♪♪ -The Roman Empire stretched from Hadrian's Wall in Britain east to Babylonia, in present-day Iraq, and across the northern coast of Africa.

Expanding and contracting, the empire lasted nearly 1,000 years, influencing politics, art, and architecture for millennia to come.

Historians and archaeologists have long theorized about what caused the collapse of such a powerful civilization.

Did surrounding enemies become more ruthless while Roman emperors lost their ability to lead or was it something far beyond the control of any emperor or military leader?

Today, archaeologists and scientists speculate that a series of virulent pandemics took their toll on the mighty realm.

New scientific advances have allowed researchers to identify the pathogens responsible for decimating Ancient Rome's population.

Did mass disease weaken the military and allow adversaries their first victories?

Or were other forces involved?

Centuries of climate data from all over Europe and Russia has shown how the climate changed across the empire.

How would a sudden temperature drop have affected the health of Rome's inhabitants?

Cooling temperatures likely provoked the great westward migrations that changed the history of Europe and brought Rome into conflict with numerous groups.

And then, at a moment when it seemed the empire might return to its former glory, the emperor's troops failed to reconquer lost territories.

Could a violent natural disaster be the real reason Rome fell?

What destroyed one of the most powerful civilizations in history?

♪♪ ♪♪ ♪♪ -The decline of Rome begins several centuries before the empire actually falls.

In the year 165 A.D., two men lead the vast dominion -- Marcus Aurelius and Lucius Verus.

That year, Marcus stayed in Rome, while Lucius left to fight the Parthian Empire in the East.

Rome is victorious over the Parthians, but in the coming years, many in the empire wonder whether an excess of violence brought disfavor from the gods.

Rome takes control of the Parthian capital city, Seleucia, which is located on the Tigris River and is home to an important temple to Apollo.

Initially, Roman troops enter the city peacefully, but then looting begins.

-So, it was the year 165, after four years of war between Romans and Parthians, in the disputed Middle East.

The victories achieved by the Romans gave them access to Mesopotamia, and they were able to sail in several columns down the Euphrates and Tigris rivers, to the heart of Babylonia, where the large capitals of the Parthian Empire were located.

The largest one was Seleucia-on-Tigris, a huge city with hundreds of thousands of inhabitants, the equivalent of Rome or Alexandria from the Parthians' point of view.

Although it was a cosmopolitan city, its history was mainly Greek.

It was like a Greek city.

♪♪ The city gates had been opened, and the Romans were able to enter peacefully.

But fairly quickly, things got out of hand.

The city was pillaged and set fire to.

A massacre took place, and the Romans left with all the riches they could find.

What happened was that the city's high temple, dedicated to Apollo, "Long-haired Apollo," as he was known at the time, was where the plundering took place.

And everything considered valuable or prestigious was taken, notably the huge statue of Apollo, which ended up back in Rome.

And so all the Romans were able to see it at the end of the war when victory had been declared.

Long-haired Apollo was the god of plagues, the god of healing and medicine, and also the god of archery, who sent death and disease.

And so, without doubt, the Romans associated this statue with the shameful plundering and pillaging of the city of Seleucia and with the sort of divine retribution which befell them afterwards, the epidemic which hit Rome at more or less the same time as the triumphal victory.

♪♪ -Lucius Verus and his army return to Rome victorious in 167 but have unknowingly brought back with them a dangerous enemy.

The size of the Roman Empire reaches its historic peak.

Estimates put its population between 50 million and 90 million people, roughly 20% of the world's population at the time.

But at this moment of triumph, the empire is rocked by an epidemic of unprecedented strength, spread by the returning military.

Is Apollo punishing Rome for the destruction of Seleucia?

The Antonine Plague arrives in Rome in the last years of the Pax Romana, when the empire is at the height of its power, control, and population.

The first-known major epidemic to hit the Roman Empire continues for the next several decades, and between 5 million and 10 million people lose their lives.

Galen, an early physician famous for his medical investigations, recorded the unfolding tragedy.

Véronique Boudon-Millot is an expert on Galen's writings.

-I spent three more years in Rome, and when the great plague broke out, I quickly left the city and hurried back to my country.

To my knowledge, there was no sufficiently powerful medicine to fight this plague, which spread everywhere before dying out.

