64

I'm going to respond to this with a framing challenge, because as it stands I think you're asking the wrong question. The issue is that the modern industrial world is a product of population DENSITY rather than total population. This may seem semantic but in a post-apocalyptic environment it's very meaningful. Modern industrialism depends on and benefits ...


42

/Their lives are to be miserable./ I know a recipe for miserable. Flies everywhere. https://en.wikipedia.org/wiki/Black_fly The flies are a genetic modification attempt gone wrong. They are everywhere and they attack anything that moves, biting and chewing and drinking. They cause blindness and carry other diseases as well. Attempts to control them ...


36

A year isn't going to cut it, you're looking at about a generation at least for any noticeable progress. The problem is the lack of industrial base. Medieval society is a hair's breadth above subsistence farming. Technology is largely driven and supported by population pressures, without that they don't have the need to progress nor to maintain anything you ...


32

The situation you describe is almost exactly (part of) the plot from the book "A Fire Upon the Deep". Which in my opinion presents a very plausible level of success. In fact the specifications you offer (1 year-ish time period, lack of homeworld contact for the high tech people) match the conditions in the book very closely. https://en.wikipedia.org/wiki/...


30

Presumably the insects swarm, gorge, breed, then either die or hibernate. Then come out in force 20 years later after the eggs have hatched and the larvae mature into adults who can fly and who need human blood in order to support breeding. Humans have 20 years to find and kill the eggs or to kill the larvae. If the eggs are grouped together, then fire ...


30

Wood You said they don't have access to fossil fuels, but not to a forest. Using wood as fuel can be a good way to start gathering energy, in form of steam engine if you are looking for mechanical energy instead of heat. For the link: A common hardwood, red oak, has an energy content (heat value) of 14.9 megajoules per kilogram [...] Wood as fuel can be ...


30

Gray Goo Nanobots that can break down anything and use the raw materials to make copies of themselves go out of control. They literally devour the surface of the Earth. Things get back to normal when for some reason the bots decide to stop replicating and decay naturally. Source: https://xkcd.com/865/


30

Your question includes the answer: they have prehensile feet. As long as their body plan allows for balancing on one foot and manipulating objects with the other, or sitting and using both feet as we'd use hands, there are few barriers to their ascent to a technological civilization. Carrying objects in their mouths will have to suffice until bags and ...


27

I've used a personal analogy for this, which I call "The brake pad problem." I estimate (without any serious research) that maybe a few hundred people around the world really know what it takes to manufacture an automobile brake pad. If you kill them all off, we're in serious trouble at least until a new process (or a reconstruction of the original) can be ...


26

No advancement at all Let's assume "very nearly destroyed" (a very vague description) means less than 5% of the population survives every attack. Those attacks occur every 20 years. Let's make an outrageous assumption that the only remaining people are distributed 19:1 in favor of women and all the women are prime childbearing (18-20 years old). In 10,...


25

Measurement units have followed a fairly standard pattern. Originally things are compared to something that both parties recognise. It could be a specific thing (the Washington monument) or a general thing (a banana). No one is really measuring anything accurately, so this is good enough. Next people start to measure things against their own standard. ...


24

Open ocean. With the still-mysterious possible exception of the Australian aborigines, up until 1500 BC everywhere humans got they walked. Europe, Asia and Africa are contiguous, and during the ice age you could walk to the Americas because sea levels were low. The Polynesians made the first convincing long distance overseas colonization in 1500 BC which ...


24

If you limit population everything else follows; specialists, and the technology they create and maintain, require the support of high density populations. There are several things going for you already: a post-apocalypse population crash, and there will be one, will effectively end the oil industry, more or less permanently, this sharply limits the yield ...


23

How can a sapient species build an advanced civilization, while remaining on one continent? Firstly let me describe your relationship to your home continent: - Sapience does not signify a lack of superstition, the question refers to Ancient-Egyptian technological level culture. This is congruent with a pantheistic, or polytheistic civilisation. Your other ...


21

Maybe you can get some inspiration from the Uru People that live over Lake Titicaca. They create artificial floating islands, with small houses on it, with a plant called Totora. This plant commonly grows at a water depth of 2.5–3 m (8.2–9.8 ft) but occurs less frequently as deep as 5.5 m (18 ft). https://en.wikipedia.org/wiki/Uru_people https://en....


