Many other questions, as well as a few online sources, all agree that in order for me to have multiple sapient species in my world, they need to be either separated by a natural landscape or not a source of competition to each other. The easier solution here is to just set up a natural barrier in between the two species. What natural barriers can I use to keep my developing sapient species away from each other?
Barriers are tricky. They are only going to last until one of the species develops technology sufficient to overcome them. If these species are anything like humans, they will probably seek to overcome the barriers just because they are there if for no other reason. Real world example: the Pacific Ocean is an incredible barrier. It is something close to a third of the entire planet's surface, has huge waves, and until modern times, was very hard to navigate. Well, with something like stone age technology, Polynesians managed to not only explore most of it, but turn it into a highway and colonize numerous islands across vast distances.
A better solution might be to make the species function in a way that, if not symbiotic, is at least cooperative by nature. It makes sense that a world with more than one intelligent species would function this way. For example, what if something that one species considered a waste product was valuable to the other? What if one species was much smaller than the first, and just by virtue of living like rats in their cities, acted as pest control for them (inhibiting disease) because they do not tolerate other small creatures in their territory?
Perhaps one of the species reproduces very slowly, but they are more intelligent. They need labor to keep a mutual economy running and the more common, less intelligent species need work and technology/engineering. There are actually a lot of ways to integrate different species into an economic or biological system that benefits both sides.
I would recommend watching the 80s movie Dark Crystal. Not only were multiple species on that planet dependent on one another to reproduce, there were also examples of species that literally joined with one another to form a third species, but only at a specific time. Very creative stuff.
If you absolutely MUST have a barrier, a good read is Ringworld by Larry Niven. That "ringworld" structure (basically a slice of a dyson sphere) was so immense that it had artificial "mountain ranges" that simply continued to climb all the way out of the atmosphere. These "spill walls" kept the atmosphere inside the ring, and also served as an impossible barrier to any civilization that had not yet developed space travel. Maybe your planet has incredibly steep mountains because of some kind of un-erodable ore in the rocks?
I read in an old science fiction novella once about a planet that had a gravity well that was actually different at different latitudes. As you moved up or down, the gravity became greater or less. That might go past science fiction and into the realm of science fantasy (or it might be a very interesting, freak phenomenon), but it could serve to significantly limit the area of operations of one or both species (one might be too frail to survive the heavier gravity part of the environment).
Set up a world right on the inner edge of the "goldilocks" zone of its star.
The Polar regions, the Arctic and Antarctic, start at tropical biomes around Latitude 50° N or south, progressing to temperate from the arctic/antarctic circle to the pole. From 50° to what we would call the tropics (23° 27′) it's progressively hotter desert, and between the tropics it's bare often glowing hot land, possibly fusing or bubbling around the equator, with boiling seas.
Each race evolves independently at one of the poles. Until they reach at least 1940's levels of technology, possibly better, they won't be able to cross the inferno zone to discover the other habitable pole region and its life forms.
Use temperature as a barrier.
All natural planets are going to have a variable temperature between different parts of the world. Temperature gradients exist whether you’re on land, in an ocean, or flying high.
Most natural barriers fail to be reliable because once they are surpassed, the environment on the other side is still hospitable enough that the species can survive (relatively) unimpeded. Given a species with enough reproductive ability, it only takes a few successful pioneers to found colonies and establish a long-lasting foothold. Imagine, however, what happens when a sapient species adapted to sweltering year-round heat in the tropics crosses their massive body of water and finds themselves in freezing tundra? Not only would it be a serious challenge for them to survive, but the climate difference may completely remove any interest in colonization.
Adaptability of each species is a concern for maintaining separation (see human history). You would likely want more extreme temperature gradients between species, perhaps with one residing near the poles and the other at the equator. Temperature also has the unique situation of not necessarily inhibiting both species evenly: it’s easier with basic technology to maintain warmth than it is to cool off (though your mileage here may vary depending on biology).
Like any other barrier, temperature won’t keep them separate forever. Technology can mitigate climate differences and globalization by either species will inevitably bring them into extended contact. Resources are limited and it’s likely that there will be overlap in valued resources between the two groups. However, climate mitigation is a serious expense for an invading species. It creates a substantial home-turf advantage for the defenders that will help them maintain separation for quite some time.
The primary issue is to ensure the various species are not in competition for the same resources. In practice, this means they will have to evolve in separate biomes with very clear boundaries between them, and not just physical barriers like mountain ranges either.
In fact, for a sentient species, physical barriers are hardly an issue at all. The Ancestors came out of Africa near the end of an Ice Age and pretty much walked around the world, outcompeting everything from megafauna, Sabre toothed tigers, super volcanoes and even related species like the Neanderthal and Denisovans. Little things like glaciers, mountains and oceans hardly slowed the Ancestors down....
