How long to rebuild the world if you could still travel to the current one?

Question

Premise: Modern society discovers a magic portal that leads to a parallel universe that is almost identical to our own but the earth on the other side never had humans (or any other technology wielding civilization). People who go through the portal may travel back and forth freely but there is one restriction, they cannot bring anything with them (they have to be naked). The portal is in the Brazilian Rainforest.

Question: How long would it take for a group of settlers to recreate the modern world's technology and infrastructure on the other side of the portal?

Edit: Transporting removes artificial fillings and empties the stomach. The portal only transports "intrinsic" parts of your body. Limited to cells with your DNA and supporting mitochondria and stomach bacteria (and any other necessary symbiotic bacteria). Transporting animals is allowed with the same restrictions. Transporting things that aren't living creatures is not allowed. Pacemakers would not be allowed. The idea is that settlers arrive with only themselves, their health, and their knowledge, along with animals as well. The portal effectively stops all technology and non-living matter (defined as a multi-cellular organism with a nervous system) from passing through. The portal itself is magical, but the process of rebuilding itself would not be.

Answer 1

My answer is going to take the question to the best-case extreme. We have had lots of questions of this type on Worldbuilding before, but they usually take a form where people can only make the trip once and have to bring everything they need up front. By allowing endless trips back and forth at will we can always have plenty of experts and skilled workers in every field and plenty of help. Disallowing non-living items also puts an interesting spin on this.

TL;DR: On the order of 1-10 years if it just pops in and enough skilled people agree to try and rush-build a new modern society there and they coordinate well. Or on the order of 1 month - 1 year if we know it's coming and can prepare and practice for it and have a colossal level of support and expertise from thousands or millions of people.

Assumptions:

• The area on the other side will have reasonably close access to all necessary resources.
• Concerning only living matter passes: I'm going to bring plants, but only if they are fully mature plants still alive enough that you could plant them in the ground and they could keep growing, that should count as alive.
• Any potential hold-ups such as "What is this thing? Is it dangerous? Let's study it first" have all happened and are not counted in this answer. The answer starts at "We know what's going on, let's do it now and quick."

This answer will follow two different cases as I go along.

Case #1: Earth evacuation. Maybe we find out aliens are going to invade Earth sometime soon, or whatever. We have a fire under our butts. Most of the people on Earth are willing to support the effort, and millions of people give their time and skills.

Case #2: Gameworld. The portal was designed and created by the most advanced of the ET races we have met, and they created it as a huge game. Each time the game is started the portal shows up on the participating race's home-worlds. You have advance notice about when and where the portal will appear, and you have time to prepare. We have a line of people waiting at the portal location who know exactly what their jobs are and what they plan to do each day. As soon as the portal appears, the single-file line can rush in and everyone can hurry to their required activities. It is the largest-scale multiplayer game in the history of the universe. Everything happens as fast as it possibly can, only taking as long as it takes to physically perform the minimal amount of actions necessary for the fastest method of reaching the goal.

For many of the actions in the gameworld case, imagine groups of people who have practiced working together to perform one type of task as fast as possible, like race car pit crews who can work together flawlessly to do several things to a car and have it driving away all in seconds. And they will have lots of teams making lots of everything and cutting corners and hurrying things; it's ok if most things fail or break early on - as long as we have hundreds of teams rushing the same thing a few of them should succeed.

Let's get building!

Go!

The first thing we need to do is set up our foundation.

Starting with primitive tools

Fortunately there are crazy awesome people who create primitive tools from natural resources just for fun or research who we can use as guidelines. Go check out the YouTube channel Primitive Technology if you haven't already done so.

Some of what follows is also from personal experience. I consider myself to be a hobby bush-crafter, or hobby "primitive survival" practitioner. I've made stone tools, started fires with flint and steel and with friction (ie: "rubbing sticks together"), made rope from plants, fired pottery the old fashioned way, dug garden beds with a stick, and done other things you typically associate with this type of activity.

