In my world, I have a reptilian overlord theme, with humanoid slaves. The planet revolves around a binary star system. What are the basic requirements of the climate and the ecosystem, for the reptilians to have evolved more than the humanoids?

The setting I have in mind right now is an arid world with lack of vegetation.

EDIT: What are the star temperature and atmosphere required to create a superior reptilian race? This should be kept in mind, that the humanoid race's evolution is hindered by the same atmospheric condition.

  • $\begingroup$ can you try to clarify your question? So far it is unclear to me what you are asking. $\endgroup$
    – L.Dutch
    Mar 17, 2017 at 5:44
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    $\begingroup$ Take me to your LIzard. zenbullets.com/blog/?p=1637 $\endgroup$ Mar 17, 2017 at 6:03
  • $\begingroup$ On a more serious note: Do you want humanoids, or mammaloids, which are filling the role of reptiles on Earth? Are your reptiles identical to the Reptilia therein — excepting their humanoid intelligence? $\endgroup$ Mar 17, 2017 at 6:06
  • $\begingroup$ Humanoids are not filling roles of reptiles on Earth. Rather, human beings are mostly stuck in Neanderthal phases of evolution. The reptiles are identical to reptilian, they have outer body scales, cold-blood and have slithering locomotion(but bipedal, in this case, the feet are converted into webbed feet) $\endgroup$
    – Elkosh
    Mar 17, 2017 at 6:22
  • $\begingroup$ You may want to consider giving the community a little more time before accepting an answer. $\endgroup$
    – Burki
    Mar 17, 2017 at 9:02

8 Answers 8


First and foremost, you need a world with two sentient species. On Earth, mankind is the top predator in most environments. We might not be able to wrestle a bear, but hands to grab pointed sticks and brains to coordinate the hunt are part of our evolutionary heritage.

The scientific understanding how homo sapiens replaced homo neanderthalensis has evolved over time, but it became clear that there could be only one. So there needs to be an explanation why the prehistoric ancestors of the reptiles didn't wipe the prehistoric ancestors of the humanoids out, or vice versa.

  • Two very different biotopes, one for the reptiles and one for the humanoids. They fight on the borders, but each has a core area that is unhealthy for the others. I'm thinking of dry/wet or hot/cold, but it could be lowlands/highlands or the area where insects transmit illness.
  • In the borderlands, warriors from one species would enslave peasants from the other species. They find it too dry/wet/hot/cold to live and breed there, but warriors overseeing slave pens are possible.
  • Then technological deveopment allowed the reptiles to travel in humanoid areas. This tipped the balance, the reptiles still had a sanctuary (which did not have to be defended by troops) but the humanoids had to fight everywhere. He who defends everything, defends nothing.
  • The more developed reptiles found ways to let humanoid slaves live and work in their heartlands, but by then the power relationship was clear.

Follow-up: The coincidence that both species evolved at the same time might be mitigated very slighty by assuming technological and cultural interchange which kept the two species roughly in sync. Mankind has a few hundred years with electrical power, but there were hundreds of thousands of years with the homo erectus. What if those early hominids had witnesses another species climbing out of the stone age, nearby?

  • $\begingroup$ I would really like some suggestion on the two different biotopes you mentioned. Specifically, the surface temperature, availability of water and seasonal cycle. $\endgroup$
    – Elkosh
    Mar 17, 2017 at 8:49
  • $\begingroup$ There is still the problem of the massive coincidence of two intelligent species evolving at the same time. In the history of macro scale life in Earth, humans went from rodent-sized critters to us in almost a blink of an eye. Why would two different unrelated intelligent life evolve at roughly the same time? It stretches credibility. $\endgroup$
    – Yakk
    Mar 17, 2017 at 18:52
  • $\begingroup$ @Yakk, see my follow-up. $\endgroup$
    – o.m.
    Mar 17, 2017 at 21:17
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    $\begingroup$ Geographical isolation could also work. The Bering see ice bridge is a bit of fluke in our history. Hominids existed for hundreds of thousands of years in Africa/Eurasia, but came to the Americas only 12K years ago. $\endgroup$ Mar 18, 2017 at 4:07
  • $\begingroup$ @o.m. can you please suggest the biotope feature? I was more interested in that. $\endgroup$
    – Elkosh
    Mar 18, 2017 at 18:53

Harry Harrison actually already wrote that story. You might want to raid the books for ideas. Or not.

