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人类如何从土地中获取能量,人口密度为何因此受限. Sal Khan 创建




for us modern humans with our easy access to the local supermarket it's easy to forget that throughout human history and even today the amount of humans or the human population has been limited by our ability to get calories or get human consumable food from the land so what I do in this video is give us a little bit of a framework for thinking about how humans have been getting calories from the land and what that how that's placed an upper limit on the number of humans that can live in any given area or the the population density of humans so right over here you have some gentlemen looking for food they are hunter-gatherers I'll say HG for short and the H part the hunter part might actually find some animals I think these guys right over here are trying to trap some rabbits and the gathering part they've just literally looking for food maybe they find fruit of some sort or some nuts or some or some maybe some roots that are that are edible by humans so literally they just walk around either try to kill things or find things that they can consume so I'll call this I'll call this right over here stage one unless you let me right over here so this is hunter-gatherers and this is what most humans have done through most of human history and just to give us a little bit of a framework for maybe how much they could get from the land and I looked at some of the that are best sense of studying hunter-gatherer populations in in land like this maybe they can get about 200 calories 200 200 and I'll make this whole column right over here this whole column right over here is the amount that they could get in terms of calories per square kilometer per calories per square kilometer per day now it's obviously going to be hugely dependent on the number of animals that are there the type of land that's there if they're next to a stream or maybe fish are just jumping out of the stream this number would be much higher if they were in some type of a desert this number would be much lower but this is actually fairly in line with some of the studies of hunter-gatherer cultures now if this is the number of calories that can they can get from each square kilometer per day how many humans can live in a square kilometre per day or what is the density of humans well to figure that out we have to know on average how many calories does a human need to survive and for the sake of this video I'm going to make the assumption that a human being needs 2000 2000 calories 2000 calories per day to survive in a non malnourished state and obviously it's usually dependent on how active this person is or how large they are and one other note this whole video I'm going to be using calories with a capital C and the calories are that with the capital C are the calories that people are used to referring to when you go to the gym and you run on the treadmill and says how many calories you burned or you look at the back of your candy bar and it says 200 calories these are the calories I'm talking about they are a slightly different notion than the calories that you encounter in chemistry class those calories are cap calorie with a lowercase C and just so that you can be optimally confused it turns out that one calorie one calorie with an uppercase C is equal to 1000 1000 calories with a lowercase C 1000 calories and the lower case C calories is the amount of energy needed to heat one gram of water 1 degrees Celsius and so this is what you see in your chemistry class but this is not what we're going to be talking about in this video we're talking about the capital C the calories you talk that that dietitians are always talking about so with this assumption that the average human needs 2,000 calories a day to not get malnourished and obviously men would need more women would need less children would need eat what children would need even less but with this assumption what is the density of humans that could be supported by this culture right over here well 200 calories is one tenth of the average daily human requirement if you believe this assumption so the population density so the density in let me write it humans humans per square kilometer you can only support one-tenth of a human with this with this calorie output so you can only support one-tenth of a human per square kilometer so one human would actually need 10 square kilometers of hunter-gathering uh to hunt from and gather from in order to support just themselves they would need maybe 30 or 40 square km to support an entire family so they could wander around and kill the animals and find whatever they need to find on that land now let's go to kind of you can view it as maybe the next stage although it's not always the case that herding is going to be more productive than hunter-gatherers especially in the case where the fish are jumping out of the water but let's go to this scenario right over here so this is we can call this a pastoral lifestyle so this is - I'll call it pastoral and over here is the realization that look you have all of this all of this vegetation that maybe humans can't consume but there are other animals that can consume this vegetation and they can turn those calories into calories that can consumed that can be consumed by human and namely them that the calories are themselves so this gentleman right over here is after he gets these these these sheep to be nice and fat he can either eat the Sheep or he can drink their milk so one way to think about is these these this these this cattle or these sheep right over here by hurting them and letting them eat the grass he's turning non-human consulted non-human consumable calories into human consumable calories and so for the sake of our thought experiment let's say we get a ten times increase in the human consumable calories per square kilometer so now instead of 200 we're up to 2,000 and so instead of one human per square kilometer we have enough calories per square kilometer per day to support one human so instead of I start instead of 0.1 we can now support one human so in that 10 square kilometers we can now support 10 people and 100 square kilometers we could now support a hundred people now the next stage and I'm skipping a bunch of stages so because you have things like substance subsistence agriculture and and various forms that they're not going to be equally productive and it depends what the land is like and it depends what the tools are at at your disposal but the next stage that I'll just kind of jump to we can call traditional agriculture so this right