Hand Tools

As long as you have various kinds of rocks you can make some very crude stone tools useful for a few tasks in less than 1 hour. If you get experienced flint nappers in your group who find flint, you could have sharp knives and axes very soon. Also, if we are allowed to bring an animal across such as a beaver and then extract its tooth, those can make good carving tools so you could have one almost immediately, but I'll just assume you wait a day for the good flint tools. By the way, flint tools can be made sharper than iron tools, they just have other drawbacks.

Earth evacuation hand tools: 1-2 days

Gameworld hand tools: A few hours

Clothing

As soon as sharp tools are available make clothes out of animal hides. If you skin it and wash it you can start wearing it almost immediately. It might seem gross, but I'd rather be wearing a hide that still smells like its previous owner than to keep going naked. We'll want better clothes later, but this is sufficient for now.

Both cases: First day

Fire

With good fire starting materials, even natural ones grabbed from the wild, the best fire starters can walk out into the wilderness, grab what they need, and have a fire on day 1 if there are some very dry materials available. It could take a few days or weeks if the materials need some processing such as drying in the sun or carving with the stone tools.

Earth evacuation: 1 day to 1 week

Gameworld: First day. Even if it's not dry enough, with enough muscle power from lots of people we can dry things out by friction.

Manufacturing

Very primitive manufacturing was done by shaping wood or making pottery. With the right people, you'll be chopping down wood and carving it on your very first day here. Pottery will take longer since you have to find a good clay source, shape it, dry it, then fire it.

You can also create water resistant objects using only wood fire ash and water. You wet the ashes, make a thick goop out of them, and use it similar to clay. It does not even need to be fired, you just leave it out for days to dry. I just learned this yesterday from one of the newer Primitive Technology videos where the guy does it and shows that an ash-bowl filled with water still has half the water left the next day.

Earth evacuation: Wood manufacturing will start on day 1 or 2. Clay firing will start after a few days for small items if you have fire by then, up to a few weeks for larger items that take longer for the clay to dry.

Gameworld: Wood manufacturing first day. Clay firing can take a shortcut: with enough help we can create so many clay objects that we don't care if most of them fail to fire, just fire them after a day or two and hope 1 in 1000 works. For larger clay objects you'll probably have to wait at least a few days. Time to finished pottery obects: a day or two for small objects, a few days to a week for large ones.

Farming

Someone can start working on a garden bed immediately on day 1. Even if you want to tear up the ground (not necessary for all farming techniques) you can still do it with nothing more than a stick you pick up or break from a tree.

Both cases: Food is available on day 1, some gardening is done and mature plants in the ground on day 1, and food is not really a problem.

Build Time: Primitive Technology

Before long you will have:

• huts for shelter, a shed full of knives, axes, adzes, rope, and other simple tools.
• a garden bed full of plants transplanted from Earth
• whatever they want made of clay including plates, bowls, bricks, tiles, etc.
• stone hearths and fireplaces

Earth evacuation: Less than a month

Gameworld: A week

Metal Age

They will have already started on the building blocks for more advanced technologies before they complete the "primitive" section above. Someone will already have started working on a clay furnace and forge. Someone else will already have started working on a bellows. Others have already been searching for ores. I'm just calling it the metal age since they can start on them all at the same time, even iron. And I'm going to concentrate on iron here.

As soon as 1 or 2 furnaces are ready, someone will already have found at least a few rocks with ores in them. Even if it takes a few tries, there will be smelted metal in less than a month, especially if a bellows is completed in time.

The first metalworking tools are sticks and stones, so the first tools made out of metal will have lots of impurities and be very crudely made, but they can function. The first iron pokers, hammers, and knives on wooden handles will be hastily made all still inside that first month.

Using those tools, better tools will be made. These tools will include all of the conventional blacksmithing tools and some knives, axes, adzes, saws, horseshoes, barrel rings, wheel bindings.

Yes, even horseshoes and wheel bindings: we might as well get the beasts of burden over here now if they aren't already there. Make sure you've been working on carts and carriages in the meantime.

