pyramid

There are historical examples of naturally arising pathogens with some of these features although fortunately for us, not all of them. Bubonic plague and smallpox and HIV all had very high lethality before treatments were developed. Smallpox and HIV have affected multiple species. Mad cow disease naturally propagates up the food chain even in the face of normally sufficient sterilization methods. HIV has a long latency period. A perfect storm pathogen that randomly ends up combining the various features of these historic plagues could give rise to a mass extermination event if it operated by impairing the functioning of genes that are shared by almost all species.

It is relatively straight forward to tweak the theoretical lethality of a pathogen in a manner that would cause it to be thwarted by resistance mutations just in time for about 50% of any given set of species to be wiped out, although estimating the lethality in terms of numbers of species rather than numbers of members of a species is a bit of a leap and covers a domain of applicability of infectious agents that are much more lethal than any such agent now know on Earth.

Some of the foundations of this kind of analysis are set forth, for example, in:

Frank Hilker, et al., "Modelling virus coinfection to inform management of maize lethal necrosis in Kenya." Phytopathology (2017); DOI: 10.1094/PHYTO-03-17-0080-FI

Like most infectious agents that are highly lethal, sooner than later it would burn out, having killed all life that isn't immune to it.

Then, relatively quickly (a few centuries to a few million years - our scientific tools use to study previous events of this kind don't have the resolution needed to cabin the time range more exactly), the temporary oxygen shortage caused by the mass die off would be overcome and returned to stasis by the species that survived and expanded into the ecological niches of those that went extinct.

  • $\begingroup$ This reads as manmade, which I have made clear is not what I want. $\endgroup$ – JohnWDailey Aug 17 '18 at 23:02
  • $\begingroup$ @JohnWDailey This was not my intent. I'm suggesting that some simple pathogen like this could evolve naturally, for example, when genes resuffle between billions of viruses over and over again until this one deadly combination randomly emerges and has a dominant effect, not unlike the event that gave rise to life on Earth in the first place. When I talk about tuning it to fit your parameters, etc. I am simply talking about what you as an author in the world building process might do if you wanted to reverse engineer what the necessary pathogen to have such an effect would look like. $\endgroup$ – ohwilleke Aug 17 '18 at 23:16
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    $\begingroup$ FWIW, there a plenty of examples historically of naturally arising pathogens crossing species barriers and having immense lethality that is sometimes transmitted up a food chain (e.g. smallpox and HIV and mad cow disease), so it really isn't a terribly far fetched possibility, just an extreme version of something that often happens naturally to a less extreme degree. $\endgroup$ – ohwilleke Aug 17 '18 at 23:29
  • $\begingroup$ crossing one or two species barrier is very very different than lethally infecting a significant fraction of all species. there is a reason we have not seen it before becasue you are talking about millions of drastically different biochemical pathways this virus mus somehow be highly compatible with, and yet not be affected by the billions of different antiviral adaptation. Its like saying you have a simple key that can open every lock ever made from mechanical to digital. $\endgroup$ – John Aug 18 '18 at 4:59

I don't know what kind of extinction event would leave 50% of both land and aquatic species, but about the question as to how long it would take the atmosphere to recover; it wouldn't ever.

A gamma ray burst would kill off so many species that all the different atmospheric cycles would be irreversibly altered. The current rates of the carbon cycle, the nitrogen cycle, and the water cycle all depend heavily on the current biosphere. Change the biosphere too much and you'll change the cycles. The atmosphere will eventually settle back into an equilibrium after a GRB, but it won't be the same equilibrium.

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    $\begingroup$ This does not answer the question. At all. $\endgroup$ – JohnWDailey Aug 14 '18 at 1:41
  • $\begingroup$ You asked two, "but that raises another question--how long would it take for the atmosphere re-establish itself?". I answered that one. My point is that "atmosphere recovery" is poorly defined. If by recovery, you mean when will the atmosphere go back to the way it was before the event, the answer is never. Every mass extinction event we have seen has coincided with a permanent change to our atmosphere. Consider the Oxygenation Event for the best example. $\endgroup$ – Ryan_L Aug 14 '18 at 2:20
  • $\begingroup$ How long would the atmosphere re-establish itself so as not to be bombarded by any more UV? But that still does not answer the main question. $\endgroup$ – JohnWDailey Aug 17 '18 at 23:01

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