A history of population within the circle would be interesting.
The population of China since 2000 years ago appears to have been a series of plateaus with brief periods of increase and a couple of brief decreases. The plateaus represent populations at the limit of agricultural systems. The increases are due to introduction of new systems, such as rice cultivation further north, double-cropping of rice and so forth. The decreases are periods of famine, disease and war. http://www.china-profile.com/data/fig_pop_0-2050.htm
Possibly other civilizations within the circle follow the same pattern of stability, innovation and crises.
If I recall correctly, rice can sustain a much greater number of people per unit of land than any other crop, and this explains why East Asia is more densely populated than any other part of the world.
The big difference with rice is that you aren't dependent on the weather nearly as much as you are with corn and potatoes. Corn and Potatoes put out more calories per acre when the rains fall at the right times in the right amounts, but rice puts out those calories regardless of the weather.
Ultimately you can only sustain a population as large as your food output at the end of a two or thee year long drought.
Sugar cane and sweet potatoes also produce similar calories per acre, depending on the soil, climate, etc. Sugar also has the advantage of being durable to ship. It could be produced in high volumes on tropical plantations and shipped to Europe.
Before bulk ocean shipping of food, population was generally limited by calorie production in the local area.
Until the industrial revolution, the human population everywhere was in equilibrium with agricultural technology. Improvements to technology would lead to population growth until wealth per capita reached the same old level and people stopped having as many kids. Check out the book "A farewell to Alms" for a solid 10 chapters of historical records on this from around the world.
The first person to notice this universal truth about the human population was Malthus. He had the misfortune to make the observation right on the eve of era when it stopped being true.
A universal truth doesn’t stop being true... So it’s probably better to revise it to say that it behaves that way under certain conditions and those conditions have changed.
The population of China since 2000 years ago appears to have been a series of plateaus with brief periods of increase and a couple of brief decreases. The plateaus represent populations at the limit of agricultural systems. The increases are due to introduction of new systems, such as rice cultivation further north, double-cropping of rice and so forth. The decreases are periods of famine, disease and war. http://www.china-profile.com/data/fig_pop_0-2050.htm
Possibly other civilizations within the circle follow the same pattern of stability, innovation and crises.