Now you have homes, gardens, metal hand tools, flocks of animals, vehicles like carriages and carts, barrels, and lots of other things. Since the heating technology is already well advanced at this point you could make glass too as long as you've acquired the materials for it.

Evacuation: 1-2 months

Gameworld: 1-2 weeks

Manufacturing Age

At the end of the previous section, you've already had everything needed to start making the first woodworking lathes. Most of it can be made out of wood, and the cutting edge can be as simple as a blade or a sharp point. I have done rough lathe work before by putting a stick in a drill, attaching something to the stick to spin it, and applying a knife held in my other hand to the wood. For our group, they don't have the hand drill, but they could make something very similar.

The first woodworking lathes could come before you even have your iron work going since you could do it with flint cutting tools. But once you have iron tools it will be even more reliable.

Now you will have lots of lathed wood objects, and as soon as a blacksmith makes an iron drill bit you can get a woodworking mill going too, so your mass manufacturing base has essentially started.

What you can not do well at this time is mill or lathe iron objects, but you can use iron mill and lathe cutting tools on some other metals like copper, tin, or lead. And you'll have various other tools at this point for rolling metal, bending metal, etc..

Your metal industry is mostly limited by how much metal can be acquired. If you can get lots of it, this world will have transformed into something that looks like Earth in the 17-or-1800s. Lots of things could be mass manufactured at this point, even non-iron metal objects.

At this time iron objects, including steel which is made from iron, will probably still be made on an anvil until you can make cutting tools of steel-cutting quality.

Both: Many tool bodies can be made of wood and already crafted and waiting for their cutting edges ahead of time. So this point should be only days or weeks after the metal working starts.

Chemistry

We have all this cool manufacturing, blacksmithing, glass, etc., but to continue making this colony into something resembling modern Earth we need chemistry.

We're not just now starting on the chemistry, instead the work should already have been ongoing since the beginning. Ever since we had fires, primitive chemists will have been starting to create lots of chemicals. Once there were clay vials, they can be working on lots of liquid chemicals too.

Before the end of the previous section, there will already have been gasifiers, beakers, tubes, and various other tools created and we'll be at a chemistry level of technology similar to the 1800s or early 1900s, except that we'll only have relatively small quantities of the chemicals so there will be an effort to simply mass produce as much as we can before we can actually start using it to do much of interest.

Two of the more useful things that might come out of this are fuel and aluminum. Fuel might come from gasified wood or from plant ethanol. Aluminum will be made using the Bayer/Hall-Heroult Processes.

Aluminum is important because it has a much lower melting point than iron, it makes a fine conductor (though not as good as copper), and it can be worked on easier than steal. Also the ground is full of tons of the ingredients for aluminum.

Another big one to try and push for soon would be plastics and rubbers. Then we can start making covers for wiring to get ready for our later electronics. Plastics and rubbers will also make it easier to do chemistry on gases in addition to the liquids and solids we've already been working with.

With our manufacturing in full swing, aluminum available to compliment our blacksmithing steal work, and fuel from plants, we might be able to start working on some automotive prototypes, the first ones probably wood frames with the simplest engine we can make.

A lot of the chemistry initial buildup will happen at the same time as the previous sections, but some will have to wait until we're manufacturing stuff. We might have some prototypes for some of these things likes automotives and wiring soon, but we would have to create a factory before we could mass produce these things. The initial versions will be crude and potentially dangerous, but they could be rushed into existence around this time.

Both: Most of this was done concurrently with previous sections. Some will have to wait until after and will take some days to months to build up various useful chemical stocks.

Electricity

It might sound like a bit of a stretch, but it would not be unreasonable to start generating some electricity as soon as we have conductors that we can start connecting fruits and vegetables with. Most people have heard of using citrus fruits as power sources, but you can do it with other thing too such as potatoes. As long as you bring a still-live one over, so it goes through the portal successfully, you can use it as soon as you can wire them up. In some stores you can even buy kits that power things this way. My kids had a potato-powered clock kit at one time.

