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I want to tell a story about two sentient species inhabiting the same planet, but never meeting. Essentially, some kind of random event (every couple of days up to every couple of months) causes species A to disappear temporarily (until the next event) and species B to appear. On the next event they switch. It could be some kind of process where their bodies actually disappear or temporarily dissolve, but a scenario where the affected species retreat, e.g. to the bottom of lakes or into caves, would also be possible. I don't want a scenario where one species just stays in their houses and sleeps through the event, as that would mean that the other species would find their bodies. If the result is a process that causes the species to hide (instead of just disappearing), ideally the process would work in such a way that it would be hard to adapt to both extremes. One species would adapt to one end and the other one to the other end. Otherwise both species would just be present all the time.

So far, I've mostly thought about the air rapidly and radically changing its chemical composition. Both species adapted to one type of atmosphere and developed some kind of hibernation in the course of their evolution. One could retreat to the bottom of lakes, while the other one goes underground. During their "absence" they don't breathe and are not conscious. Which natural process on Earth (or a similar planet) could explain this? By "similar planet" I mean that I would like a setting that feels as close to earth as possible (similar climate zones, day-night cycle, seasons). Differences are okay, but a pure water or lava planet wouldn't be.

You can also give other answers as long as they satisfy these conditions:

  • Causes one species to be absent while the other one is there, either because their bodies actually disappear, or because they voluntarily retreat to an inaccessible place
  • It must be possible for plants to have adapted to it. The idea is not that the surface of the earth gets regularly sterilized by gamma rays. When one species returns, they still recognize the face of the earth, roads are still there etc
  • Both species share more or less the same environment (it's okay for places to exist where only one species can live), without actually meeting and only having indirect hints about the other one's existence
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  • $\begingroup$ Even if they never met, wouldn't they be able to find traces of the other, and leave messages? Even ignoring that they might find the other species purely by accident as a result of mining/research. $\endgroup$
    – techno156
    Aug 12 at 17:39
  • $\begingroup$ Yes, they're supposed to find traces, but never actually meet directly. $\endgroup$
    – C4X
    Aug 12 at 23:21

4 Answers 4

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The simplest thing would be radical temperature changes. During the day temperatures get very high, at night they plummet very low. One species thrives in the hot temperatures but can't survive the cold, and the other vice versa. Both retreat to caves or underground shelters when it's not "their time", as these would have a relatively constant, middle temperature. You can make the day/night cycle of the planet as long as necessary to give the time frames you want. Easy enough to posit plants that can survive both extremes. Plants can often survive greater environmental ranges than animals.

You could certainly get more exotic. Like, periodically it rains some chemical that is toxic to one species but that the other tolerates or even eats. This happens in the real world. For example, cyanide is toxic to humans but red spider mites eat it. (I'm not sure if they actually get nutrional value from it or just tolerate it.)

Maybe: The land periodically floods, either high tide or high water mark of a river. An aquatic or amphibious species thrives during flood stage. A mammallian species thrives during dry stage. The aquatic creatures retreat to the ocean during the dry stage; the mammals retreat to high ground during flood stage. I'm not sure if this would meet your requirements, but if not, maybe it can serve as a springboard to something that will. :-)

I think your bigger problem is when you say these are intelligent races. You specifically mentioned that they build roads. If species A comes out of hiding or hibernation and sees that a new road has been built while they were away, surely they would realize there must be another intelligent species somewhere to have built this. Or do they suppose that it somehow evolved by random processes? Maybe they say, "It wasn't here before we went into hibernation, it's here now, clearly it must have evolved in that time"? They could develop complex theories of how this happens. Seems a stretch to me, but whatever.

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  • $\begingroup$ temperature: see comment worldbuilding.stackexchange.com/a/248872/105335 toxic rain: Sounds promising! Do you know any natural process that could explain why a certain chemical only starts raining down randomly? I guess I can't just replace water with that chemical, as the other species would also need some kind of "water", and two different types of clouds sounds impractical. flooding: This would prevent the species from meeting indirectly intelligence: Seeing the road and realizing that someone else has built it. The mythology around this is part of the story. $\endgroup$
    – C4X
    Aug 12 at 23:18
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    $\begingroup$ @c4x Maybe the chemical is spewed into the air by a volcano or geyser. This doesn't happen every day, but at times that are difficult to predict. Or maybe the chemical is in a comet that passes by the planet periodically and it gets deposited in the atmosphere from the comet's tail. $\endgroup$
    – Jay
    Aug 13 at 4:39
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Take a large and hot star, with the liquid-surface-water zone (Goldilocks zone) relatively far out. The orbit of the planet is somewhat elliptical, ranging from the inner limit to the outer limit over the course of years. One species is optimized for the hot times and and hibernates through the cold times, one species is optimized for the cold times and aestivates during the hot times.