In any case, you specify that the reptilians are cold blooded. This is problematic as brains generally require stable temperatures to work reliably. But problems are just unrecognized opportunities.

Let's start by assuming that your planet has large areas with stable temperatures and your reptilians evolved to take advantage of that. Stable here means that climate has been stable for millions of years and that the areas the reptilians live in have low seasonal variation in temperatures. No winter.

Advantage would be that with less energy spent on maintaining stable temperature, the reptilians would be much more energy efficient. This would mean less food required, which would enable higher population densities. It would also mean less oxidative stress and lower metabolic rate so the reptilians might reasonably live two or three centuries?

I am mentioning this here because if you decide to assume larger typical group size and longer life spans it will have effects on the reptilian society and civilization. Which might be fun flavor to have.

As for humans. I think the simplest explanation for having two sentient species is to assume that one, the reptilians, evolved naturally and the second evolved as a result of the first one meddling with its development.

So the reptilians would have domesticated monkeys evolved for night time activity or possibly apes evolved for climates with seasons depending on what was available. Domestication would have been for either pets or as food. Possibly both as with dogs. You'll have to decide based on what gives the proper relationship for your story. You might want to have reptilians consider humans food or not.

I think the simplest origin is to have monkeys stealing food from reptilians during night time when the cold blooded reptilians have difficulty responding to it and the reptilians domesticated some of the monkeys to guard their homes and food stores in exchange for certain food.

Over time the reptilians then bred the monkeys for larger size, better intellect, and not to shed fur on furniture and the result was "humans".

The reptilians would have wanted to use humans as proxies during night and cold weather so there is reasonable rationale for breeding them (us?) large and smart enough to use reptilian doors and tools without supervision.

At some point humans would have been "good enough" and reptilians would have started to spread away from their original tropical environment with heated houses for reptilians and humans for labor. This would eventually lead to autonomous human societies on temperate lands.


I don't think it's possible for these two species to have co-evolved together.

When people imagine reptilian overlords they pretty much imagine humans in a different form, I think that's more or less how you're describing them as well. Our high intelligence strategy is an ecological niche, and those cannot be shared by two species for long.

Geographical isolation is the get-out clause from this situation. This is what most of the answers here seem to be based on.

Perhaps the humans live in the cold areas etc. I don't want to duplicate that, but I'll suggest another possibility.

Perhaps you've happened upon a world where this geographic isolation has only recently been broken. The reptilians are very much a combination of conquistador and grey squirrel. They'll probably eradicate the humans in the end, but they're overloads for now. They don't actually have to be a 'better' species, just one that got dealt a better hand in the Guns Germs and Steel sense, .

Note that two sapient species have co-existed before irl. For most of the history of homo-sapiens, we've existed alongside other types of humans. But we won and they're all gone now.


It isn't plausible for two different relatively unrelated intelligent species to evolve intelligence at the same time, at least naturally.

The time it took for humans to evolve from rat-sized creatures to intelligent beings is a short one. 66 million years ago, mammals where tiny critters. Reptiles first arrived on land 300 million years ago. The ancestor of humans and apes was about 10 million years ago. We looked like modern humans (with the same voice box etc) for about 200,000 years.

Assuming you want civilization with cities and the like, that took place in the last 10,000 years. The one to reach civilization "first" might find and domesticate the other species. So you need two relatively random windows of 200,000 years to overlap with one ahead of the other over a domain of many many millions of years.

For this to work plausibly, you'll need coevolution. That means something about one being intelligent drove intelligence in the other.

The easiest way is genetic engineering. The "Reptiles" evolved first, and they genetically engineered a slave race. This requires "Reptiles" to be relatively high tech.

Slightly less plausible, but more so than accident, is that the "Reptiles" bred the humans to be smarter (but not to aggressive); a bit like how we bred dogs from wolves, but moreso. You could imagine the "Reptiles" finding the rough equivalent of Chimpanzees or Ouranopithecus and breeding "Humans" from them over their prehistory.