Galen left Rome as soon as the epidemic had been declared and took refuge, so to speak, in his native town, where he spent two years in relative peace and quiet, according to what he wrote at the time.

He went about his usual daily life.

And then he received a letter from the joint emperors, Marcus Aurelius and Lucius Verus, ordering him -- it was indeed an order -- to join them in Aquileia.

And when he arrived in Aquileia, he found an army camp where the soldiers were crammed in.

The camp was overcrowded, and no sooner had he arrived when a terrible epidemic, stronger than before, broke out and decimated the soldiers.

The two emperors, Marcus Aurelius and Lucius Verus, were terrified and left -- "ran away," in the words of Galen -- and returned to Rome.

And Galen stayed there alone with a few other army doctors and had to deal with thousands of casualties, he tells us, among the soldiers.

Pestilence was like a savage beast, destroying its victims horribly, devouring them -- not in small numbers, but whole towns of them.

Most people died not only because of the plague, but also because it all took place in the heart of winter.

Despite his long experience as a doctor, having witnessed many things in his career, he said that the extensive pestilence was something he'd never seen before, and he described unheard-of symptoms.

The theory which is most prevalent today is that it was smallpox, the very first smallpox epidemic in the Roman world, with a population who had no immune system against a disease never before encountered.

The devastation was clearly significant, but it's still very difficult to put a figure on it.

-Galen notes the devastation the disease causes at the military camp in Aquileia.

The soldiers are stationed there in preparation for battle against barbarians from the north and east.

This fighting would go on for years, as a variety of tribes tested the strength of the Romans along the Danube and Rhine rivers.

The ongoing conflict takes a toll on the empire.

As the pandemic rages, Rome, desperate to keep their military fully staffed, begins recruiting soldiers from across its vast territories, a decision that may have weakened the cohesion of the once-mighty army.

Barbarians crossing into Greece, deep in Roman territory, defeat these soldiers, one of the empire's first losses.

In August 170, enemy forces attack one of the empire's most sacred sanctuaries, the Temple of Eleusis, a place of pilgrimage famous throughout the Roman world.

This stunning act of violence has repercussions across Ancient Rome.

-Every year, hundreds of initiates came in procession through this gate.

The initiates were young, old, men, women, free citizens, and slaves.

The initiation was open to everybody, on two conditions.

They had to be able to speak Greek and not have blood on their hands.

If these two conditions of purity were fulfilled, anyone was entitled to receive the blessings of the goddesses.

-According to Greek mythology, Eleusis is where the goddess Demeter gave humanity the gift of agriculture.

A temple was built to honor the site, and every year, pilgrims arrived to be part of secret religious rites, known as the Mysteries of Eleusis.

The site's importance for the empire and the sacred objects within make the temple an obvious target for the barbarians.

-It was an enormous space, a jumble of 42 columns 54 meters wide, 20 meters more than the Parthenon.

And in the center of this huge area, on the side of which were the seats for the initiates, there as a special chamber, called the Anaktoron.

This was the heart of the sanctuary.

It's still in the same place today, despite all the changes to the site over the centuries, and the centerpiece of the ritual.

It's the area where the sacred objects were shown to the initiates.

And it's here that the initiates discovered something, which we know very little about, as they were sworn to secrecy.

So we can only surmise as to what happened in this spot and guess at what the mysteries were.

♪♪ -The Costoboci warriors, a tribe from the Southern Balkans, overrun the shrine, plunder its sanctuary, and then burn the temple to the ground.

This attack on such an important site signals Rome's changing fortunes.

♪♪ -This is Emperor Marcus Aurelius.

It was his armies who couldn't prevent the Costoboci getting through and causing the catastrophic pillage and destruction.

There's a statue of him because he was the one who had everything rebuilt.

♪♪ Between the years 171 and 176, in the Danube region, there were five years of bitter war to keep the barbarians at bay and bring about peace.

♪♪ From the Greeks' and Romans' perspective, it would seem that the situation had returned to normal and that the affront of having been violently plundered had been forgotten.

But from the barbarians' point of view, they saw things differently.

They now knew that they could break through.

The warlords could now spot the weaknesses just across the empire's borders in the hope of once again seizing valuable booty and returning triumphant to their own people.

-The unimaginable has happened at Eleusis.

