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The world I'm building contains two sapient species; one is about Mesolithic-level in terms of technology, while the other is about as advanced as the ancient Egyptians or Sumerians. However, they're only found on one of three continents on the planet*, and almost all of the population has access to the same level of technology.

*There have been travellers who have explored the other lands, but only in small parties - there are no permanent settlements elsewhere.

How could this possibly be true? By the time the Egyptians were around, there were humans not only in Africa but also Europe, Asia, the Americas and Australia. Some clarifications:

• The other continents are not especially difficult to reach


• The continent the species is found on is about two-thirds the size of the Americas together


• The other continents have mild, relatively un-harsh environments

So, how could an intelligent race progress technologically to
Sumerian/Egyptian-level technology, while remaining on a single 30,000 sqkm continent, even though they have the capacity to colonize other lands and survive there?

Note the last part of the question, which rules out "they're too far away" or "it's too dangerous" as answers. Ideally, an answer should suggest a cultural ideology or decision so that the sapients deliberately choose not to expand, despite the obvious economic gains.

If I have left out any details, please ask and I will provide them.

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 contains a boring pretty much same but dull in comparison with the primary continent, as they also end up back home. Or exiled that wouldn't last 3 generations as these sapiens is even more fragile in birthing process our real world counterpart, nobody in their right mind would even try to bear offspring outside the complex healthcare support that primary continent have.

Even if all this condition will fail to prevent colony in the future, this scenario could halt colonizing process by few generations.

This is more WHY not HOW.

And I'm not sure it's possible. Both technological development and migration take place over multiple generations. Hundreds, thousands, tens of thousands of years.

How long has your community existed and been able to move around their own continent? That's how long they would have needed a reason not to go to a different continent. Or at least when their tech got to the point where those small groups of explorers did it.

Cultural ideologies don't last that long. We're talking at least thousands of years, right?

Your only hope is some sort of physical barrier that requires more advanced tech than they have (though the fact that some people have made it there and returned will belay that). They'd have to be reporting that the other continent isn't suitable for settlement. And you've said that isn't the case...

The physical barrier could be geographic, climate, foods that are indigestible (so travelers can bring their own food but can't live there...though this explanation wouldn't really happen), other peoples or animals who prey on this species, alien tech that stops them from migrating, I don't know, something.

But culture morphs a lot over the millennia and that just won't work as a reason.

One of the main problems in Ancient agriculture is that land gets less fertile after repeated harvesting. As civilization advances and villages turn into cities, they need large quantities of land to feed the population. And those lands can be too far away because a loaded cart can do only 25 km in a full day, so farmers need to be able to get to their lands and into the safety of the walled city each day. So the same lands are worked each year, without letting them regenerate.

In fact, empoverished land is cited as one of the causes of the Bronze Age Collapse (c. 1200 BC) and also the reason why the Egyptians suffered that collapse a bit less: the Nile brought each year new fertile land with its floodings.

So your civilization with Egyptian-level technology remains in the 30,000 sqkm continent because it's incredibly fertile. Maybe there is a yearly flooding, maybe a massive migration of insects that die and fertilize the terrain... something that keeps the land productive.

When your civilization made atempts of creating colonies in other continents, the land around the villages stopped being fertile after three or four years and famine and disease followed.

Maybe the sages of your civilization call it a curse. Maybe they think they are priviliged because they live in a land that it's always productive while the rest of the world doesn't want to yield food.

Whatever explanation, they don't leave their continent because they would die of hunger.