Let's imagine a thought experiment.

An aggressive viral plague has struck humanity. Spreading astonishingly quickly through our modern world of dense cities and international airliners, we'd already lost the fight in a matter of weeks. Civilisation has collapsed and the vast majority of humanity has died. But you've survived. You fell deliriously ill, but through some innate immunity you lived through the raging fever, and have woken up in your cold house, with no electricity, no water in the taps or gas feeding the boiler or stove. The streets are eerily quiet, and no airplane contrails criss-cross the sky. You're a survivor in a post-apocalyptic wasteland.

These are all tropes we're familiar with from books like Canticle for Leibowitz or The Road, recent computer games like The Last of Us, and films like I Am Legend or Mad Max. On the whole, these narratives feature protagonists wearing a little too much tight leather, and a lone hero striving through the wilderness. But how realistic are these scenarios?

Fictional end-of-the-world narratives feature protagonists wearing a little too much tight leather  

If you did ever find yourself a survivor of a global catastrophe that wiped out most of humanity, what could you do about it? What would be the most vital knowledge you'd need to survive, and eventually thrive? It's here that the lone hero trope falls down. There's safety in numbers, and of course, we were only able to progress through history and build the modern world in the first place by working together; humanity is an inherently social, collaborative species. So while there will undoubtedly be a period of turmoil following a collapse, people will once again settle down into communities soon enough.

The question is, what next...? What will be your immediate priorities, and what capabilities should your community aim to recover over the following years? This is one possible chronology.

FIRST FEW DAYS

Once people stop monitoring and maintaining the power stations, the grid will go down pretty quickly. But by scavenging solar panels, or portable generators from a building site, you'll be able to keep your life electrified for the time being.

The internet will evaporate as soon as the servers behind it start dropping-off as the fuel in their automatic back-up generators runs out, so don't think that you'll be able to rely on Wikipedia for knowledge. But this doesn't mean your smartphone will become a useless brick. The compass uses an internal magnetometer so you'll still be able to find your way around, and in fact the last map you loaded will continue to help you navigate with GPS.

The GPS satellite network will continue working well for a few weeks after the collapse, but after about six months the position accuracy will have degraded until it's all-but-useless. Your priorities in the immediate aftermath will be to ensure you find a stockpile of bottled water and canned food, and also a set of decent outdoors clothing.

FIRST WEEKS

In the first few weeks you'll probably have encountered pockets of other survivors. Treat strangers with a wary caution until you've found a small band you can trust and rely upon for mutual protection, and this will also greatly improve the effectiveness with which you can forage for supplies and scavenge what you need.

By now, the urban area you started in is beginning to get pretty unpleasant. The stench of innumerable rotting bodies fills the air, and unfed pet dogs have formed into increasingly aggressive packs. In any case, a modern city is a grossly artificial bubble, supported only by the civilisation that constructed it.

Without mains electricity to run lifts or lighting, natural water sources likely contaminated, and the ground itself smothered in tarmac and concrete, you'll find life easier in a more rural setting. A traditional farmhouse with fireplaces for heating and cooking will be far more comfortable after the collapse than a modern high-tech apartment. You can always make scavenging forays back into the crumbling urban areas to restock supplies while you try to relearn how to make and do things for yourself.

FIRST MONTHS

Your main concern is going to be how to secure safe drinking water and avoid the water-borne diseases that have been the scourge of humanity for millennia. Boiling is a sure-fire way to kill pathogens but uses a great deal of fuel. Purification tablets can be scavenged from camping stores but sooner or later you will need to apply some basic chemistry to ensure the water you put to your lips isn't going to kill you.

You will need to apply some basic chemistry to ensure the water you put to your lips isn't going to kill you  

Water can be chemically disinfected by scavenging kitchen bleach or even swimming pool chlorine (sodium hypochlorite and calcium hypochlorite) and diluting it enough so that it kills microbes but doesn't poison you. Here you are exploiting the chemistry of chlorine, which also underlies the tap water we drink today – historically, it was such developments in hygiene and public health that enabled us to live in fabulously dense cities.

Until you've worked out how to make chlorine yourself, a very low-tech method for water disinfection can be used: solar disinfection. This is a technique being taught around the developing world by the WHO, and simply involves filling a plastic bottle with suspect water and leaving it in bright sunshine for a day or two. The ultraviolet rays from the Sun will pass right through the bottle and kill any pathogens.

Simply washing your hands is also exceedingly effective at blocking disease transmission. Soap can be made by hydrolysing animal fat or plant oils; by boiling with alkalis. Alkalis are one of the most crucial classes of chemicals throughout history, and can be extracted from your natural environment. Potash (potassium carbonate) can be extracted by trickling water through ashes from a hardwood fire, and soda ash from burned seaweed or other salt-tolerant coastal plants like samphire or saltwort. Collecting seaweed for soda production was a huge industry along the Atlantic coasts of Scotland and Ireland for centuries. 