If sentience arises at nearly the same time on land and in the oceans, then the two species might discover each other as the terrestrial species takes to boating or the oceanic species starts exploring land. They will probably relate to each other as some sort of spirit being from the other realm which can talk after a fashion and can sometimes be persuaded to trade amazing things. There may be some limited competition between the species at the interface between land and ocean, but since they are so different and have so little overlap of resources, genocidal wars of extermination don't seem to be in the cards.
I’ve read about a hypothesis that the Americas were not successfully colonised by humans—not for lack of trying—until certain megafauna died out.
You could have a region that is impassible due to what lives there, with normal barriers around it. The sapent species can conquer the barrier but can’t diffuse into the new area.
Only once open-ocean voyages become possible would they skip that area, or try settling in earnest rather than just expanding a few miles per generation.
Make one species adapted to and dependent extreme conditions
If you have two species with modern technology, they will both spread across the natural boundaries of their terrestrial limits. The way to avoid this while living on the same planet is one species is adapted to extreme conditions (using humans as a baseline) and thus cannot survive outside of that extreme condition easily. I think the most salient example would be a deep-sea species, whose bodies are designed to live at certain pressures and derive energy primarily through geothermal methods. This would basically force non-interaction between them and a normal terrestial species.
The 1950's British weekly science fiction comic strip, Dan Dare, Pilot of the Future, had the planet Venus separated into two independent hemispheres by the Flame Belt. Each hemisphere had its own sapient species, the evil Treens in the Northern hemisphere and the extremely nice Therons in the Southern. By the time Dan Dare and Friends reached Venus the Treens and Therons had been at war for millennia. Actually the Therons developed technology first and sent expeditions north where they discovered the primitive Treens. Once the Therons gave Treens technology things turned nasty. The Treens built an insanely technocratic and militaristic society bent on the conquest of the entire universe. Just shows how good intentions can go wrong, doesn't it?
The important bit here is the nature of the Flame Belt. This was essentially a seismically active equatorial zone girdling the planet Venus. Boiling lava, hot bubbling mud, volcanoes and steaming geysers. Something like the North island of New Zealand wrapped around the midriff of the planet Venus, but turned up to eleven.
This example was used to show one way this notion has been done before. Now to tackle the problem of keeping two sapient species apart so they can evolve independently.
The answer is, of course, to choose the right sort of planet that place it in the right orbit around its primary star. The planet will have deep oceans. This suggests it could be a high gravity planet because there are studies that indicate high gravity planets will have deep oceans. The planet orbits close to its primary star, so the star needs to be cooler than our Sun. For example, a red dwarf star. The planet will be in an orbital resonance such that it isn't tidally locked. So it will enjoy a day-night cycle, perhaps, something to that of Mercury. The timing for this will depend on the OP's requirements for his story. The planet's orbit and star can be adjusted accordingly.
Our two sapient species will evolve on separate island continents. Each island continent will have its own independent biogeography. This does not preclude the possibility that the two island continents were part of a supercontinent but due to continental drift had broken apart hundreds of millions of years ago.
One important geographical factor is the location of the two island continents on the surface of the planet. They must be situated such that there is circumnavigation zone for winds and weather to circulate around the globe, both above and below each island continent, without any major land masses to act as "wind or weather breaks". This means both island continents will be caught between weather and wind systems like the Roaring Forties but supersized on steroids that help seal them off from the rest of their planet. Since most of the rest of the planet is deep ocean that won't be much of loss, except possibly for islands scattered here and there.
These powerful weather systems will be driven by the heat of the primary star acting on the deep oceans and setting up permanent storm and wind systems. This is reason for placing the planet in close orbit with a cooler star, but a star that is still hot enough to produce the turbo-charged weather patterns to keep the two sapients separated on their own island continents.
The OP can decide whether or not there should be, say, another major island continent, but without sapient life just to give the planet some extra variety. Other islands can be scattered about according to taste and any need for additional color and excitement in the story.
Connecting the two island continents as part of an ancient supercontinent way back in deep time, anchors the development and evolution of the two separate evolutionary domains to what was originally a common evolutionary history. Their biogeographical isolation will have enabled their divergence and propagation into two distinctly different biological domains.
The best geographical features for separation of intelligent human like species would be large oceans. Desert, mountains, swamp, deep forests; humans walked across all of them as lowly hunter/gatherers. The ocean boundaries were slightly more effective.
Looking at unique areas of Earth biology, they are mostly large islands (or isolated continents): Madagascar, Australia, or the American Continent.
Early Homo Sapiens expansion was aided by the lower ocean levels and associated land bridges. Your continents would need to be better isolated, such that no land bridges or winter glaciers would allow direct walking travel between them.
There is still going to be a lot of leakage across the boundaries once even rudimentary sailing technology develops in either group. The vast distances covered by the various Pacific Islanders proves that once boats are invented the species will spread.
