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(Related to this question about the smallest land mass to support a civilization, possibly dropped in fully-formed by magic or advanced technology. This question is asked in response to comments there asking that the questions be separated.)

Assume a planet of Earth size and general composition. Presumably if the planet was entirely covered in water, then mammalian/primate life as we know it could not have evolved, and human civilization as we recognize it could not have developed. I imagine that there is some lower-bound for the dry land mass that would be required to support that development. What is it?

I'm looking for an alternate historical sequence that could lead to human society to the technological level of ancient cultures with organized cities with populations of tens-of-thousands (e.g.: Babylonian, Egyptian, Roman; pick whichever has the smallest dry-land requirement, if it matters greatly). Assume an expanse of land with optimal natural resources (whatever that would be; please specify if possible).

Edit: Assume specifically that we're looking for the evolution of a humanoid, intelligent, and landbound lifeform.

Daniel R. Collins
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    Anything is possible if you the creator of your world want it to be. Unfortunately, the development of technology is a very iffy subject. Many discoveries were accidental not inspired, and its extremely hard to accurately link all the different industries required to build a civilization up to any age. There is often a simple test you can run to determine if a question is too broad called the book test. If the answer would take a book to explain, its too board. Any answer to this would take up an entire library... several libraries. – Shadowzee Jul 17 '19 at 02:33
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    What constitutes a 'civilization' or an appropriate level of technological advancement? What is considered technologically advanced depends on the needs of the environment, and what constitutes a civilization depends on the definition. – Andrew Fan Jul 17 '19 at 03:11
  • You're aiming too low. The issue is not, how much land mass is needed to support a civilization, but how much land mass is needed for humanoid, intelligent and landbound life to evolve. My guess is: at least Africa. – Erik Jul 17 '19 at 08:32
  • @Erik: I've edited the question to include that, thanks. – Daniel R. Collins Jul 17 '19 at 14:12
  • @AndrewFan: Unfortunately, I'm unclear what kind of extra detail you want. I've noted the need for built-up cities and given three society examples from which the answerer can pick. Can you provide more detail on what parameters you think should be specified? – Daniel R. Collins Jul 17 '19 at 14:13
  • What exactly constitutes the lower bound for a civilization? You mention Babylonia, Egypt, and Rome, but all of those originated from somewhere, and their control fluctuated greatly over their existence. In addition, would a sea and trade based society such as Venice be considered 'human civilization'? It had very little land mass and depended heavily on the presence of other civilizations but was incredibly prosperous. – Andrew Fan Jul 17 '19 at 15:34
  • IMHO this question is not broad, but rather very specific. However, it is hard to argue scientifically and therefore would be prone to opinion-based answers. – Alexander Jul 17 '19 at 18:10
  • @Alexander: The Venice area alone, if dependent on outside trade, would not count. The question here is asking for the total completely isolated land area needed to evolve human-type organisms and civilization. Or in other words, the data-point of the Venice civilization would count if we added up all the outside land area needed to support its development. – Daniel R. Collins Jul 17 '19 at 21:17
  • @Alexander: I agree with you that the proper answer should be very specific: e.g., ideally a single number: X square miles (with justification). I also agree that this is a hard question. – Daniel R. Collins Jul 17 '19 at 21:19

2 Answers2

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Given a historical sample size of exactly one? 84,980,532 square kilometres.

Humans evolved in Africa, and we have no way to know whether or not the same species could have evolved on a smaller landmass.

Due to the fact that Africa is joined to Eurasia, I have considered them together as a single whole.

Arkenstein XII
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Assuming an Early human population of ~20K homo sapiens And 40 hectares per person for a hunter gather culture under ideal circumstances. Given the abundant water on this world, i’d think the sea would provide enough food that ideal circumstances would be a reasonable assumption. Otherwise use 100 hectares per person

Then to give rise to an early civilization you need a minimum of ~1000 square miles of land in the form of islands or continents.

You’ll likely need more since the population will grow larger before agriculture can gain efficiency and generate more food per acre. But, I can’t imagine a science based way to estimate how much more. So my answer of ~1000 squares miles for early homo sapiens is just a starting point.

EDL
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