Having a climatic barrier against Reptiles being functional (for any length of time) where the Humans could be useful would both permit bred intelligent Humans to be useful (you train them to go into that territory and come back with resources), and permits "wild" human evolution (feral Humans not wiped out in that territory).

Now, metabolism wise, warm-blooded animals are going to be doing more stuff. Warm blooded creatures heat up their body in order to make some biological processes work better, and also have cooling mechanism to prevent overheating; in exchange, they burn more resources. So you might want to make your Reptiles warm-blooded, or at least less cold-blooded than many Reptiles. Reptiles might come with a cooling system but not an effective heating system, and somehow have "hot" territory where the Reptiles can function. Outside of there, Reptiles need both lots of heavy insulation and fire to keep their dwelling warm enough. In effect, what we'd call "temperate", they'd call "arctic".

This would restrict the Reptile civilization to the tropics for much of history. The domestication of the soft-body and training them to do hunting & gathering tasks would allow Reptiles to spread further away from the Tropics. They would initially be bred for being both docile and intelligent by marginal Reptile tribes.

Feral soft-bodies escape, and continue to evolve more intelligence. Reptiles breed the feral ones with their domesticated ones, breeding out the aggression and getting increased intelligence out of the wild genes.

Eventually there are 3 regions; the Hot central Reptile lands, the "Temperate" lands where Reptiles rule over Humans, and the "Cold" lands where Feral Human tribes roam.

The feral and tame humans are mostly on a one continent, isolated from the larger landmass like how the Americas is isolated from Eurasia+Africa.

Reptiles develop farming and civilization and tool use first in a "Hot" region of the super-continent, probably due to a mass climate disruption (like it apperas it happens on our planet). This also happens on the "Americas" continent, but due to the usual "Guns Germs and Steel" rules the larger continent advances faster.

They develop civilization, and during an age of exploration discover the "new world" with a mixture of soft-bodied Humans and primitive Reptiles. Where they interact, the soft-bodied Humans are slaves; they learn about how to enslave and domesticate the soft-bodies from the Reptile civilization over there. Lacking the "cold" region that the New World has, they have less fear of Feral soft-body packs. In addition, better Old World technology makes short work of the Feral soft-body geographic advantage in the cold parts of the New World.

A slave state starts its spread over the cold parts of the New World. Soft-bodies are exported to the Old World and used as servants in the Temperate parts of it. The New World Reptile civilization is conquored by Old World civilizations and somewhat merged, with its own slave-state habits.

The diseases from the high population density and trade Old World decemates the New World Reptile empire. Soft bodies exploit this. The war on them is harder than it was on the native americans of our world (as in our world, native americans where massively weakened by epidemics), but the technological edge remains huge.


Firstly the reptilians would need to become intelligent, this doesn't require anything fancy that we know about, interaction with their environment and normal flukes of evolution could have caused this over time. It's quite feasible that having high intelligence isn't just a matter of brain size, there are several animals with bigger brains than humans who don't have our reasoning power. It could have started with a more efficient use of reptilian brain through evolution and then as an advantageous trait led to increased brain size and more efficiency.

But to have humanoids as well you could have the following.

At least two continents separated for a long long time or always separate would be more interesting as they would have evolved totally differently from the first mud crawling fish.

If you want climate to be a factor in retarding humanoid evolution, make their continent like Australia, mostly desert with humanoids fully occupied with grubbing around for food, and scarce easy to get to natural resources and land not suitable for large scale agriculture without additional fertilisation. This means that they couldn't form large cooperative groups with a stake in their locality and would have to remain scattered living off the land around them and moving when they had exhausted it and caused conflict between the small groups over resources.

Reptilian Overlords became dominant on one and were so superior in technology and perhaps brute force that when they invented sea travel and found the other continent in which the humanoids were evolving they just took over and used the local fauna for whatever it was best suited for in their terms.

Breeding the humanoids into the forms best suited for them would depend on what they needed. Most likely they would breed for docility, loyalty and obedience as we have with dogs. Labelled all others as untameable animals and killed them off and thereafter killed any that became feral or otherwise dangerous.


