Does every society have a use-by date?
The fact that we're not bumping into Aztecs or Byzantines around the place does suggest that they all end at some point.
States can slowly decline and fade, or morph into new entities, while a few simply collapse in on themselves.
"This is something which has haunted even the most powerful of empires," Luke Kemp, a research affiliate at the Centre for the Study of Existential Risk at the University of Cambridge, tells ABC Radio National's Late Night Live.
"It's natural to look over our shoulders at the wreckages of the past and wonder if we're going to end up the same way."
What is a societal collapse?
Human history stretches back around 300,000 years, but we only made the move from hunter-gatherer communities to states about 5,000 years ago.
Dr Kemp, whose upcoming book is Goliath's Curse: The History and Future of Societal Collapse, calls a state "a set of centralised institutions that impose rules on and extract resources from a population in a territory".
They appeared in Egypt and Mesopotamia (modern-day Iraq) before spreading around the world and becoming modern day "Goliaths" like the United States and the People's Republic of China.
All states across history have eventually ended and a societal collapse is perhaps the most dramatic coda.
It's "when you have multiple systems of power all go down. Not just the state but also the economy and potentially the population [as well]", Dr Kemp says.
And for anyone worrying about the imminent societal collapse of Australia, there's a bit of good news.
"It's actually surprisingly rare for all [of a state's] systems to go down at once," Dr Kemp says.
America's forgotten city
One of the best examples in history of societal collapse is perhaps one of the least known.
Around the year 1000, Native Americans created a city called Cahokia on the Mississippi River near today's St Louis. This was the first proper city in the present-day United States.
With 10,000 to 15,000 people, Cahokia was bigger than many European cities of the time, including London. It was a place of kings and elites overseeing a great experiment in urbanism.
But when Christopher Columbus and other Europeans started arriving in the Americas from 1492, the city had disappeared and no comparable cities replaced it.
"Within about 150 years, it starts to fall apart," Dr Kemp says.
Coinciding with a period of drought and flood, the city was entirely abandoned.
"All the key power structures you would associate with a modern state fell apart … [and] much of the culture changed as well," Dr Kemp says.
"But even more interesting is that the people there seem to intentionally forget about this experiment with urbanism … There are no oral traditions, no stories about it.
"It's almost like they wanted to forget about it and leave it to rot in the swamps of the Mississippi."
The fall of Rome: Not that bad?
After centuries of European domination, Rome — or the Western Roman Empire — fell in the 5th century.
Rome was sacked by Germanic Visigoths in 410, and then in 476, Germanic leader Odoacer deposed the last Western Roman emperor, Romulus Augustulus.
It seems to fit the brief of a societal collapse. The military fragmented. The economy plummeted. Trade broke up into local levels. The elites fell apart.
But Dr Kemp says it's a much more complex case than say, Cahokia.
"Some power structures did continue … The ideological basis of Rome, which by that stage was the church, actually continued to grow in power. It became the connective tissue across all of Europe after Rome fell," he says.
"Many of the cultures, customs and even garbs that the Romans had were carried on by the [new] Germanic rulers."
And in Dr Kemp's eyes — even with Pax Romana and all those aqueducts — the collapse of Rome wasn't necessarily a tragedy.
He explains the Roman model: Conquer a territory. Use the riches and resources of the new territory to conquer more territory. Repeat.
And he says meanwhile, most of the benefits of the empire were channelled back to the capital.
"It was a very large-scale pyramid scheme," Dr Kemp says.
"Its collapse was in many ways a good thing for a lot of people. If we look at skeletons in Latin Europe after the fall of Rome, people seemed to get taller and healthier.
"So I think we've been handed down a set of stories which have emphasised collapse — and probably overemphasised how bad it is."
The life span of a state
Dr Kemp and his colleagues have crunched the numbers and found the average life span of a state throughout much of history has been 326 years.
But there is a big range on either side of this number. For example, the Byzantine Empire lasted for 1,000 years, while China's Qin Dynasty lasted for just 15 years.
And the largest states, or "mega-empires covering over a million square kilometres", were more fragile, with an average life span of 155 years.
From Rome to Cahokia to many other examples of collapse, Dr Kemp says there's one key culprit.
"We tend to get a bit too preoccupied with looking at big external shocks," he says.
These can be things like plagues or enemies at the gates, which can be part of, but not the whole story.
"You find that different societies handle these [external shocks] pretty well, but then they seem to become weaker … This is largely due to the fact that inequality seems to increase over time."
So Dr Kemp calls inequality "the master variable behind crisis and collapse".
"Inequality tends to make states and societies much more vulnerable … [and] much more fragile."
On the flip side, after picking through the details of states from the Hittites to the Spanish Empire, Dr Kemp has a theory about what gives a state longevity.
"Democracy and inclusive institutions seem to be the big things that encourage states and societies to often last longer."
He cites research from Yale University that looked at how different societies coped with the Late Antique Little Ice Age of 536–660, when three different volcanic eruptions led to cooling temperatures around the Northern Hemisphere.
It found that some societies were more resilient to temperature swings than others — and that a big predictor of resilience was how democratic and inclusive their institutions were.
The next collapse?
So of the world's roughly 200 countries, which is next in line for a societal collapse?
"It would be very easy for me to say one of the countries in the Horn of Africa … Or Yemen for instance, which is in a state of civil war," Dr Kemp says.
"But today when we do have a collapse, such as the collapse of Somalia or Afghanistan, it tends to be very short-lived. Resources are mobilised to make sure that a new state is propped up very quickly."
Instead, Dr Kemp paints a picture less about individual states collapsing, but the possibility of something far bigger.
He is much more concerned about the "world as a whole", given we all live in a "global, interconnected system".
"If we're really thinking about a global collapse, then the things we have to start worrying about today are things like nuclear war, climatic change and dangerous new technologies," he says.
"All of those, once again, are built upon large systems of inequality."
And he says, while we can't put clear numbers on the likelihood of this kind of a collapse, the possibility of a "catastrophic" global event in the coming century has him worried.
