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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 human population has been limited by our ability to get calories or get human consumable food from the land, so what I want to do in this video is give us a little bit of a framework for thinking about how humans have been gathering calories from the land and how that's placed an upper limit on the number of humans that can live in any given area, or 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 that they might actually find some animals. I think these guys are trying right over here are trying to trap some rabbits and the gathering part, they're just literally looking for food maybe they find fruit of some sort, or some nuts or maybe some roots that are edible by humans so literally just walk around either try to kill things or find things they can consume. So I'll call this right over here stage one. So this is hunter-gatherers, this is what most humans have done through most of human history. And just to give us a little bit of a framework for how much they could get from the land, I looked in some of the best sense of studying HG populations in land like this maybe they can get about 200 calories. I'll make this a whole column, this whole column is the amount that they can get in terms of calories per square kilometre per day. Now it's obviously going to be hugely dependent on the number of animals that live there, the type of land that's there, if they're next to a stream where maybe fish are just jumping out of the stream this number would be much higher, if they were in some type of desert this number would be much lower. But this is actually fairly in line with some of the studies of HG cultures. Now if this the number of calories that they can get from each square kilometre per day, how many humans can live in a square kilometre per day? What is the density of humans? Well, to figure that out we have 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 2,000 calories per day to survive in a non-malnourished state. And obviously it's hugely dependent on how activ this person is or how large they are. And one other note: this whole I will be using Calories with the capital C. The Calories with the capital C are the calories that people are used to refering to when you go to the gym and you run on the treadmill and it says how many calories you've burnt or you look at the back of your candy bar and it says 200 calories. These are tha calories I'm talking about. They're slightly different notion than the calories that you encounter in your chemistry class. Those calories are calories with a lower case c. And just so that you can be optimally confused it turns out that 1 Calory with an upper case C is equal to 1,000 calories with a lower case c. And the lower case c calories are the amount of energy needed to heat 1 gramme of water 1°C. So this is what you see in your chemistry class but this is not what we are going to be talking about in this video. We're talking about the capital C Calories that dietitians are always talking about. So with this assumption that the average human eats 2000 Calories a day to not get malnourished, and obviously man would eat more, women would eat less, children would eat 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 1 tenth of the average daily human requirement if you believe this assumption. The the population density, humans per square kilometre you can always support 1 tenth of a human with this Calory output! So you can only support 0.1 of a human per a square kilometre. So 1 human would actually need 10 square kilometres to hunt from and gather from in order to support just themselves! They would need maybe 30 or 40 square kilometres 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 as maybe the next stage although it's now always the case that herding is going to be more productive than HG, especially with that case with the fish jumping out of the water, but let's go to the scenario right over here. So this is, we can call this a pastoral life-style. So this is 2, I'll call it pastoral. And over here is the realisation that, look, you have 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 be consumed by humans and, namely, the calories are themselves! So this gentleman right over here after he gets 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 this cattle or these sheep right over here by herding them and letting them eat this grass he is turning non-human-consumable calories into human-consumable calories. And for the sake of our thought experiment let's tell we got 10 time increase in the human-consumable calories per square kilometre. So now instead of 200 we're up to 2,000. And so instead of one human per square kilometre per day to support one human ... oh sorry, instead of 0.1 we can now support 1 human. So in that 10 square kilometres we can now support 10 people, in 100 square kilometres we can now support 100 people. Now the next stage - and I'm skipping a bunch of stages because we have things like subsistence agriculture and various forms that are not going to be equally productive, it depends what the land is like, it depends what tools at 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. For the purposes of this video the difference between traditional agriculture and modern agriculture - in traditional agriculture you didn't have mechanisation, so you had very primitive mechanisation, you definitely didn't have fossil fuel based engines, you didn't have modern pesticides, you didn't have modern genetically engineered crops, but you have some of the basic science of breeding crop 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 are actually producing. Let's assume that we've got a hundredfold increasse in the productivity. And looking in some of the historical records it looks like, it's depending on once again where you are, that's not out of the realm of possibility. So you have a hundredfold increase, so insdead of 2,000 calories per square kilometre per day you can get 200,000 calories per square kilometre per day, and now you can support 100 humans per square kilometre - if you wanna to! So you might not have 100 humans, one on all of the land, you might not be able to farm from, or there are other limits on the population for whatever they might be, but the important thing is to think about this upper bound. In this traditional you are able to get type of productivity from your land and you are able to in theory support 100 people per 1 square kilometre, that means if all of a sudden you have 200 people living there, they maybe every once migrate to this land because it seems as specially fertile or there are really small farmers living there, then all of a sudden not everyone is going to get 2,000 calories a day, some people might get malnourished, other people might actually starve. There is this upper bound on the actual number of people that can be there based on how productive the land actually is. Now let's move over to modern agriculture and we want to talk a little bit about what exactly is modern agriculture. You have machines like this combine over here that does a lot of the human labour. One human can... I'll talk about the different dimensions because there are actually two dimensions here: how much calories can you get from the land, and how much energy and labour can one human input into the land using tools at their disposal. So in this case cattle, these oxen pulling this plough, or in this case this combine that's fueled by fossil fuels. But in modern agriculture, because 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. Let's say that you have another factor of 10 from traditional agriculture. so now you can get 2,000,000 calories per square kilometre per day, or you can support 1,000 humans per square kilometre. So once again this right over here is an upper bound. And just to give a sense, I picked these numbers just so that nubmers would be clear I looked in some historical records, these are completely out of line with what it looks like human have been able to do in the past, but to give you a sense of what human populatin densities look like right now and why this upper bound seems to be right about correct, in a place like the U.S. the population density is 30 people per square kilometre, in a more dense country or a significantly more dense country like India the population density is 300 humans per square kilometre. And in the most population dense country in the world, which is where I come from, or actually I was born in New Orleans, but where some of my ancestors came from, which is Bangladesh, so there's a lot of people like me, I guess, Bangladesh has population density of 900 humans per square kilometre! 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 in what now on 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 I reffered to a little bit more, is just think about those two dimensions because sometimes it get a little bit muddled. One is the kind of the productivity of land and at the other there is productivity of labour. So right over here in HG, they're not getting many calories from their land so they're right over there, and the humans have to do all the labour, they don't have animals helping them in any way, they definitely don't have robots or any types of engines helping them in any way, and so there 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 progress with things that aid humans, so for example if you all of a sudden have cattle helping you or you have other tools that help you, you get more human productivity, so less and less of human labour has to be used to get that productivity from the land so maybe other humans can 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 HG to a pastoral life-style to traditional farming with irrigation, to modern farming. And so in this graph kind of tools for the individuals move us up, getting productivity from the land moves us right, modern argriculture gets us right over here. So we're getting mach more calories per unit land and we're getting much more calories per unit labour. So you need much smaller percentage of the human population actually involved in the farming. Anyway, I let you go there, hopefuly that gives you at least an undesrtanding of that food doesn't just come from nowhere, it really is a rate-limiting factor on humanity's population.