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Let's say we have an Earth-sized planet and the same general solar activity and climatology. However, the continents are much smaller, and it's effectively impossible to accomplish travel between them (due to distance, inability to navigate, lethal oceanic weather, or something). That is, any island continent is effectively cut off and isolated. What is the smallest land-mass that would be required to support a human civilization with cities and technology to the level of an ancient culture (e.g., Sumeria, Egypt, Rome)? And what is the optimal terrain and natural resources to achieve that minimum?

Note this is different from this question which asks for the smallest planet that could support a civilization, in which the answers focus largely on issues of gravity difference. In my question I'm stipulating Earth-like size and gravity, but wondering what the minimal dry land mass would be required to support an organized human civilization approximately similar to something in our history.

Edit: As a point of clarification, assume that this question is about only the support of a human civilization with ancient-city-level technology. That is: Possibly the human civilization has been set down fully-formed by magic or advanced technology. The question subsumes the requirement that the population be large enough to maintain a healthy genetic pool over a long period of time.

A separate question has been made on the topic of the smallest land area to develop (evolve) such a human civilization.

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    $\begingroup$ Sumerian, Egyptian and Roman culture spanned over few thousands year, and all of them started from small villages. It would help if you could give a more precise temporal reference. $\endgroup$ – L.Dutch - Reinstate Monica Jul 15 at 5:18
  • $\begingroup$ my guess is the area (and thus supportable human pop) would be below the lower bounds for both evolution and healthy gene-pool for procreation... $\endgroup$ – bukwyrm Jul 15 at 5:40
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    $\begingroup$ Easter Island was pretty isolated during most of the time people lived there. The culture there was pretty advanced at one point, but degenerated, possibly because all the forests were cut down (though that part may be oversimplification). This could taken as evidence that an island this size is large enough to sustain a civilization, but also that such a civilization would not be sustainable. Similarly, some evidence have been found on the Azores.suggesting that there might have been a more advanced civilization there than there was when Europeans rediscovered the Islands. $\endgroup$ – Klaus Æ. Mogensen Jul 15 at 8:33
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    $\begingroup$ I can't recall exactly which island it is, but it's an example of "too small." There's a small-ish island near Australia where the population was insufficient to maintain the culture. Gradually they were losing features. The first thing they lost was the ability to build boats big enough to get to another island. The population was not big enough to sustain all the different job skills, so when a family didn't have enough children make it to adulthood, and that was the family that traditionally had a skill, then that skill would get forgotten. $\endgroup$ – puppetsock reinstate Monica Jul 15 at 13:29
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    $\begingroup$ I see two different questions here. One is the smallest landmass to develop a civilization (from apes to bronze age humans), and another one is the smallest landmass to sustain a civilization (arriving settlers already possess the technology). It appears that the question being asked is former rather than latter, right? $\endgroup$ – Alexander Jul 15 at 16:05
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There is a view in philosophy that civilization start to grow when you have everything in excess. I think it was Władysław Tatarkiewicz who said that Roman, Greek and Egyptian culture and philosophy flourished because they had warm long days, olives and cheese to eat and wine do drink. And they just go around and wonder.
Of course they were all having wars, and a lot of seafaring which added to their resource sources.

Regarding just food here's a question about how much people you could feed from 1 km² of land. How many people can you feed per square-kilometer of farmland?
Now, you have to remember that Romans, Greeks, Egyptian relied HEAVILY on slaves. It's estimated that in city state of Athens where they had 5000 men eligible for voting there was 50k of slaves. Conservative math of a family of 2+2 would give you 70 thousand people.
From linked question (and answer) and just simply crop rotation you would need 56 square kilometres.

Now a small reality check - Crete, an island that was a cradle of Bronze Age Aegean civilization, the Minoans only have 8 sq km.
So now you can conclude that there was way less people on the Crete than those estimated 70k for Athens. OR the Minoans relied heavy on sails. With the sea the thing is that you may fish in one spot maybe 100 metres square, but everything below is moving. So much larger ecosystem is providing food.

And now back to our first thought. Excess. People crave a little different buzz than just potatoes. Or Fish & Chips. So your island would need to be mild in weather, allowing to grow different types of food all year round in excess for humans AND animals. I don't know the food requirement of sheep (the most popular source of meat in Greek mythology) but pig for example eat the same amount as human do daily (per Cows, Pigs, Wars, and Witches: The Riddles of Culture by Marvin Harris) so you can exclude those as breeding animal.