Your two species could develop like this, but once large scale contact is made, based on Earth history, it is not likely to be very nice for the less advanced species. Coexistence is unlikely to last, wholesale enslavement or extermination of the other species is likely.
For comparison, the other known intelligent species to develop on Earth, Neanderthals, only lasted a few thousand years against the only slightly more advanced homo sapiens.
Thinking outside the box, how about a binary planet? Two lobes close together: in Rocheworld they were close enough to share an atmosphere. This would alow for a common biosphere with exchange of microbes, seeds, etc. but any animal would have a very hard time crossing, even up to 19th century technology.
Call it a real air gap.
Differing biomes come back to mind possibly with some innately built in mistrust. Say one specie cannot live for long or well outside of adaptation to tundra based climate. Simply overheats and dies when below a certain longitude.
Much the opposite for the next specie super adapted to hot dry desert biomes unlikely or unwilling to travel outside desert.
You could go on and on with planetary "bands" or biomes at different longitudes or rings up and down the planet each with its own over specialized inhabitant(s) unwilling or unable to explore past its own biome.
If you want to keep them unaware of one another natural barriers wouldn't be enough. As mentioned in another answer because technology grows and we tend to expand and explore there has to be another factor involved. If you want to keep them separate for a while longer even after technology has advanced to the point of allowing them to cross the oceans and mountains, and allows them to survive in habitats they aren't suited to there is a very simple solution.
Make a religion. It's the oldest trick in the book. And if you don't want a full on religion, make myths about it. Have you ever heard of the Boogey Man? Not as popular anymore, it was once one of the easiest ways to get kids to behave. If you make this elaborate enough adults will believe it too. This might require a conspiracy where the leadership of the two species know about each other and agree that it would be best not to be in contact.
It is also possible that a long time ago the two species were aware of each other and the elders decided it was best not continue co-habitating (for whatever reason, possibly a terrible war that threatened both with extinction) so the species split and they created the myths and didn't tell their children that they were only myths. Over generations these myths would be expanded on and new stigmas would arise around them.
There are some species based physiological barriers you could use depending on your setting:
- Diurnal-nocturnal split, one species only comes out in the deep night and the other only after the sun is well up.
- Pressure differential, a species that relies on a certain level of atmosphere cannot necessarily survive at higher or lower pressures and so species can be separated that way.
- Chemical reliance, a species that relies on particular plants or mineral deposits as a dietary staple that are both highly toxic to other lifeforms and restricted to a particularly environment will not have a lot of competition for that environment nor company in it.
Now separations of any sort will not be completely effective, you will get some interactions, or at least stories of interactions, and in many cases these divisions will fall down as civilisations advance but if the rare interactions between species result in traditions of "drastic location avoidance" this may not be an issue. "Don't go into the woods or the (insert name of boogeyman here) will get you" only based on a real creature, or creatures, and actual disappearances.
Evolution proceeds normally up to a proto-sapient species and then a very large comet comes very close and is captured (the world must have a big moon for this to happen.) It's in a retrograde orbit with a periapsis not very far outside the atmosphere. Since it passes within the Roche limit of the planet it's shattered. Rather than one cataclysmic impact it (and the gases it releases as it boils) rain down on the planet in a tolerable fashion.
The lowlands now have an atmospheric density that is not tolerable to the proto-sapient. For humans this would require about quadrupling the atmospheric pressure to cut the world apart into a bunch of highlands. Short excursions into the lower altitudes would be possible but trying to walk to another highland is death.
Note that in time tolerance would evolve. I do not know how fast this would proceed.
A deadly disease.
Consider a world that has evolved as Earth has up to 5 million years ago. At this point two things evolve:
1) A bloodsucking insect. It can cope with a lot climate-wise but it's not tolerant of thin air and thus lives only in the lowlands. It's quite small, it's bites generally go unnoticed. It's also small enough that short of a magnifying glass nobody is going to specifically identify it as something different than the various gnats and the like.
2) A disease that's fairly benign to most things but extremely deadly to all apes. It's a blood vector only disease, but the insects carry it from host to host.
Result: The lowlands pretty much everywhere are death to any ape, but since the insects can't function in the highlands those are safe for apes. The world is thus divided into a set of highlands, each with it's own ape population.
As time passes sapients evolve in each highland but to attempt to travel to another area brings death. Eventually they will figure out the problem but it's still not going to be easy to cross--you will need the equivalent of chemical warfare gear except the contamination threat is airborne, not merely surface contact.
I would expect first contact to be by radio, first physical contact via airships.
Note that with both of my answers to this you have a big problem that your sapients are extremely unlikely to evolve sufficiently in parallel to have two sapients coexisting but with neither having the tech to reach the other.