Barbarian tribes in territories surrounding the empire now know it's possible to defeat the most powerful army in the world.

Despite the military losses, Rome pushes forward with an immense public-works project.

Emperor Caracalla inaugurates a new complex of thermal baths in 216 A.D. -The huge scale and luxury of the Caracalla Baths demonstrated the prosperity of an empire that had recovered its strength and filled its financial coffers.

The 11-hectare baths were the largest in Rome at the time -- a gigantic construction built with amazing technical prowess.

Building huge vaulted ceilings and, at the same time, ensuring that they would resist the heat and humidity was a masonry nightmare.

-The thermal baths are central to life in Ancient Rome, serving as important locations for socializing.

Most cities and towns throughout the empire have at least one.

But each bath is also a dangerous place, where germs are spread and epidemics take hold.

-The emperor created an environment that was completely controlled, as the water and air temperatures were regulated.

People came in big groups to meet up, look after their well-being, and have a healthy lifestyle, which was important to the Romans.

But from our modern point of view, this hygiene was deceptive, because it didn't take into account microorganisms.

Bathing in the same water, which was not chlorinated, purified, or filtered, meant that everyone shared the same germs and the same intestinal parasites.

The baths inevitably contributed to the spread of diseases in the city.

-In 251 A.D., a second pandemic, likely a hemorrhagic fever, starts in Alexandria, Egypt, and sweeps across all of Ancient Rome.

-Just like in the Marcus Aurelius period, there was a correlation between the disease and the military problems on the empire's borders.

But, in some ways, from the year 250 onwards, the danger to the empire was more serious than before.

The enemies really forced their way through.

Pillaging was rife and more extensive, and the stakes for the army were more serious.

Barbarian invasions led to a greater need for security in the empire's provinces.

This security emanated from the emperor and his armies.

But since there were simultaneous and multiple front lines and one emperor couldn't be omnipresent, the regions would set about choosing their own.

♪♪ It came about that three, four, or more emperors co-existed for brief periods of time.

So, along with defeat came civil war.

-For the next 30 years, a succession of emperors and usurpers leads the empire, most only in power for a few months and often stationed with their troops far from the capital.

The empire remains fragmented until the end of the 3rd century, when a series of military victories and reforms unites the empire once again.

But this calm will not last.

-In the 2nd century, the outer limits of the city of Rome weren't very clear.

It was Aurelian, in the 270s, who decided to fortify the city by building a wall around it.

♪♪ It was a sign of the concern that was weighing on the empire, because they knew that the barbarians, as well as the Goths and the Alemanni, were descending on Italy from the north time and time again, threatening Milan, and Rome could be next.

-But the new walls offer only limited protection, and the city remains an easy target for Germanic and Eurasian enemies.

Just 30 years later, the empire's seat of power moves east to Byzantium, located at the passage between the Mediterranean and the Black seas.

The new capital is closer to the most dangerous enemies and easier to defend if invaded.

But the move and the division between the eastern and western territories of the empire is another sign that Ancient Rome's strength is slowly fading.

And with barbarian attacks increasing and political control fractured, a new danger threatens the empire -- climate change.

Today, scientists are finding evidence in the Czech Republic that shows significant fluctuations in temperature and rainfall, just when Rome was already weak.

-All plants that live for more than one year outside the tropics, so where we have a seasonal cycle in the climate system, they produce a growth layer.

We call it a ring.

And since most of the plants that are of interest for us are trees, we call them tree rings.

♪♪ ♪♪ And depending on the growth conditions, if they are favorable or less favorable, the ring will be wider or narrower.

So we are basically able to see the fingerprint of the environment and climate captured in the tree-ring sequences.

[ Grunting ] So, I expect about 80 to 100 rings on this oak tree, so we would basically go back over most of the 20th century.

♪♪ If we take a core sample, we know the outermost ring, just before bark.

So, the ring that was produced in this summer has the date 2021, and then we just count the years back.

-The climate data that scientists collect from the tree rings can stretch back several centuries, depending on the age of the tree.

This is the first step in a process scientists will use to understand the climate of Ancient Rome.

The next step is to look at samples from the oldest wood they can find.

-Go around this one.

The main timber here in the middle looks good.

This old windmill here is dating approximately in the beginning of the 18th century, so it's roughly 200 years old.