Barring plant-power, we can start making batteries if we find the necessary materials. If you have the correct materials at hand, you can make a battery just by piling the right materials together in the right way.

Both of the previous two electricity-generating methods could be done as soon as you can make the wires.

As soon as we have wires and magnets we can start making electricity the way we actually do it in power plants. Once we get to this point, we can set up larger scale power generating plants. We could do this soon after arriving as long as we have magnets. To actually do much of interest with the electricity will require the manufacturing from the previous section.

Evacuation: Some months

Gameworld: Some weeks

1900s level

We're already building devices to use with our electrical grid and we have manufacturing and chemistry. How long it takes to have various useful devices depends on their complexity.

Some examples of things you could expect rough versions of:

• electric motors/engines
• lights
• heaters, stoves
• radios
• air-tight containers
• airplane prototypes (early Wright brothers level)
• 1800s style "submarines" (they are not what you imagine when you say submarine)
• rocket prototypes
• 1800s or early 1900s style automotives
• pumps (both water and air)
• telegraphs and maybe telephones

By this point you should not expect:

• electronic computers, at least not useful ones yet
• TVs, monitors
• useful rockets

Evacuation: Less than a year

Gameworld: Some months

We might be able to have some form of crude 8-bit electronic computer soon. It would probably be just a few of them built, they would be designed for their specific purpose, and they would be used only to help bootstrap the rest of the computer revolution. These initial ones would be slow, mouse-less, monitor-less things whose time would be very valuable until they were used to help design and create the next generation of computers. Remember, this is being assisted directly by Earth computers and engineers on the other side of the portal, we only need the ones here just to get some form of automation going and then that world's computing revolution can explode.

Since we are assuming limited previous life on this planet there might be no major source of fossil fuels. That is just as well: it may speed the development along, but it's all the better that we start right out developing renewable, sustainable, cleaner energy sources. It would probably start with wind and water first, and we might be trying to make solar panels after that.

Evacuation: I think we would see prototypes and some more functional thing we associate with 1900s level technology before the end of the second year.

Gameworld: Gameworld is still rushing headlong getting everything done literally as fast as humanly possible. We're probably still not approaching the end of the first year yet.

2000s level

We can get up to 1800s and some early 1900s level by cutting a lot of corners. But going that last step to having very precise machining and manufacturing of precision parts and having microscopic-scale manufacturing for computer parts is going to be one of the trickier parts.

How long this would take is a hot topic of debate among some people. I feel the research is inconclusive.

It certainly should not take decades though. It took decades on Earth. If we have the push for this that I described initially, we should be able to do it many times faster than it took us on Earth. After all, we already have it and understand it, we're just recreating it, and we're doing so with all experts readily available.

So going from the end of the previous section to being fully 2000's style modern should take on the order of years to redo it, 1 year for Gameworlders who know exactly what to do and just do it. I would say months, but there are activities which need to be physically done, tested, possibly redone, such as physically creating computers, rockets, planes, etc.. It's not just a matter of drawing the design up in your first computer you make.

Conclusion

All of the initial setup checkpoints will take days, weeks, or months. Then to get to the level of a few hundred years ago will take months, setting up mass-production (relative to what is already there) will take months.

Then progress will slow down as you approach modern Earth level and you try to make everything very precise, very large scale mass production, build up a large quantity of essential components, and recreate complicated structures like modern rockets, modern cars and planes, and modern computers, medical equipment, etc..

The entire process from day 1 to the day you could almost mistake it for Earth today would be on the rough order of...

Evacuation: 1-10 years

Gameworld: 1 month to 1 year

This is the best case scenario for fastest time, and it is borderline crazy but might technically be possible, especially since there is advance notice and preparation, and maybe even practice runs.

Now I feel like I want to start a club who actually does this type of thing. What government will let us do all this in their border?

Answer 2

Brazil's tourism industry would boom.

Even with the mandatory nudity, this is perhaps the easiest settlement in history, because of the free travel to and from the new world.

