What other areas could match India/China's population?

So, I was reading up on some information about the population of India and China, and the question in the title came into my mind. They both have the advantage of highly developed river valleys, and they're not hurt by surrounding desert like Mesopotamia or the Nile. The only places that come to my mind are the Mississippi Valley, if it had the same history of development that these two had had, or possibly some sort of hyper-developed Europe. But would even those be able to support such massive populations, or are there other places which could do so?
 
Maybe a more developed Ukraine or a more developed Amazon river delta? If you introduced rice and bananas and goats into the area, along with the fishing, it could support a large population. Idk.
 
Maybe a more developed Ukraine or a more developed Amazon river delta? If you introduced rice and bananas and goats into the area, along with the fishing, it could support a large population. Idk.
Ukraine is too cold to support a really large population (multiple cropping is impossible here). The Amazon valley with its hot and wet climate, on the other hand, seems plausible, especially if ATL Amazon peoples manage to cultivate most of it without causing an ecological disaster (tropical forests are notorious for their vulnerability to human impact - their fertility potential is contained in the trees and bushes rather than in the soil, and so deforestation often leads to an abrupt fall of biological productivity).
 
Ukraine is too cold to support a really large population (multiple cropping is impossible here). The Amazon valley with its hot and wet climate, on the other hand, seems plausible, especially if ATL Amazon peoples manage to cultivate most of it without causing an ecological disaster (tropical forests are notorious for their vulnerability to human impact - their fertility potential is contained in the trees and bushes rather than in the soil, and so deforestation often leads to an abrupt fall of biological productivity).

Except they did - they invented the notoriously fertile soil called terra preta, although they seem to have lost the technology a few centuries before contact with the Europeans.
 
Except they did - they invented the notoriously fertile soil called terra preta, although they seem to have lost the technology a few centuries before contact with the Europeans.
I know about terra preta, but to become really numerous, the Amazonians would need to cultivate the entire valley (or better still, the entire rainforest area) in this manner, and never lose this technology. AFAIK, it was used in narrow strips immediately beside the river OTL.
 
I suspect the Fertile Crescent without the Mongol invasions could be like this.

I don't know if it might reach the levels of China and India, but indeed. I remember to have read somewhere that the Mongols destroyed the centuries-old irrigation systems in Mesopotamia, which had been improved by the Persians. It was the only thing separating the genuine "Fertile Crescent" from a barren desert.

Perhaps somewhere in Africa can sustain a large population? I'm not sure about the Nile Valley (as its too a thin stretch of land, excepting the parts near to its source and the river delta, which always housed the majority of its population), but a more developed and environmental conscious Western Africa, south of the Sahel and the Congo Basin.

What about Equatorial East Africa, around the Great Lakes region? It seems to have a suitable climate, geography which allows for expansive and numerous settlements, and plenty of natural resources. I also have the impression (might be mistaken) but its populace has a bit less exposure to deadly tropical diseases than the western part, inside the jungles.

EDIT: I think this map might be helpful - its shows the percentage of arable land by country.
Arable_land_percent_world.png
 
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