By then, the material, the oaks that were used to build this construction, are again about 150 to 200 years old.

So that means we are getting back with the tree rings, with these oak rings, almost 300 to 400 years back in time.

♪♪ We need many, many, many rings, rings from many trees.

So that's why we are not only taking one or two cores from this construction here -- we will likely extract 20 to 30 cores from individual oaks.

♪♪ -Finding wood 2,000 years old requires a visit to archaeological dig sites, like this medieval well in the Czech city of Brno.

It dates from the 12th century and contains a rare resource -- partially fossilized wood, preserved in the ground for centuries.

♪♪ ♪♪ But a new discovery takes the scientists even further back in time.

♪♪ -Gravel pits are a particularly useful source for such old subfossil material because the trees must be buried under anaerobic conditions.

So they are somehow in the gravel below the water table.

And you see, we are here in a very flat area.

So, the river was meandering, changing its bed, over the last centuries and millennia.

The dead trees will fall in the water and they get buried.

We don't know how old the material is.

It can be between 1,000 and 8,000 years before present, so there is a huge range of possibilities.

And it is so important because this is the material that really extends our oak chronologies the furthest back in time.

-For this study, dendrochronologists collect hundreds of samples of oak trees, without knowing which historical time period they may be from.

At the Mendel University in Brno, each of these samples is carefully sanded so that their rings can be highlighted and precisely measured.

[ Beeping ] -Once this is done, we start a process, a technique, that is called cross-dating.

[ Clicking ] With enough overlap between the different sources, from the living material to the relic wood, the historical timber, later on, the archeological wood, and the subfossil one, if we have enough overlap, if the tree rings match, if they fit together, we can absolute date all to the historical part.

-These curves show the variations in the width of the wood rings through time.

Each curve overlaps, creating a timeline that stretches back centuries and millennia.

-Once we have all these tree rings together and we can say, "Ah, this ring is the year 1258, this ring is the year 505," we take the wood that is absolutely dated, we cut the material with a small knife -- have to be very careful that no material from the previous and the next ring is included in this probe.

So, we talk about thousands and thousands of small probes, and then the process starts.

We extract the cellulose pure.

We homogenize the cellulose so that we have a very homogenous mass, put it in a silver or tin capsule, and then we have two samples per tree ring.

One we're gonna use for carbon, one for oxygen.

-The carbon and oxygen levels from the samples tell scientists about historical temperatures and rainfall that enable them to create an accurate picture of plant-growth conditions.

-So, if we start in the Roman period -- so let's say 100 years BC to about 150, 180 years Common Era -- we see this was generally dry period.

Dry means, for us, it was likely also warm.

One could say it was favorable.

And it was less fluctuating, so that's an important thing.

Independent of warm or cold, it was more stable.

And then we enter a period around 200 where fluctuations kick in.

In this case, it was getting wetter.

Wetter and potentially also cooler.

And these fluctuations are always unfavorable, because societies are not able to adapt to them if they are too fast.

And they are unpredictable.

-The data from the tree rings indicates that after several centuries of relatively mild temperatures, a period known as the Roman Climatic Optimum, the climate in the empire began to cool.

Cyprian, a writer who was also bishop of Carthage in the 3rd century, described a world where the sun's rays were less bright and harvests less abundant, "Only a pale old man on the verge of his grave."

And the empire's problems continue to mount.

With the cooler temperatures, Eurasia's nomadic tribes move west into Eastern Europe, in hopes of finding more hospitable land for agriculture.

The Huns, a formidable adversary, begin attacking the empire's eastern border.

Again, tree rings provide the evidence for understanding these westward migrations.

-In addition to our research in Central Europe, we also spent much effort with our Russian colleagues in Southern Siberia.

♪♪ So, this is the great Altai region at the border between Russia in the north, Mongolia in the south, China in the southwest, and Kazakhstan in the west.

♪♪ ♪♪ -The present-day Altai Forest starts at an altitude of 2,400 meters.

We can start from there and go even higher.

♪♪ -So, the Altai is particularly important for us to reconstruct summer temperatures because we find very old living and relic dead trees at the upper tree line.

♪♪ There is a reason why they can't grow higher -- because it's getting too cold.

So even small changes in summer temperature will leave a distinct signal in the tree rings.