What you do first is you set up a city at the Earth Prime-side of the portal. That would be easy enough: an interdimensional portal would draw a lot of attention and investors hoping to profit off of it. And profit they will!

You bring over survivalists, Bear Grylls and his colleagues, who have actually trained for this sort of thing. Surviving alone in the wild is hard and dangerous, but if you bring a hundred people, who can just leap back whenever they feel ill or stressed out, you would very quickly establish some sort of "base camp". I'm talking tents made out of leaves, access to fresh water, a supply of food, and rudimentary cooking vessels. Not a four-star hotel, but enough for the less trained fellows to survive for extended periods of time. This should take less than a month.

Then, you bring scientists: people who know how where to find iron ore and how to smelt it, people who know how a loom works and how to construct one. Engineers to the rescue! And these people will spend some time doing scouting for the things they need, but most of their efforts will be spent writing books of what they know (assuming you cannot just bring over books). Leaving behind instructions for when they grow tired of the place. This stage is ongoing but in just another month, you should have a good start on bringing over knowledge to get others started.

And then… tourism time! In our world, castles are regularly built by enthusiasts for free. People love dressing up as people from the past, and living like they did. And, the idea of a whole new world would entice many people dissatisfied with the current state of Earth, luddites and nudists alike, to travel to Brazil and brave the new final frontier. Manifest Destiny 2.0, except this time they're speaking Portuguese.

People would, for very affordable prices, book a holiday to this place for a week or two. They would help bake some bricks, weave some clothes, pave some roads, craft some tools, etcetera, using the knowledge that the scientists and engineers brought over. Each individual wouldn't contribute much, but with the manpower of millions, you would have a city on both ends of the portal in just a year.

Of course, the Mirror City would be a pretty crappy one: Uruk meets summer camp. But the sheer manpower would get anything done over time. They built pyramids with this level of technology and far less knowledge. I think that the level of progress would slow down once you get at a level of sophistication that a tourist would need more than a day's introduction to understand, which would slow down different technologies at different stages. But the engineers, the enthusiasts, and the survivalists, they would each set up their own goal. Who will make the first steam engine in the Mirror World? The first combustion engine? The first computer? This would be another space race, and Latin American countries would compete for those pioneering achievements the way they do with football.

I think it would possibly take as much as ten years for the first computer, and only because it is a milestone people would be dying to achieve. You would have an incredibly uneven level of progress - few are going to want to be the first to invent sewage in the Mirror World - and life there would generally suck. But even when the initial enthusiasm has died down, when the tourism income is slumping and the most obvious scientific measurements have been made: then you're talking settlement. The international community would map the place and divide it into countries, so they do not have to go to war over natural resources.

The place would become an Australia in the end. Used to be the hot new thing, and now there's infrastructure and a permanent population but most of it remains empty.

Answer 3

That's really an interesting question. Even in the TV show "naked survival", the participants were allowed to bring some things with them. In your scenario, the people will start with literally nothing.

(tl;dr: If the portal won't be hidden from everybody and the other side is survivable for humans, it will take about 50-100 years to reach our technological standards and maybe another 300-600 years to have the same population count)

Well let's start from the beginning. A weird wormhole-looking thing appears in the brazilian rainforest. One of the most unlikely things that might happen immediately after this event is some people shouting "Hey, let's get naked and jump into that thing".

So, what might happen is, that they call the police. The police will maybe call the ABIN (the Brazilian Intelligence Agency) and they will most probably secure that unknown object. Maybe they will just build a research institute around it. They won't give normal people access to this thing.

From here on, there are two main questions: How does is look (i.e. can you see through it? Does it look like a portal to a different world or just like a big iron sphere)? And what happens to the things that can't pass it? Do they just vanish? Do they not interact with the object at all? Do they explode?

The research institute will invite scientists from all over the world, who will try to find out what that object is. They will try if it's safe to stick sensors into that thing. Then they will stick sensors into it. If the parts of the sensors that touch the surface of the object just vanish, the object will be considered dangerous and the scientists won't let any living creature touch that thing. On the other hand, if the sensors don't interact with the object and just appear on the backside of it, the scientists might think, that it's just some kind of hologram and maybe won't recognize it as a portal to a different dimension.