I think your question is a non-starter. Reptiles came first, and when they became more highly evolved they became mammels and birds.

“old” reptiles are still around, too. Crocodiles are successful because they work well.

So, you might have reasons why none of the new designs lasted but crocks just kept going. But the crocks would not be more advanced, they'd be crocks.

Maybe you have a new evolutionary branch that, to us, seems “reptilian” somehow, even though they are just as far removed from reptiles as mammels and birds. That could be because their skin covering resembles that of lizards (even if it’s really different) rather than having a new non-reptile covering. Their outward appearance may look more like lizards.

Now why? The liniage that gave rise to them might never have developed a different skin, but got internal differences that led to their success and propagation of the line. Perhaps the heart and lungs, or digestive system, or anything really.

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    $\begingroup$ They are cold-blooded, but mainly walks on two legs. Not only the external skin that has not 'evolved'. I am asking about a reptile with bigger cranial capacity. I want to understand the facts which would have facilitated a higher brain-to-body ratio for a reptile, instead of human beings. $\endgroup$
    – Elkosh
    Mar 17, 2017 at 6:19
  • $\begingroup$ I think they would not have. They need the higher metabolism and far better oxygen delivery before getting big brains. $\endgroup$
    – JDługosz
    Mar 17, 2017 at 6:51
  • $\begingroup$ Asking specifically how a cold-blooded creature could support (and evolve) a large brain would make a good more-specific question (if it isn’t posted here already: here and there are nuggets scattered among these answers even if it’s talking about dragons or gill people). $\endgroup$
    – JDługosz
    Mar 17, 2017 at 6:53

Good answers here.

I have a bit of a problem with 'binary star system' You need to nail this down, and my astrophysics was 40 years ago.

  • Are planets stable around a binary system.

  • A long period binary with the planet circling just one of them would give you two superimposed sets of seasons. A normal one corresponding to the planet's year, and a much longer one corresponding to the orbit of the two stars. This provides the periodic 'super heat' year when the planet is effectively illuminated by two stars. This in turn with the changing environment puts a big premium on adaptability.

  • If you really want to give these guys grief, give them a fairly massive moon in an highly eccentric orbit. Earth's moon if perigee were just outside Roche's limit (40,000 km or so) would mean that one set of tides a month would be enormous (Tides go up with the 3rd power of distance, Put the moon 8 times closer, and you get 500 times the tidal force. This is not long term stable. Planetary engineering by a third race. With this sort of flexing there would be lots of volcanic activity too. Even if it was enough to have a monthly 50 meter tide, it would make oyster farming a much more challenging task.

This sort of set of conditions may mean that you have one species that is more warlike and dominant, but it gets getting destroyed down to a small alcove with the regular catastrophies. (In general this would knock both speices back.)

Now they both have legends of their previous encounters.


The reptilian homeworld(s) would probably need to be on the inner (closest to its star) edge of the Goldilocks zone. Hot enough to keep cold-blooded creatures from spending all their time chasing warmth to avoid torpor but not so hot that your slaves are dropping dead from heatstroke. Basically, both species need a mutually survivable ecosystem and climate. It'll have to have diverse flora and fauna to support the different diets of the reptilians and humans (at the very least, to keep the costs of feeding slaves reasonable).

If your reptilians and humans didn't co-evolve or otherwise can't take one another's climates, the slaves are probably being imported. From the slaves point of view, it might be considered going to a harsh life in a hellish environment. Mothers might discourage misbehavior with a warning that "the reptilians will drag you screaming to hell!" If the human climate is inappropriate for your reptilians, some humans may make a living capturing, transporting, and selling their fellows.

Finally, I don't think a species in an arid climate without much vegetation would adapt webbed feet (not large or deep enough bodies of water to need help swimming). If you meant a desert by your description, I don't know why--even if partially erect--the reptilians wouldn't develop more of a sidewinder style of locomotion (up to eighteen miles per hour, according to YouTube; but I couldn't find another source). If the two species are being held back evolutionarily for whatever reason, I'd suspect that it would end up like Earth: even if crocodiles became "intelligent," humans are going to be more versatile.




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