So the tree rings are almost like a thermometer.

♪♪ ♪♪ -The natural conditions in high altitudes are characterized by extremely low temperatures.

And these temperatures prevent the wood from decomposing.

♪♪ ♪♪ Also, very often in mountainous zones, dead wood doesn't lie on the ground, but on stone, which also prevents the destruction of the wood.

♪♪ -Once the Russian scientists finish collecting the wood, they travel to Novosibirsk State University in Siberia, where the samples will be dated and measured.

The data is then sent to the University of Cambridge, in England.

-Very interestingly, the summer temperature reconstructions from the Altai, so from Inner Eurasia, which is about 5,000 kilometers east of the European Alps, are very similar.

[ Beeping ] ♪♪ So we have evidence for a more stable, warmer Roman period, and around the 2nd century A.D., temperatures become to be more variable, so there is more fluctuation.

Past agricultural systems were pushed towards their limits, and nomadic steppe empires -- those that we know from Inner Eurasia -- were most likely affected in a way that they just started to move, to migrate further distances.

♪♪ ♪♪ -So, here, we're at an archaeological site called the Kuraika Cemetery.

It corresponds perfectly with the historical era of the Huns.

♪♪ -The Altai has always been a territory which one could call a crossroads for different populations.

It's a territory where we find traces of nomads from all eras, nomads who came one after the other over several millennia.

[ Shouting ] When a cold snap happened and the climate graph indicates a drop in temperatures, we can see that people migrated and the Altai territory became depopulated.

In general, people left this region because farming conditions were deteriorating.

-Once the Huns make it to the eastern edge of the empire, they discover the region has already been settled by nomadic tribes from Northern Europe.

Smaller groups, like the Goths, crossed the Danube and settled in the Balkans and north of Greece, in present-day Bulgaria.

-They traveled right across the Russian plain, along the Volga River, turned south towards Crimea and the Black Sea, and encountered barbarians well-known to the Romans -- the Goths and the Sarmatians -- and fought them.

The barbarians neighboring the empire found themselves caught between the hammer of the Huns and the anvil of the empire and asked Rome if they could be integrated into their territory.

The Goths were not at all welcome once they crossed the Danube.

They found themselves exploited by the greed of Roman officials in the province.

So they decided to revolt.

-The Goths had hoped Rome would annex their territory and offer protection from the Huns.

The Romans agree to provide the Goths with security, but taking advantage of the group's vulnerability, they also treat them poorly.

In 378 A.D., the Goths revolt.

In Adrianople, northwest of Constantinople, the Eastern Roman Empire, led by Emperor Valens, faces off against the Gothic rebels, led by Fritigern.

Believing his forces outnumber the Goths, Valens goes on the attack... a decision that proves fatal.

[ Shouting, clanging ] The Romans find themselves overwhelmed by the enemy cavalry and reduced to hand-to-hand combat.

It is the greatest defeat in the history of the empire.

Tens of thousands of soldiers are killed, and the emperor Valens himself dies in battle.

With this major defeat, the Eastern Empire no longer has the resources to support its other half in the west.

Left vulnerable to attack and more easily reached, it is only a matter of time until barbarian tribes conquer the Western Empire.

The Gothic victory reveals Rome has weaknesses that can be exploited, bringing its enemies ever closer to the capital city.

No longer the seat of power, the Eternal City is sacked in 410 A.D., though battles in the west continue.

Finally, when the Vandals invade North Africa in 429, the Western Empire disappears altogether.

Well-armed and less exposed to invasion, the Eastern Empire in Byzantium continues on... until the beginning of the 6th century, when Emperor Justinian sees a chance to get revenge and reconquer lost territories.

But his military strategy fails to consider a force much stronger than any of the barbaric tribes -- the climate.

-Let's imagine we're in the year 535.

The eastern provinces of the Roman Empire are doing fine.

They have a new upstart emperor, who was born in the western provinces, who speaks Latin as his native tongue, and has a dream to reconstitute the Roman Empire.

And he targeted as his first goal to reconquer Africa.

Africa fell almost in a matter of months.

The Roman Empire had reconquered the richest breadbasket of the western half of the Roman Empire and was poised to now move on Italy.

In 536, they cross from Sicily into the mainland of Italy and start marching towards Rome.