It will take a few years just to build that research facility and do some research. The press will find out about that object. There will be many conversations on the internet, there will be conspiracy theories about that object. Maybe some people will try to destroy the research facility.

Meanwhile in the facility, something might happen that identifies the object as a portal. Maybe the scientists watch insects that fly through the portal or an animal from the other side jumps through it and appears on our side.

Now things will become a little bit harder to explain. Why can extradimensiomal animals pass the portal, but research instruments can't? The first idea of the scientists may be that the portal only works in one direction. But it won't take long until they throw a rat or some other test animal into that portal and see that it survives the journey. You could explain that with "only living biomass can pass the portal", but why are dead things like hair, fingernails, tooth enamel etc. unaffected? Why are wooden constructions excluded from traveling through that portal? Why does the water in living cells or blood pass the portal while the water in your stomach can't pass it?

If the scientists can find a physical reason, why only living biomass can pass the portal and the test animals survived long enough, they might find it safe enough to let humans pass the portal. Maybe the first test persons will be death sentenced prisoners. They will be pretty surprised that they appear naked on the other side and they will be hungry, but they survive and can return to our side after a few minutes. They will have some medical examinations to see the effects on the human body that the travel and the other side made. If they can't find a reason for why this can pass but that can't pass, they might wonder if the portal is made by intelligent aliens and be too afraid to send people through it. Also, you don't know if the portal will suddenly disappear.

After that, you need some survival experts that examine the new world. You need to know how dangerous the animals are, if they are edible or poisonous, which plants are edible and if there are other dangers. Maybe the other world is highly radioactive or the atmosphere is poisonous or the sun radiation is unhealthy or it is too hot/cold to live there - especially when you are naked.

So the first people who go there will maybe only stay there for a few minutes. Then they will begin to build simple tools from wood and stones, they will produce some simple clothing, they will build shelters and maybe sleep a night there. They will eat some plants. If they feel sick, they can simply return to our world and anything poisonous they might have eaten, vanishes from their bellies.

After the first explorers found enough food sources you need a lot of craftsmen to build wooden houses, fireplaces, better tools etc. So, in our world you have to think about how to find the right people. You need to find people who are good enough in what they do and who are willing to work for food and shelter only. In the first years there will be no money on the other side. Maybe nobody wants to work in ore mines for free, so there might be a new slave culture.

The more people go to the new world, the faster technology will improve. First there will be machines that build basic survival goods, like fabric, clothes, bows and arrows, knives, pots, dishes, wooden boards for houses and furniture etc.

When the first people start to build iron tools or machines, it won't take long until electricity will become available. That will make it easier to craft more machines.

The people need to establish a financial system. Maybe they build a "National Portal Bank", that links to their bank accounts on our side or everybody starts with an initial amount of money. They also need some extradition agreement between the old and new world, so people from the old world can't just travel to the new world, destroy the city and jump back to the old world.

Bit by bit there will be new jobs available. You will still need a lot of farmers and craftsmen, but there will also be a lot of people who copy books from the old world and who establish more and more modern technology. The progress will become faster and faster but it will depend on the willing of technology transfer between the old and new world. The big businesses from the old world can't just sell their new tech to the new world people. They have to rebuild it on the other side.

The portal will become the biggest international travel hub of the world on both sides and maybe the favourite target for terrorists.

So a short timeline of how I imagine the progress:

1 week: discovery of the portal and installation of a safety zone around the object

1 year: building of a research facility

1 year: research of the object without anything travelling through it, then accidental discovery of it being a portal

1 month: testing if animals survive

6 months: testing if humans survive and what influence the journey has to their bodies. Of course everybody will need a quarantine after coming back.

1 year: survival experts testing the survival conditions and digestibility of the most common plants and animals.

5 years: craftsmen building wooden houses

10 years: building of basic machines for food, clothing and woodwork

10 years: establishing an electric power supply and building simple electric machines.