The problem with that, of course, is that empty houses and fields may be found during the 'off' season. Here the size of the star and the length of the local year come into play. Both species have developed the habit since their prehistorical times to take all their possessions with them into their hiding place. The change in seasons takes long enough, and is violent enough, to have natural processes like floods and storms eradicate most homes, so what isn't taken along would be smashed.

The explanation becomes less and less credible as the species develop technology. Even if monster hurricanes smash a castle, the ruins would remain distinctive. And both species might have underground mineshafts which run into each other. Even if they are temporarily unoccupied, each would have distinctive ways of doing things, and the found artifacts would be 'weird.'

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  • $\begingroup$ I like it! Would extreme temperature changes that still allow plants to survive both extremes be extreme enough to force people out of their houses? It's important that they don't regularly see the other species bodies. $\endgroup$
    – C4X
    Aug 12 at 23:08
  • $\begingroup$ @C4X, assume that there are scavenger animals which would eat a hibernating sentient unless that sentient gets into a snow cave in the mountains or a loam cave in a lakebed and shuts the access from the inside. That means most would pack up and move to their hideouts. But some would always be late in each season, and die on the way. You cannot reasonably keep both species ignorant. $\endgroup$
    – o.m.
    Aug 13 at 4:17
  • $\begingroup$ No, the idea is not to keep them absolutely ignorant, but neither should the existence of the other species be completely revealed. I want to keep them at a mysterious distance so to say. $\endgroup$
    – C4X
    Aug 14 at 11:37
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Here's my answer:

Think of the relationship between Trees and Animals - Trees need CO2 to grow and produce Oxygen, Animals needs Oxygen and produce CO2.

Lets take that idea to an extreme.

Species A as part of their respiration cycle produce a chemical that is toxic to them, once it reaches a certain concentration, they have to withdraw from the area and go into a hibernation-type cycle. The reason why this event isn't consistent depends on factors such as: Temperature, what they've eaten, how much they've had to work and a myriad of other things.

Species B needs the chemical produces by Species A to survive - but they require it to be in sufficient concentrations for them to consume it. The concentration they need is also the concentration at which it forces Species A into Hibernation.

Same as with Species A, the time it takes for them to ingest to the point where the concentration is lowered to where it's safe for Species A and insufficient for Species B is variable.

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  • $\begingroup$ This is a very elegant idea IMO as it directly couples the appearance/disappearance of the two species. But would it be possible to produce sufficient quantities of any toxic gas in a time frame of a couple of months max? AFAIK it took plants a geological time span to get to an oxygen concentration similar to today. $\endgroup$
    – C4X
    Aug 12 at 23:12
  • $\begingroup$ @c4x - it's your world, they can if you need it to ;) $\endgroup$
    – TheDemonLord
    Aug 13 at 0:30
  • $\begingroup$ Hehe I know :) I meant: Would it be physically possible without resorting to magic? I.e., would plants produce more oxygen if they had more CO2, light, and water? Or are there pyhsical "hard limits" where it becomes hard to believe? $\endgroup$
    – C4X
    Aug 14 at 11:39
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There are some cicadas whose life cycle is based on a 17 (or 13) years cycle: they reproduce, lay eggs and then the larvae stay underground for 17 years before surfacing and reproducing again.

Your species can do something similar. For conspecific creatures it makes sense that they have overlapping cycles, but two different species can have non overlapping cycles, so that when one is in the active stage, the other is in their quiet stage. They would never meet, and there wouldn't need to be any change in the environment for it to happen.

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    $\begingroup$ That's an interesting idea! AFAIK, with the cicadas, their life cycle developed as a way to make it harder for the life cycles of predators to align with theirs - when the cicadas hatch in the millions, the predators, still used to a normal year can only eat a small amount of the cicadas. Do you know of any animals that do something like this without being preyed on? Also, with the life cycle of cicadas it would be hard to have a real society, as the time they spend "awake" is much shorter than the time spent uderground. $\endgroup$
    – C4X
    Aug 12 at 16:17

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