India and China, together, are home to over 2.7 billion people. That's as many humans as in the next 20 most populous countries combined, or in the whole other 170 countries in the world.
Over the last 300 years, India and China each added more than a billion people to their populations, way more than any other country, but they have actually been growing at about the same rate as the rest of the world. They have a lot more people today because they had a lot more people a few hundred years ago, when the world began its period of modern, and rapid, population growth.
It's like comparing a big bank account and a small one. If they both grow at about the same rate over many years, the big one gains a lot more money, simply because it started with more. So the billion-dollar, er, billion-person question, is why did India and China have so many people when they started their rapid growth? We can’t know for sure, since so many variables factor into long-term population dynamics, but the two most plausible explanations are food and area.
Having lots of fertile land and good access to fresh water makes it possible to grow lots of food, which in turn makes it possible to nourish a lot of people. Even today, the 10 most populated countries in the world all have a relatively large amount of farmland. And Asia - and in particular south and east Asia - has tons of farmland, lots of river valleys, and the ability to grow food year-round.
Plus, domestication of plants and animals essentially started in Asia, giving populations an early leg - or wing - up. As such, Asia’s been the most densely populated region of the world for a really long time. Area matters too; countries like Pakistan and Bangladesh may be filled with farms and densely-populated, but because they’re smaller in area than India and China, they simply cannot contain as many people.
Of course, a few other things happened in India and China in the last few thousand years, and some of them helped the populations grow, and some of them shrank the populations, but through it all, the large and fertile lands of India and China were able to sustain lots of people, such that when the era of modern population growth came around, they had a head start. Or rather, a hundred-million-head start.
中国和印度一共有超过27亿人口，超过人口数排名第3到第23名国家人口的总和，甚至比剩下的170个国家和地区的人口总和都多，过去三百年，中印都增加了超过10亿人，虽然增长数量没有其他国家能比得上，但实际上他们的增长速率都差不多。
那究竟是为什么同样的增长速率，中印却增长了如此多的人口呢?其实很简单，他们古代的人口基数大，就像银行里存100万和100元的不同 相同利率却获得不同利息，巴拉巴拉(道理都懂不翻译了) 但是为什么他们在古代能拥有那么多的人?
在历史的长河里有太多的影响因素，我们还不能完全解释清楚，但有两个关键因素是能确定的——食物和生存空间，大片适合种植的土地和充裕的水源，可以养活很多人，甚至在今天也是如此。
在亚洲特别是东亚和南亚，有很多的农田和河流，有些地方能全年耕种，而且，有很多动物和植物的驯化起源于亚洲，这让人口的增加如虎添翼。
因此,亚洲成为世界上人口最稠密的地区很长时间了。生存空间也很重要，巴基斯坦和孟加拉国也有很多农田人口也很稠密，但是他们的国土面积比中印小，不能承受太多的人，当然，还有很多其他因素 有些让人口增长，有些使人口减少。综上所述，中印人口最多就不足为奇了。