10 years: finding chemical elements for medicine and semiconductors and building first simple computers. Establishing an optical data connection between the new and old world.

15 years: old world companies building facturies in the new world

At this point, we might reach the technology of the old world. Kids who were born in the new world might be overwhelmed by the fast progress and maybe can't operate all the new stuff. There will be mainly one big city in the new world with a few villages around it. It will take a long time to colonize the whole new world. At least a few hundred years. Travelling between the worlds might become really expensive because the portal is too small to handle millions of travellers per day. (Also you need to buy new clothes and take a meal everytime you travel)

So I think if the people of the old and new world work hand in hand, it might only need about 50 years to reach our technology level. If travelling and exchange of data is somehow prevented, it might take hundreds or thousands of years for the new world people to research the technology again.

The growth of population will depend on how many people moved to the new world. Let's say a million people went through the portal and the population doubles every fifty years, then it will take 650 years to reach our world population again.

Answer 4

Parasites, Bacteria, Biting Wildlife, and Apex Predators Would Make It Take A Really Long Time

I think a big question that hasn’t been brought up in the previous answers is how are these people going to deal with bacteria, parasites, and interactions with other native wildlife like large predators, venomous snakes, and stinging insects. If only living organic matter can pass through the portal that means the people coming through will have no clothes, no footwear, no camping supplies, and most important no medicine. Antibiotics have to be carried in containers, after all, and may not be able to pass through since they aren't living matter.

When people go to the Amazon or other rainforest regions today they often wear heavy clothing to prevent mosquitoes and other insects from biting them, closed-toed footwear to prevent them from being stung or bitten by venomous snakes, fire ants, or even just cutting their foot on a rock and getting infected, and often bring special camping gear to keep out wildlife while sleeping and to purify water to drink. Mosquitoes, bullet ants, fire ants, poisonous plants, spiders, scorpions, kissing bugs (which spread Chagas disease), assassin bugs, poison dart frogs, venomous snakes, South American army ants, are all concerns without proper footwear, clothing, and bug spray.

While people do live in these areas without modern conveniences, many societies do use clothing or footwear of some kind and most of them are not people used to modern life who have lived in controlled, indoor environments all their lives. And in many cases their gut bacteria have adapted to the local water microbes (this is why one of the golden rules of travel is never drink unfamiliar water wherever you go unless you have lived there a long time). Even if you only send people who are used to living without clothes or shoes and drinking river water, a world without humans could result in very different parasites and diseases in the water supply for which modern immunity means nothing.

Disease will cause massive casualty rates without access to medicine. Viruses may not be that big of an issue because many viruses are thought to have transferred to humanity from domestic animals, and if you’re in South America the only easy source of viruses would be New World monkeys (compare Africa, where you might get viruses from apes or even non-human hominins depending on the setting). The big issue will be parasites, and in pre-urbanized societies parasites are a much larger issue than viruses. Some examples known from South America can be found here. The big one? Malaria, which would be worse without any anti-malarial drugs you can bring with you. There are a lot of parasites in water bodies around the world, especially in tropical climates, and others can be transmitted to humans if they eat the local wildlife (our tapeworms originally came from a species that infests lions and hyenas, for example). Parasites reduce the quality of life for the settlers, may require other individuals to care for them, and can cause disability or even death, especially in a world without anti-parasite drugs.

The Amazon is about the worst place in the world to try and settle without the aid of a pre-existing technological base, as its high biodiversity and warm, wet climate means that parasites and stinging insects are everywhere, compared to somewhere like a grassland or desert (and it's actually been suggested this is one reason why human civilization first developed away from heavily forested areas). At least in a desert or grassland you don't have to constantly worry about parasites and disease-carrying insects.

Food is also going to be a major problem because as per OP you can't bring dead organic matter through the portal. So all food has to be gathered on site and you likely can't gather enough in the local area to feed a large group of people without tools to increase efficiency. At the least you might strip the area bare of edible plants. Catching game is going to be difficult without tools or tools with which to build tools (scrapers, appropriate striking tools and material to make arrowheads, sinew or sap to attach arrowheads onto shafts).