20

A significant increase in global volcanism would satisfy your requirements: The devastation would be widespread, varied, and continuous. Think of something on par with the formation of the Siberian Traps. Such eruptions can last millennia. The resulting volcanic winter would wreak havoc on the biosphere and agriculture, necessitating greenhouses and ...


20

Sure, why not? The Soviet Union had mandatory military service pretty much EXACTLY like what you're describing with a population that size. The factors that would make it difficult to have nationwide combat training have everything to do with politics (your population refuses to participate) and economics (you can't afford to have all those people not ...


19

Their birthrate is so low while their primary area is so abundance. So the population is already scarce and they already in comfort. Why you need to built a new settlement when you and your peaceful life is guaranteed inside your kingdom since your grand-grandfather? The one who venture outside will be just a small band of thrill-seeker;their journal also ...


19

You could start with the panda pattern. Giant pandas are carnivores which have adapted an all vegetarian diet. Your elves are also pandalike fallen carnivores. https://en.wikipedia.org/wiki/Giant_panda Despite its taxonomic classification as a carnivoran, the giant panda's diet is primarily herbivorous, consisting almost exclusively of bamboo. ...


18

Nearby Supernova. Dr. Mark Reid, a senior astronomer at the Harvard-Smithsonian Center for Astrophysics stated: … were a supernova to go off within about 30 light-years of us, that would lead to major effects on the Earth, possibly mass extinctions. X-rays and more energetic gamma-rays from the supernova could destroy the ozone layer that protects ...


15

Too Much Tampering with Genetics Stargate SG-1 had a species called the Asgard. The Asgard modified their own genetics for multiple reasons - curing diseases, better life spans, smarter, more adaptable. The baseline Asgardian genome had become lost to time. At some point, the species recognizes that there is some subtle flaw with their modification ...


15

You give your own answer! When that size variation was a problem, they could standardize, somewhat, by using the body part of a ruler/monarch/lord/etc. instead of just some random individual. There is some historic individual who has made a big impression on all of these involved races. Measurements reference body parts and famous exploits of this ...


14

Poop! Poop! And more Poop! But seriously, dried animal dung is a very common fuel, even today in some places. You can burn it like wood. Dry dung fuel (or dry manure fuel) is animal feces that has been dried in order to be used as a fuel source. It is used as a fuel in many countries around the world. Using dry manure as a fuel source is an ...


14

It doesn't matter Both will be able to get manual labor jobs unavailable to the other. The elephant will be favored for jobs requiring strength while the mouse will do better with jobs requiring fine detail. But neither of those jobs will pay well. The best paying jobs will be those that require thinking. Unless the physical size comes with different ...


14

Basic sanitation, including the germ theory of many diseases. Animal husbandy and plant improvement based on Mendelian genetics. If they have the data and sensors, mid-term weather forecasts. Imagine a serf knew that there would likely be two more dry and sunny weeks before the harvest is due. Food preservation technologies like canning (this requires jars ...


13

Perhaps on your world, seaweed grows up to the surface like in the Sargasso Sea. If so, that seaweed could be harvested and dried into a building material. Perhaps its natural saps form a resin which make it almost wood-like once dried. So the harvesters pull it up, and lay it out in overlapping crisscrossed layers on open topped rafts. Need a one inch ...


12

Insects are both bane and boon... and much worse for low-tech than for high-tech. They're not ever going to knock people back down the technological totem pole, because 20 years is well within living memory. Any technology is going to be build with the assumption of once-every-20-years horrible insect swarms. Mostly, then, it adds another thing to the ...


12

First, welcome to Worldbuilding, halp! Let me preface this by saying that this answer @rek gives about volcanism is a good start, and it was my first thought, as well. But in considering the additional details you've given, you may want to go another route (that is, depending on how realistic you want to be). I'll expand on some of the finer points below. ...


12

The New Scientist article "Why the demise of civilisation may be inevitable" from Issue 2650, which is based on the work of Jared Diamond, in particular Collapse suggests that killing as few as 2% of the workforce could initiate a cascade of failures in infrastructure and transportation systems sufficient to bring down modern western civilisation; if those ...


11

Gamma Ray Burst. It would have pretty much all the same effects as the supernova in Fay Sugger's answer, except it affects a narrow beam. Nobody really knows how narrow the beam is, but I don't know of any reason why it couldn't be small enough to only hit the Earth. Fun fact, this may have already happened on Earth once. Gamma Ray Burst is a popular ...


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