And they're marching up successfully in the spring of 536, when something entirely unexpected happened.

The sun stopped shining for a period of between 12 and 18 months.

A contemporary says that at noon, the strength of the sun was about like that of the moon.

-What could have made the sun stop shining in the spring of 536 A.D.?

Was the constant darkness why Justinian failed, when the whole empire seemed within his reach?

-Do you want to flat out switch to... -At the University of Maine, scientists have identified physical evidence of how the climate was shifting at the same time Justinian set out to expand the empire.

-...when there's a small iron peak, but a much higher sulfur peak.

-A unique team of climate specialists and historians has come together to study how a natural climate event could have brought down the Roman Empire.

♪♪ This large freezer inside a University of Maine lab holds an ice core, which offers insight into what happened in the 6th century.

-This piece is about 60 meters deep.

We keep it in our freezer at temperature minus-25 Celsius.

And right now, I'm preparing this ice for laser-ablation analysis.

I'm scraping the top part off to remove any contamination and make the surface flat, as flat as possible.

-This ice core serves as a record of the climate in Europe for the last 4,000 years.

Scientists retrieved it from a mountain peak on the border of Switzerland and Italy, in the heart of what was once the Roman Empire.

-Up until now, most of the great ice-core work has been done in Antarctica.

There, the technology is perfectly adapted to ice cores that, as in Greenland, are 3 kilometers long.

In Europe, the situation is completely different.

The ice core, from today to bedrock, is 72 meters.

And the technology was not yet established which would allow one to see the patterns in ice that was so heavily compressed.

The Climate Change Institute, under Professor Mayewski, has perfected a new instrument which allows one not to measure 100 times in a meter, but up to 40,000 times.

♪♪ -The remarkable thing about this instrument is that it uses a laser to collect the sample.

And you can actually see it on the screen here.

It almost looks like a worm moving along.

What it's actually doing is, like a tiny jackhammer, poking away, flaking off chips of the ice.

Once those pieces of ice fly off, they get incorporated in a gas.

And that gas then takes the sample to this instrument, which is capable of measuring very, very low concentrations of things like copper, lead, sodium.

Every single one of these has a different story about the climate, about human activity, a variety of things.

-Once they've identified the area of the ice core that corresponds with the year 536, scientists find a higher level of sulfur than is normal -- evidence of a volcanic eruption.

They also find volcanic glass dust in the core sample, which allows them to locate where the eruption took place -- Iceland.

But would this volcano have been enough to stop Justinian's military campaign?

-Determining the magnitude of the eruption is hard from an ice core, except if you take a look at the sulfur, the bismuth, and the tin levels, they're remarkably high for the last 2,000 years.

So that would tell us that the 536 was a very big event.

[ Rumbling, explosion ] -The eruption threw so much sulfur dioxide into the atmosphere that, in the years following, temperatures drop by about 2 degrees -- one of the coldest periods of the last 2,000 years.

-And so, 536 -- that's the beginning.

Another volcano in 540, another one in 547.

It gets colder and colder and less and less appealing in terms of crop growth.

And then, in 541, in the Nile Delta, there explodes a new pathogen.

-In 541 A.D., an epidemic of unprecedented virulence changes the history of the empire forever.

Today, archaeologists are searching for traces of this epidemic all across Ancient Rome.

Here on the island of Maguelone, near Montpellier, France, they suspect the epidemic caused the collapse of a Mediterranean village.

-This is a magical place.

It's a cathedral built on an island.

It's an island very close to Montpellier, not far from the mainland, situated between the marshland and the sea.

It's surprising, because today, we can just drive across, but in the past, this island was separated from the mainland.

♪♪ -We've found Byzantine-style weights and measures, suggesting that there had been spice merchants here, trading across the Mediterranean.

♪♪ -The first thing you would see on arriving on the island, probably, were the boats.

There must have been a whole flotilla of seacraft -- smaller boats and flat-bottomed boats for navigating the marshes.

♪♪ ♪♪ We suspect that they were storage depots, mainly that.

It's likely that there was trade in sheets, wool, cheeses, and all sorts of goods that don't leave archaeological traces.

♪♪ -In the years 560 to 580, it's noticeable that the site became depopulated, and we discovered some rather strange things.

We found corpses, makeshift burials, which were really just bodies thrown into the rubble of demolished buildings.

