I am imagining a world where an astronomical event occurs, like Earth collides with a comet, and humanity is all but wiped out. In their desperate attempt to recover, humans enter a form of stone age. Most of the existing life forms are still there, though also decimated by the event. Modern humans apply their existing knowledge to set out hunting, gathering, farming, and ranching. But something is different; the bioavailability of iron in all the Earth's arable soil is diminished and consequently becomes a highly coveted resource. The entire planet is anemic. The drive for iron-rich foods drives humans crazy. They drink blood from their dead and hunt each other for it. New species with copper based blood like the horseshoe crab rise to prominence.

What kind of event might have led to these conditions?

  • $\begingroup$ Welcome to worldbuilding. We enforce a strict "one question per post" policy, while here I count 3 question. Please edit accordingly. You can find out more in the help center and taking the tour. $\endgroup$ – L.Dutch Jun 26 at 19:10
  • $\begingroup$ @L.Dutch edited! $\endgroup$ – AdamO Jun 26 at 19:18
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    $\begingroup$ en.wikipedia.org/wiki/… Maybe something related to that goes a bit wacky? $\endgroup$ – puppetsock Jun 26 at 20:32
  • $\begingroup$ @puppetsock an alien bacteria that, otherwise harmless, renders endogenous iron inaccessible is a good idea. Although I don't think there would be any benefit to drinking the blood of other infected people/animals. $\endgroup$ – AdamO Jun 26 at 20:52
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    $\begingroup$ Iron is the fourth most common element in the earths crust you can't have an earth like planet without massive amounts of bioavailable iron. And it only gets more abundant as you go deeper. To deplete the planet of iron you would literally have to remove 35% of the mass of the planet. There is nothing you can do that would significantly reduce iron without wiping out all life on the planet. $\endgroup$ – John Jun 27 at 1:59

Iron is irreversibly chelated into non-degradable molecules.

Consider PCBs or polychlorinated biphenyls.

These were useful industrial chemicals, but unfortunately persist indefinitely in the environment. Somehow they also cause health problems, which I would not expect for a biologically indestructible molecule but there you are.

The earth is showered with PCB-like indestructible chemicals which have the property of being extremely iron avid. Perhaps they are like PCBs but contain sulfur? The provenance of these chelators is up to the author - maybe space molecules, or products of an alien civilization. Or perhaps like PCBS they were produced in great quantities on earth (bioengineered crops?) before their troublesome properties became known.

In any case: a lot of that stuff, because there is a lot of iron in the crust. The indestructible chelator molecules mop up the iron and keep it. Maybe these chelators form rusty looking blobs that people and animals eat and then excrete unchanged.


Iron is rather abundant in solid land, but no so much in significant parts of the ocean. Furthermore, plants have difficulty absorbing iron when soil pH is too high, has too much clay, or is overly wet (source). To make matters worse, high amounts of plant particles or manure (which would likely be a rudimentary attempt at fertilization post-apocalypse) can also cause iron deficiency.

When you put all this together, a giant comet slamming into the ocean could cause a significant redistribution of water across most inhabitable land. In the right conditions a higher pH-soil or more clay-like soil might cover a significant part of the earth's crust, making stone-age style soil aration difficult, not to mention unlikely to penetrate deep enough for plants to be able to reach more iron-rich soil. Just enough clay and acidity, mixed with rudimentary farming, and plants will suffer from iron deficiency, which in turn means most humans would as well.

It all comes down to what was in that comet - anything on/in the comet that can rice the pH level of dirt or cause some type of residue that alters pH in soil would do the trick.

As iron deficiency gets worse, it can cause "unusual cravings for non-nutritive substances, such as ice, dirt or starch" can occur. Although drinking blood is not commonly associated with iron deficiency, I suppose iron deficiency plus a sudden restriction of the gene pool (read: inbreeding) would be at least somewhat believable.

  • $\begingroup$ The problem is that a comet large enough to do that is large enough to do that worldwide is one large enough that you're talking a planetary mass extinction. There won't be any humans left who have to worry about it. Also, you'll note that comets have slammed into the ocean and what you've postulated has never been observed as having happened. $\endgroup$ – Keith Morrison Jun 26 at 21:18
  • $\begingroup$ @KeithMorrison small comets are probably part of the source of our oceans. A large enough comet (with as much mass as Chicxulub) would have the effects described. I agree with you that whatever is big enough for the disaster OP wants is big enough to kill everything as well. $\endgroup$ – Renan Jun 26 at 21:20
  • $\begingroup$ Except I was specifically thinking of Chicxulub and it did not have the effect suggested. $\endgroup$ – Keith Morrison Jun 26 at 21:28
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    $\begingroup$ Drinking blood would be less driven by actually craving it, but rather by the knowledge that there is iron in it: part of the importance of "modern knowledge in a resource limited setting". My story is partly inspired by stories of moms and profound pika after delivery. $\endgroup$ – AdamO Jun 26 at 21:28
  • $\begingroup$ no matter how big a tsunami a impactor generates the water will run back into the ocean within a few weeks. Everything else on your list are local condition that can;t be generated planet wide. $\endgroup$ – John Jun 27 at 1:46

A miracle, or Alien Space Bats.