And while you can bring livestock through to kill for meat and leather, you will have to bring them through alive and have no tools with which to kill and butcher them. And when you do you have no way of keeping them from wandering off or trampling you in a panic when you try to kill them because all halters, leadropes, and reins will disappear the minute you go through the portal. It is hard to restrain a horse, cow, or other large livestock without a halter, because even docile livestock tend to do what they want when not restrained and given how big they are it is difficult to stop them.

Apex predators would be a huge problem. The nature of the portal means no guns or even spears and bows and arrows to defend against attacks from jaguars and caimans. At best weaponry has to be made from scratch. If this is a world where humans never got to South America it also means Smilodon, dire wolves, short-faced bears, and Protocyon troglodytes, all of which have been found in Amazonian Brazil and are known to eat large (read: human-sized) mammals, would still be a major issue, as these species are thought to have been wiped out by humanity (e.g., humans competing with them for food). Nothing short of a firearm or atlatl is going to stop them.

Exactly how common large predators are without humans to reduce their numbers by hunting them and competing with them for food is often greatly underappreciated (for example, 20,000 years ago carnivorous mammals were more common on the humanless North American prairies than on the human-filled African Serengeti). People often forget that the world we live in has been massively shaped by humans even before the invention of agriculture, and that without humans the world would likely be swarming with large predators and megafauna. Without human interaction these animals would have no fear of humans and would see them as new and potentially interesting food (like polar bears in the Arctic). Something an absence of weapons would do nothing to dissuade.

How this relates to the larger question of how long it would take to rebuild modern society is that constantly having to spend time caring for sick or injured individuals or send them back through the portal for treatment, losing people who could be working due to disease, having to constantly deal with predators (or even something like a big, aggressive gomphothere or ground sloth that wanders into camp), and replacing manpower losses from predators and the associated psychological effects (think Tsavo) would slow rebuilding society to a crawl until a proper technological base is built up.

Given in this scenario people are trying to rebuild civilization in a region that is not amenable to traditional agriculture, has lots of parasites, few exposures of rock to mine iron and stone, lots of small biting wildlife, extreme floods, and dangerous megafaunal predators it might take centuries. It's possible it may not ever happen or if it succeeds it succeeds due to sheer luck, sending people through the portal likely resulting in massive casualty rates. Sending people through and trying to rebuild civilization without medicine, food, tools, or means of protection sounds like either a desperation move or a particularly cruel form of execution.

Even in cases like Discovery Channel's Naked and Afraid many of the contestants have to abandon the challenge due to injury or illness or be sent to the hospital shortly thereafter, and even those that complete the challenge are usually suffering from starvation and dehydration by the end. Humankind's biggest weakness is that without tools we are pretty much helpless or at least on the defensive until we can get tools, and we often rely on already-existing tools to make more tools. There's a reason why in most of these survival shows the goal is "get back to civilization" rather than the more ambitious "rebuild civilization". On the other hand, surviving in this world and actually trying to build this technological base could make good grist for your story.

Answer 5

I think it will take long.

First thing first, you are naked and with an empty stomach: you need water and food, and maybe some clothes immediately afterwards.

You know how to hunt, farm and sanitize water, but you have no tools. If you are lucky you can find some stone around you for concussing some animal or excavating the soil searching for edible roots, else you will be searching for fruits.

You won't be able to carry any seeds, so there it goes your ability to set up farming in the short term. Congratulations, you are now a hunter-gatherer!

You now how to make SiC and other fancy carbides and alloys, but you have your bare hands and nothing more. If you are lucky you will be able to light a fire before dusk, and make some sort of weapon for the next day. Yes, again that rock, or a wooden spear.

Then while you wander seeking for prey you might explore to see if you find exposed ores, from which you might attempt refining some metal.

You can go back and forth to eat and hydrate yourself, and use the time there only for surveying. But once you need to set up something, you will be forced to stay.