over here let's call that traditional agriculture and that's this one over here as well so both of these I'm going to call traditional agriculture and for the for the purposes of this video the difference between traditional agriculture and modern agriculture in traditional culture you didn't have mechanization so you didn't you had or very primitive mechanization you definitely did not have fossil fuel-based engines you didn't have modern pesticides you did not have modern genetically engineered crops but you did have some of the basic science of breeding crops and irrigating and using animals as tools so in this stage right over here and once again it completely depends on where you are on the planet how fertile the land is how good your tools are what crops you're actually producing let's assume that we got a hundredfold increase in productivity and I'm looking at some of the historical records it looks like depending on once again where you are that's not out of the realm of possibility so you have a hundredfold increase so instead of 2,000 calories per square kilometre per day you can get two hundred thousand two hundred thousand calories per square kilometre per day and now you could support 100 humans per square kilometre if you wanted to so you might not have a hundred humans one not all the land might you don't have to be able to farm from or they you just that's you there are other limits on the population for whatever they might be but the important thing to think about this upper bound in this traditional if you are able to get this type of productivity from your land and you're able to in theory support a hundred people per square kilometer that means if all of a sudden you have 200 people living there maybe everyone's migrated to this land because it seems especially fertile or all the really smart farmers live there then all of a sudden not everyone's going to be able to get 2,000 calories a day some people might not get malnourished other people might actually starve there's this upper bound on the actual number of people that can be there based on how productive the land actually is and now let's move over to modern agriculture and we've already talked a little bit about what exactly is modern agriculture you have you have machines like this combine over here that does a lot of the human labor one human can can and and I'll talk about the different dimensions because there's actually two dimensions here how much how much calories can you get from the land and how much energy can one human how much labor can one human you you know input into the land using tools at their disposal so in this case cattle knew these ox pulling this plow or in this case this combine that's fueled by fossil fuels but in modern agriculture because of all of the things you have these amazing tools you have genetically engineered crops you have modern pesticides and not everyone is a fan of all of these things but they have hugely increased our productivity so you have modern agriculture and let's say that you get another factor of ten from traditional agriculture so now you can get two million calories per square kilometre per day or you can support one thousand you can support one thousand humans one thousand humans per square kilometers and once again this right over here this right over here is an upper bound and just to give a sense and I picked these numbers just so that the numbers would be clean I looked at some historical records these aren't completely out of line with what it looks like humans have been able to do in the past but to give you a sense of what human population densities look like right now and why this upper bound seems to be right about correct in a place like the United States so in the United States the population density is 30 people per square kilometer so this is 30 humans per square kilometer in a more dense country or significantly more dense country like India the population density is 300 humans per square kilometer and in the most population density in the world which is where I come from or actually I was born in New Orleans but where my where some of my ancestors came from which is Bangladesh so there's a lot of people like me I guess in Bangladesh you have a population density of 900 humans per square kilometer to some degree this is a testament to the fertility of the land and whatever else but this is pretty near the limits depending on agricultural productivity and whatnot in the land of modern technology so it really makes you think if you don't get population under control you might end up with some of these kind of hitting the wall type of scenarios and so the last thing I want you to think about and this is why we're her to a little bit more is just think about those two dimensions because sometimes they get a little bit muddled one is the kind of the productivity of land productivity of land and then the other is the productivity of labour productivity of labour so right over here in a hunter-gatherer they're not getting money calories from their lands so they're right over there and the humans have to do all the labor they're not had they don't have animals helping them in any way they definitely don't have robots or any type of engines helping them in any way and so there's so they have to spend a lot of human time and a lot of human labour doing the work getting that productivity from the land but as we progress so as we progress with things that aid humans so for example if all of a sudden you have cattle helping you or you have other tools that help you you get more human productivity so less and less of human labor has to be used to get that productivity of the land and so maybe other humans can go do other things like paint pictures or become blacksmiths or whatever and in this direction you get higher productivity per unit of land and so that comes from moving from hunter-gatherer to a pastoral lifestyle to traditional farming with irrigation to modern farming and so in this on this graph right over here tech kind of tools for the individuals move us up getting more productivity the land movements to the right modern agriculture gets us right over here so we're getting many we're getting much much more calories per unit land and we're getting much much more calories per unit labour so you need much fewer a much smaller percentage of the human population actually involved in the farming anyway I'll let you go there hopefully that gives you at least a an understanding of you know the food isn't just it doesn't just come from nowhere and it really is a rate limiting factor on humanity's population