FIRST YEARS

As preserved food runs out, you'll be facing starvation if you've not rebooted your own agriculture. Growing your own vegetables and fruit may be straightforward, but how many of us today know how to grow our own staple cereal crops; wheat, rice or maize?

The millstones in a windmill or waterwheel are like a technological extension of our own molar teeth  

Cereals are in fact species of grass; they're fast-growing and produce nutritious grain, but the human body is biologically-disadvantaged and we don't have four stomachs like a cow to be able to digest grass. Instead, we've had to apply our brains to the problem and invent technologies to aid our bodies. We need to physically grind wheat grain into flour and then use the transformative effect of fire in an oven to bake the flour into bread and release nutrients that our bodies can absorb. In this way the millstones in a windmill or waterwheel are like a technological extension of our own molar teeth, and the oven we use for baking bread or the pot we use for boiling rice is like an external pre-digestive system.

The primary problem is how you productively cultivate cereal crops in the first place – food surplus is the fundamental basis of any civilisation. If one person can feed 10 others who are not rooted to the fields and can specialise in other skills, then your society becomes more capable. Tools for working the soil like a plough and harrow could be scavenged, or created by repurposing steel items with a simple forge. But the crucial trick, one that evaded medieval farmers, is how to consistently maintain the fertility of your fields over the years. Without modern artificial fertilisers, you'll need to replenish nitrates in the soil by ploughing-in animal manure and cycling leguminous plants – peas, lentils, clover, alfalfa – with your cereal crops. Dissolving bones in acid will provide phosphates, and spreading crushed chalk or limestone will counter rising soil acidity.

FIRST DECADES

As your community becomes increasingly self-sufficient rather than relying on scavenging what's been left-behind, you'll need to relearn traditional skills like blacksmithing and making metal tools and keeping machinery and engines running. Civilisation has advanced thanks to the growth of mechanical power: waterwheels and windmills then steam engines, turbines and internal combustion engines, to alleviate the hard toil of human muscles.

A capable civilisation also needs fuel. Before the late 1800s and the exploitation of coal and then crude oil, the source of vital chemistry – acids, alcohols, solvents, tars – was by 'dry distillation' of wood; baking timber in an air-tight container and collecting the vapours released as it was converted to charcoal. You can even run a car engine on the gases given off by pyrolysis of wood; during World War Two there were over a million gassifier-powered cars – wood-fuelled cars – driving along the roads of Europe.

During World War Two there were over a million wood-fuelled cars driving along the roads of Europe  

Without access to crude oil for the reboot (our civilisation has already sucked-up all the easily-exploited oil) you can also make biodiesel for running machinery from rendered animal fat or plant oil reacted with methanol (wood alcohol, dry-distilled from timber) and lye (made by reacting soda with quicklime from roasted chalk or limestone).

In your nascent chemical industry, other easily-extractable substances will also have multiple uses. For example, ethanol, from fermenting grain and then distilling to concentrate the alcohol, is a versatile solvent and effective disinfectant. Charcoal is useful not only for producing high temperatures for forging metals or creating bricks or glass, but is also a chemical 'reductant' and so is needed for smelting metals out of their rocky ores.

FIRST CENTURIES

In the long term, the only way for the post-apocalyptic society to advance and redevelop knowledge and capabilities is to come to understand the workings of the natural world, and to apply that understanding to exploiting particular principles in the creation of useful technologies. The best way for confidently ascertaining how things work is the scientific method; to rigorously test your theories against carefully-designed experiments or observations of the natural phenomena – the scientific method is itself an invention, a kind of knowledge-generation machinery.

There is one substance, utterly indispensible to how we've conducted science through history  

In order to effectively investigate the world you need tools, and there is one substance that is utterly indispensible to how we've conducted science through history. It is relatively strong, chemically unreactive, and completely transparent. This wonder material is of course: glass.

You need glass to make test-tubes to learn about chemical reactions, and to make thermometers and barometers to understand about temperature and pressure (key prerequisite principles for building technologies like the steam engine, and then internal combustion engine), and glass can even manipulate light itself; forming lenses for the microscope and telescope. To make a simple glass, all you need are three ingredients – silica, soda, and lime, which can be gathered as sand, seaweed, and chalk or limestone. In a true Robinson Crusoe effort, you could make your own glass from scratch on a single beach.

And so with these tools for science, and a rational and enquiring mindset, you could hope that your post-apocalyptic society rapidly recovers after the collapse and avoids another Dark Ages. It may take decades or more, but a new form of civilisation could emerge from the apocalypse. What that world would look like is anybody’s guess, but with a bit of human ingenuity, we have the potential to rebuild – and perhaps even improve upon – the sophisticated society we know today.

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Prof Lewis Dartnell is a research scientist and writer based at University of Westminster. This feature is inspired by his book The Knowledge: How to Rebuild Our World from Scratch.

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