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I want to create a world set on our Earth but with humans shrunk down as much as possible to allow a huge population.

As the population increases they genetically modify their bodies to be as small as possible while retaining human intelligence. They do not replace their bodies and minds with machines. As a result each shrunk human requires considerably less food and water, allowing the population to increase beyond what Earth could otherwise sustain.

The shrinking is limited by the need to remain mobile. I don't want the humans shrunk down to a brain in a jar. They still have arms and legs and can move around (albeit more slowly).

If they make use of the entire surface of the Earth, collecting as much solar power as possible without overheating the Earth beyond what they can survive, and maximise efficiency by not eating meat, what is the maximum population that could be sustained long term? Assume that they drive other creatures to extinction and only sustain life that they use for food.

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Looking at ant hills, the biggest ant colony ever found housed about 300 million ants on a 2.7 km² big area. for comparison, that's basically everyone living in the USA packed together in 2/3 of central park.

If we assume that an ant colony of that size needs 10 square miles to sustain, that means we can fit 15 million ant hills on the land surface, assuming the earth was flat. However, if we only take arable land, that's 1.4 million ant hills, or about 428,400,000,000,000 humans.

However, there are some problems with this.

first: lifespan scales linearly with size, while energy requirements scale less than linear. This means that humans will live far less long, but require far more energy in comparison.

second: no other living creatures means that the entire ecosystem is ruined. I assume that everything larger than us is completely destroyed.

Third: at such small sizes, technology would be far more difficult to work with, because basic molecular physics fail at that level.

Fourth, and very crucial: It's really, really hard for something as complicated as the human brain to be made smaller without affecting mental acuity. One of the reasons we're smarter than our close cousins chimpanzees is that our brain is so much bigger. I can only see 2 outcomes: either we don't shrink the brain as much, which results in implications for our spinal column, or we change the location of our brain in a way that can sustain growth. Either way, at an actually quite early point, we cannot shrink anymore without serious medical issues.

So the question is not just "what is the maximum population possible?" but "how small can you make a human before we encounter basic biological, medical, mechanical and ecological problems that would cause the population to decrease again?" I think we first need to figure out what size you are aiming for. Also, if it's a world with futuristic tech, you can basically make them as small as you want and handwave the downsides through the technology.

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    $\begingroup$ Interesting. I wasn't expecting to run into the problems encountered at microscales since I specified that human intelligence be maintained. I can't see human intelligence being housed in an ant sized head if we are restricted to biological brains. $\endgroup$ – trichoplax Oct 8 '14 at 14:45
  • $\begingroup$ The population will depend on how small humans can be and still have a human brain. I specified that the brain must remain biological, not replaced with technology. $\endgroup$ – trichoplax Oct 8 '14 at 19:11
  • $\begingroup$ @githubphagocyte The problem is that it's really, really hard for something as complicated as the human brain to be made smaller without affecting mental acuity. One of the reasons we're smarter than our close cousins chimpanzees is that our brain is so much bigger. I can only see 2 outcomes: either we don't shrink the brain as much, which results in implications for our spinal column, or we change the location of our brain in a way that can sustain growth. Either way, at an actually quite early point, we cannot shrink anymore without serious medical issues. I'll add this to my answer. $\endgroup$ – Nzall Oct 8 '14 at 21:27
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    $\begingroup$ Can we use Laron syndrome (hereditary growth hormone insensitivity, which confers nigh immunity to cancer and type II diabetes) as an upper bound for how humans can shrink and still be human? $\endgroup$ – Damian Yerrick Oct 8 '14 at 22:38
  • $\begingroup$ Yes these are exactly the kind of strong reasons I'm looking for that the human body cannot be shrunk beyond a certain point. This would address the question much better. $\endgroup$ – trichoplax Oct 9 '14 at 6:13
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Right, maths for this question too! We'll be using the US as a basis for the maths.

The US has 9,629,091km2 of land, of which 16% is farmland and if farmed correctly we will assume could support them all (on a vegetarian diet). It's population is 316,000,000, meaning if you remove farmland, comfortably you could fit 39 people per 1km of land.

The Earth has a land mass (minus water) of 149,000,000km2. Taking off our 16% for farmland, this gives us 125,160,000km2 of land.

Now, if we say humans were shrunk to 1/10th their size, so we could fit comfortably 390 people per 1km, you'd be looking at 48,812,400,000 people. But you notice how I keep saying comfortably? Earths current population is 7.125 billion, that's 56 people per 1km. With our shrinking, that'd be 70,089,600,000 people. But maybe 1/10th size isn't enough? How about 1/100th size? We would have room for 700 billion people.

But as stated in Nate's answer, this is assuming they eat the same portion of food (scaled down), if they have to eat more due to a metabolism trying to compensate, you would need a higher percentage of farmland, which would cut that figure down dramatically.

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  • $\begingroup$ The numbers are interesting but this doesn't give me a reason why 1/10th or 1/100th would be the smallest that could contain a human intelligence. $\endgroup$ – trichoplax Oct 8 '14 at 14:59
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    $\begingroup$ Also bear in mind that a human 1/10th the height would take up 1/100th the land area and 1/1000th the volume, so even before considering that smaller people would need correspondingly more food, the numbers would be considerably larger than expected. $\endgroup$ – trichoplax Oct 8 '14 at 15:00
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    $\begingroup$ None of these numbers appear to be limits. Basing population limits on one particular country with low population density doesn't tell me about the upper limit. I'd be interested to hear about energy requirements compared to available sunlight and minimum body size that could sustain a human brain. 1/100th the size seems arbitrary. $\endgroup$ – trichoplax Oct 8 '14 at 15:22
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The main point is that we want to keep the same level of intelligence. Shrking beyond a certain point would affect our intelligence.

Looking at the real world, shriking our size by a factor or 2 or 3 and keeping our brainsize seems achivable. We would basically all be dwarves.

Through some genetic manipulation let's say that we manage to make our brain more compact while retaining the same efficiency. Realistically I don't think we could shrink it more than the size of a child's brain withouth affecting it's functionality.

So I think that we could biologically shrink ourselves to have the size of a 6 year old child while keeping our intelligence but we ll never be able to shrink ourselves to the size of smaller mamals like cats or hamsters.

I disgress a bit but I really think that we can feed way more than our current population using more advanced farming IF we find a way to produce cheap renewable energy.

The solution would be soilless vertical farms. If you have energy you can mosly do anything.

-make freshwater out of saltwater

-create artificial lights to simulate sunlight

-extract minerals from seawater

You could build 100 stories skyscrapper on a 1 square KM plot of land and feed an entire city with it.

It s not scifi and is already exprimented in Singapore and Dubai. The pb is that electricity is still too expensive and it s not profitable.

However in 100 years if we crack fusion power solar power that becomes feasible.

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