Now in those ancient cultures most common metal is bronze. So you would need some space for excavation of copper AND coal (because you don't want to waste space for trees that don't breed fruits)

So I would say that 70 square kilometres is a safe estimation WITH the requirement of being able to catch and eat seafood.

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    $\begingroup$ Crete is not 8 but more than 8000 square km big... $\endgroup$ – bukwyrm Jul 16 at 5:11
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It's not entirely a matter of size, but of natural resources. An island like Britain did fairly well in developing technology up to the Industrial Revolution level without a lot of imports, because it had a diverse geology, with lots of mineral resources. OTOH, a volcanic island like Hawai'i would have little in the way of minerals.

As comments point out, you have the problem of how your people evolved on such small islands, but let's assume they were landed by space ships, which either left or stopped working.

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    $\begingroup$ It is ultimately a matter of size because you have to define the smallest number of people that could be considered a civilisation, and then the land area required to feed them. Remembering that you're looking at low intensity agriculture. $\endgroup$ – Separatrix Jul 15 at 7:16
  • $\begingroup$ Agree with @Separatrix. Assume an optimally-resourced piece of land. I'm hoping someone can describe those optimal natural resources (as posed in the question), and also how big that land is. $\endgroup$ – Daniel R. Collins Jul 15 at 16:20
  • $\begingroup$ @Separatrix: The OP asked for civilization, not low-intensity agriculture. $\endgroup$ – jamesqf Jul 15 at 20:43
  • $\begingroup$ @jamesqf: Is that not a necessary requirement for an ancient-style culture? $\endgroup$ – Daniel R. Collins Jul 15 at 22:39
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    $\begingroup$ "Low intensity agriculture" was a note to not use modern agricultural outputs when calculating the land area required for your population. $\endgroup$ – Separatrix Jul 16 at 7:34
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Easter Island-sized landmass is probably a reasonable minimum to sustain a civilization.

As @Klaus Æ. Mogensen reflected in a comment, the history of Easter Island can suggest that this landmass can be both sustainable and unsustainable for a civilization. It was a home for a sophisticated nation of Polynesian origin for 500-1000 years before the eventual collapse. The simplest explanation is that this land is just barely sufficient for a complex civilization. The surface area of Easter Island is 163 sq. km, or 63 sq. mi

The island inhabitants, Rapa Nui, were less developed technologically than Egyptians, and of course Romans, but, in our case, if we assume that inhabitants already possess an iron-age level technology and have sufficient population and resources, there is little reason to think that it would be abandoned.

At its peak, Easter Island had population of 15-20 thousands, which might have been unsustainable in long term. If the island was located in a more favorable climate, this population would have been sustainable. It is also worth mentioning that 15-20 thousand is adequate for a classical city-state.

There are some requirements to keep this civilization stable:

  1. Availability of high yield crops, like potato, sweet potato or corn;
  2. Isolation from pest and diseases;
  3. Availability of minerals and ores necessary to maintain tech level;
  4. Environmental awareness
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    $\begingroup$ +1 as point of data. However, I'm unlikely to select this as the best answer, because the Easter Island example didn't have a built-up city. Also, granted Easter Islanders only used stone tools, perhaps more geographic diversity is required to supply iron, etc. supplies? $\endgroup$ – Daniel R. Collins Jul 17 at 14:08
  • $\begingroup$ @Daniel R. Collins Yes, probably more diversity is needed here (whatever to satisfy #3 above). I don't know if Easter Island has iron ore deposits, but iron is something that is widely available. On the other hand, copper and tin is much harder to get by on one small island. $\endgroup$ – Alexander Jul 17 at 16:02
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North Sentinel Island is a 23 sq km island off the coast of India which has supported an un-contacted (and very hostile) tribe estimated to be in the population range of 50-400 individuals and believed to have called the island home for 6,000 years with contact with the outside world limited to a handful of documented incidents. Their nearest cultural relatives, the Onge people, were brouht into contact with the Islanders in the 19th century by the British but found the language incomprehensible despite very similar cutural practices, suggesting the isolation had occured at some significant period in the past to affect linguistic drift. By comparison Modern English speakers can still make out similar root words to their own language in both Modern French and Modern German, near linguistic relatives to English.

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    $\begingroup$ I don't believe they meet ancient civilization levels of tech though. $\endgroup$ – Nosajimiki - Reinstate Monica Jul 16 at 20:40
  • $\begingroup$ Note the question asks for a civilization with built-up cities. The estimated 10 total households on North Sentinel Island wouldn't count. $\endgroup$ – Daniel R. Collins Jul 16 at 21:41

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