There was an epidemic situation or a war situation.

In any case, the occupying power was in a phase of decline.

♪♪ We're trying to shed light onto some of the darker aspects of this age covering the 5th and 6th centuries.

It's the key period between the Roman and Byzantine worlds and the emerging medieval world, with the creation of the Germanic kingdoms.

-So, here, there are two femurs, the thigh bones, pointing downwards.

The individual doesn't seem to have been buried horizontally on the site, as would normally happen.

You can see it clearly here -- there are these parts, which are lying more or less flat, but when you get to these bones here, they're pointing down into the earth.

The body might have just been thrown away like that, like you throw rubbish into a tip.

♪♪ -Occasionally, the bodies were thrown away, literally thrown into a rubbish dump.

Sometimes, they were buried carefully -- no signs of wealth or caskets or any construction, but with a certain amount of care.

♪♪ -We've also found, for example, a body buried face-down, which is a terrible punishment for a Christian, since it signals absolute deprivation from eternal light.

♪♪ -Oh, look -- here's a tooth.

-Often, it's teeth which are the best-preserved parts of ancient corpses.

Sometimes, that's all we find because they fossilize so well.

What's wonderful about this is that they're all there.

And that will enable us, thanks to DNA, which is so well-preserved in teeth, to study diseases, in particular the plague, to look for the plague bacillus.

♪♪ -Numerous burials in disarray, buildings destroyed, and the village deserted.

Why was this apparently prosperous town suddenly abandoned during the 6th century?

♪♪ Answers to this question can be found at the Max Planck Institute in Germany, where a team specializing in the study of diseases found in archaeological remains is now identifying pathogens from Ancient Rome.

Bones are being collected from dig sites all over Europe.

-So, what we usually take as material to start with are actually teeth from the past, from a person that has died, potentially of an ancient infectious disease.

And a tooth is so interesting because inside the tooth, you have dried blood, and inside the blood, of course, the pathogen might have been circulating.

So, how do we get that out?

We have to basically cut the tooth open.

So, we make a cut and then we drill inside that cavity and remove bone powder -- little pieces of bone that, on its surface, might have the DNA of the pathogen bound.

♪♪ ♪♪ So, what we then next do is we have to somehow get the DNA out of that bone powder from the tooth.

So, we do that by dissolving it in a liquid.

So, what you then have is something we call the DNA extract, which is the extracted DNA from the ancient bone, where you then have little pieces of DNA of the person.

You have maybe the pathogen that has killed the person.

But the majority of the DNA that we get from an ancient skeleton is actually not from the person and it's not from the pathogen, but it's from the environment, because the skeleton has been in the ground for more than 1,000 years, so it accumulated DNA of many, many micro-organisms.

You really have a soup of DNA, and just a tiny proportion of that DNA is actually from the pathogen that you are interested in.

It's maybe something like 0.001% of the DNA is actually the pathogen DNA that we are interested in.

[ Beeping ] -In order to find a specific pathogen, the DNA from the tooth sample is broken into many segments.

The segments are then compared with the DNA of known diseases, eliminating possibilities like tuberculosis or typhus.

♪♪ In the tooth sample, the scientists identify Yersinia pestis, the bacteria that causes the Black Plague.

-We have been looking for Yersinia pestis from the time of the Justinianic Plague, so from the 6th century.

We have actually screened hundreds of human remains from the time period to see where in Europe you could find plague during the time of the Justinianic Plague.

And we were actually quite surprised because we were finding it in individuals from Iberia, so from modern-day Spain, from France, from Germany, from England, so from large parts of Western Europe.

In about a dozen or so ancient human remains, we found Yersinia pestis DNA to be preserved, and we could reconstruct entire genomes of those ancient plague bacteria.

-Gradually, scientists draw a map of where they're finding plague, noting that, based on the succession of waves of illness, the epidemic lasted roughly a century.

In Constantinople, an estimated 300,000 people died during the first year of the outbreak.

Despite its vast territory and immense power, the Roman Empire could not withstand centuries of mass disease happening at the same time that the climate was cooling and crops were failing.

The process took several hundred years, but ultimately, Ancient Rome may have fallen as a result of forces far greater than military or economic power -- perhaps a lesson about the limitations of humanity in the face of Mother Nature.

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