There's nothing regarding an impact that would be survivable (as per the requirements of your scenario) that would significantly alter the chemical composition of the crust on a planetary scale. It might add a detectable amount of something, but not remove it. You're not going to get your vampires that way.

  • $\begingroup$ Chemical composition of the crust, no, but chemical composition of the surface, yes. $\endgroup$ – Renan Jun 26 at 21:10
  • $\begingroup$ @Renan, how? How would an impact remove something from the surface and keep it removed? $\endgroup$ – Keith Morrison Jun 26 at 21:15
  • $\begingroup$ An impact like the one OP describes throws large debris around a quarter of the planet's surface, and fine dust all around it. Even if you live on the antipole of the impact point, your house will be covered in dust. $\endgroup$ – Renan Jun 26 at 21:19
  • $\begingroup$ How would an impact remove something? Your answer...doesn't answer. $\endgroup$ – Keith Morrison Jun 26 at 21:27
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    $\begingroup$ If all the plants are gone, one has slightly more pressing concerns than the bioavailability of iron. Namely, you'll be dead long before anemia becomes an issue. $\endgroup$ – Keith Morrison Jun 26 at 21:49

Iron really likes Oxygen to form rust. From geology we're know that the oceans once held far larger quantities of iron, but that when photosynthesis evolved the iron turned to rust and precipitated. I'd there were more O2 in the atmosphere, more iron would form rust, and less would remain bioavailable.

If the event caused the O2 level to significantly increase, the would be really ferocious fires and less iron.

  • $\begingroup$ What makes you believe that only elemental iron is bioavailable? Elemental iron is extremely rare. $\endgroup$ – AlexP Jun 27 at 2:32
  • $\begingroup$ No, iron in other molecules is bioavailable. I don't know if elemental iron is bioavailable or not. My claim is that iron is so strongly binding with oxygen that if there is more oxygen, more iron will be bound to oxygen and precipitate as rust, which is mostly not available. $\endgroup$ – cmm Jun 27 at 2:47
  • $\begingroup$ Richard F. Hurrell, "Bioavailability of Different Iron CompoundsUsed to Fortify Formulas and Cereals:Technological Problems", 1984. Long story short, both elemental iron and iron oxide are bioavailable. $\endgroup$ – AlexP Jun 27 at 2:59
  • $\begingroup$ Thank you. I will read the article and may delete my answer. $\endgroup$ – cmm Jun 27 at 3:08
  • $\begingroup$ The paper refers to an organic preparation they call "iron oxide saccharated". "Saccharated Ferric Oxide This compound comes as an amorphous brownish red powder containing from 3 to 10% iron. It is very soluble in water and an aqueous solution has been used in intravenous injection to treat anemic patients (32). It is made from ferrous chloride, sodium carbonate, sucrose, and sodium hydroxide. It has no fixed formula but it is reported to be a mixture of ferric oxide and saccharose. It is used only to a limited extent and there is little information on its bioavailability..." $\endgroup$ – cmm Jun 27 at 17:32

An nonnative invasive alien fungus grows over the entire surface of the earth. It proliferates and stockpiles accessible iron into mycorrhizae attached to the mycelium 100s of meters below the earth's surface, and then goes dormant for centuries. The fungus is not visible to the naked eye, or perhaps it is but is resistant to any efforts to exterminate it. Even if people know it's below the Earth's surface, efforts to mine or sequester it are fruitless.


You can't eat pure iron, it will kill you. Plants and bacteria are able to take iron oxides from the substrate and form organic and inorganic complexes that animals may then ingest.

So your iron intake is coming from either plants, or animals that ate those plants. In the event of an impact that fills the atmosphere with dust and blocks sunlight, most plants would die. This has happened before. But in that case, it's not just iron that animals would be missing.

In our case, we might thrive for decades with dietary supplements scavenged from factories and supermarkets. But at some point post-apocalyptic societies without access to silos and other long-term seed-storage solutions would be out of useful plants to grow, and then they would be in dietary trouble.

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    $\begingroup$ Yeah, no. This is trivially proven wrong. Inuit eat caribou, muskox, assorted varieties of herbivorous ducks and geese, and ptarmigan. Source: Me. I live in the Canadian Arctic in an Inuit community. $\endgroup$ – Keith Morrison Jun 26 at 21:14
  • $\begingroup$ @KeithMorrison I stand corrected, I'll remove that bit. $\endgroup$ – Renan Jun 26 at 21:14
  • $\begingroup$ Indeed we don't eat "pure" iron now, yet it is a vital nutrient in our diet, and without it we die. We get it from plants and meat. If soil is iron deficient, so are the plants and the animals who eat them. The idea of progressively whittling away iron stores over centuries or millenia without even the knowledge of it happening, is very appealing. Thank you. $\endgroup$ – AdamO Jun 26 at 21:36
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    $\begingroup$ The problem is you're not thinking of the effect throughout the food chain: if there's insufficient iron for people, that's true for animals and plants further down the food chain. The humans will die of starvation long before iron deficiency specifically becomes the problem. $\endgroup$ – Keith Morrison Jun 26 at 21:51
  • $\begingroup$ You can eat pure iron just fine iron poisoning requires injesting huge amounts of high availability iron, like iron salts, such as eating dozens of iron supplements a day. $\endgroup$ – John Jun 27 at 1:55

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