Victims of the Black Death in the 14th century, whose remains were discovered during excavations for London's new Crossrail railway line. Photo: Crossrail Ltd
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Gerbils and squirrels, not rats, may have been responsible for the Black Death

New research has found that rats alone can't have been responsible for Europe's medieval plague outbreaks - and giant Central Asian gerbils may have been an alternative accessory to the crime.

The political and cultural impact of the Black Death, which is usually seen as beginning in the period 1347-1353, on European life was immense. At least a hundred million people were killed (and possibly more than twice that by some estimates), reshaping the economy of feudalism and influencing the wars and politics that followed for at least the next four centuries, as plague outbreaks struck cities and villages over and over again. The bacteria responsible, Yersinia pestis, has long been believed to have originated in China, and to have been carried from east to west in fleas living on black rats that migrated with the merchants moving along the Silk Road, first by camel across central Asian steppes and deserts, and then across the Black Sea and Mediterranean by ship to major trading ports like Venice.

This distributional jigsaw may be missing some pieces, however - a new study by Norwegian and Swiss bioscientists and climatologists instead points the finger at some different rodents as bearing some of the responsibility for spreading plague: grey marmots, long-tailed squirrels, camels and Mongolian gerbils.

In the paper, published in the Proceedings of the National Academy of Sciences, the authors looked at 4,119 records of historical plague outbreaks across Europe, and cross-referenced them with 15 climate records derived from analysing tree rings within the same regions as the outbreaks. They were testing the hypothesis that the plague, once it arrived in Europe, dug itself in by living among rat populations (or "rat reservoirs" as the authors call them), which then experienced regular population swellings and shrinkings as the climate adjusted to become more or less rat-friendly over the centuries. They expected to see that plague outbreaks correlated neatly with warmer, drier summers, and that periods with fewer outbreaks were wetter and colder - yet it didn't quite work.

Most of the outbreaks in the records could be traced to other outbreaks nearby, but there were a select few - 16 in total, all in port cities like London, Lisbon and Hamburg - which had no connection with other outbreaks nearby, and where brand new imports of Y pestis via trading ships must have been responsible.

This, in itself, isn't too remarkable, but the researchers did notice a strong correlation between the climate records of east Asia and these 16 outlier European plague outbreaks. For each of them, certain climate conditions in east Asia would, 15 years later, be followed by a large outbreak. Something was linking the two sides of the Eurasian continent, and the researchers see a number of different rodents as responsible.

Consistently, a warming climate followed by a sudden cooling seemed to kickstart the transmission of plague bacteria across the Silk Road. The researchers theorise that warmer summer made it easier for fleas to find hosts to live and feed on - but the crash in temperature kills those same hosts, meaning fleas jump to new species, including humans. The model they propose has this happening over the first couple of years after the climate event; then, as those fleas move through the human population, they hitch onto humans on the edge of the Silk Road trading networks of central Asia.

From there, the plague moved slowly over a decade or so along trading routes, surviving in the large populations of rodents that live in those areas. Once the migration hit the eastern edge of the Black Sea, the journey was then sped up thanks to the speed of ships - within two or three more years, the plague will have successfully travelled from one side of the world to the other, to infect those living in European cities. "This association strongly suggests that the bacterium was continuously reimported into Europe during the second plague pandemic," writes lead author Boris Schmid of the University of Oslo, "and offers an alternative explanation to putative European rodent reservoirs for how the disease could have persisted in Europe for so long."

This doesn't get the black rat (Rattus rattus) off the hook - it's still the key vector for plague transmission once the bacteria arrived in Europe with each new wave. But it does mean that other members of the rodent family lose some of their ability to act all cute and innocent.

Ian Steadman is a staff science and technology writer at the New Statesman. He is on Twitter as @iansteadman.

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Don’t shoot the messenger: are social media giants really “consciously failing” to tackle extremism?

MPs today accused social media companies of failing to combat terrorism, but just how accurate is this claim? 

Today’s home affairs committee report, which said that internet giants such as Twitter, Facebook, and YouTube are “consciously failing” to combat extremism, was criticised by terrorism experts almost immediately.

“Blaming Facebook, Google or Twitter for this phenomenon is quite simplistic, and I'd even say misleading,” Professor Peter Neumann, an expert on radicalisation from Kings College London, told the BBC.

“Social media companies are doing a lot more now than they used to - no doubt because of public pressure,” he went on. The report, however, labels the 14 million videos Google have removed in the last two years, and the 125,000 accounts Twitter has suspended in the last one, a “drop in the ocean”.

It didn’t take long for the sites involved to refute the claims, which follow a 12-month inquiry on radicalisation. A Facebook spokesperson said they deal “swiftly and robustly with reports of terrorism-related content”, whilst YouTube said they take their role in combating the spread of extremism “very seriously”. This time last week, Twitter announced that they’d suspended 235,000 accounts for promoting terrorism in the last six months, which is incidentally after the committee stopped counting in February.

When it comes to numbers, it’s difficult to determine what is and isn’t enough. There is no magical number of Terrorists On The Internet that experts can compare the number of deletions to. But it’s also important to judge the companies’ efforts within the realm of what is actually possible.

“The argument is that because Facebook and Twitter are very good at taking down copyright claims they should be better at tackling extremism,” says Jamie Bartlett, Director of the Centre for the Analysis of Social Media at Demos.

“But in those cases you are given a hashed file by the copyright holder and they say: ‘Find this file on your database and remove it please’. This is very different from extremism. You’re talking about complicated nuanced linguistic patterns each of which are usually unique, and are very hard for an algorithm to determine.”

Bartlett explains that a large team of people would have to work on building this algorithm by trawling through cases of extremist language, which, as Thangam Debonnaire learned this month, even humans can struggle to identify.  

“The problem is when you’re dealing with linguistic patterns even the best algorithms work at 70 per cent accuracy. You’d have so many false positives, and you’d end up needing to have another huge team of people that would be checking all of it. It’s such a much harder task than people think.”

Finding and deleting terrorist content is also only half of the battle. When it comes to videos and images, thousands of people could have downloaded them before they were deleted. During his research, Bartlett has also discovered that when one extremist account is deleted, another inevitably pops up in its place.

“Censorship is close to impossible,” he wrote in a Medium post in February. “I’ve been taking a look at how ISIL are using Twitter. I found one user name, @xcxcx162, who had no less than twenty-one versions of his name, all lined up and ready to use (@xcxcx1627; @xcxcx1628, @xcxcx1629, and so on).”

Beneath all this, there might be another, fundamental flaw in the report’s assumptions. Demos argue that there is no firm evidence that online material actually radicalises people, and that much of the material extremists view and share is often from mainstream news outlets.

But even if total censorship was possible, that doesn’t necessarily make it desirable. Bartlett argues that deleting extreme content would diminish our critical faculties, and that exposing people to it allows them to see for themselves that terrorists are “narcissistic, murderous, thuggish, irreligious brutes.” Complete censorship would also ruin social media for innocent people.

“All the big social media platforms operate on a very important principal, which is that they are not responsible for the content that is placed on their platforms,” he says. “It rests with the user because if they were legally responsible for everything that’s on their platform – and this is a legal ruling in the US – they would have to check every single thing before it was posted. Given that Facebook deals with billions of posts a day that would be the end of the entire social media infrastructure.

“That’s the kind of trade off we’d be talking about here. The benefits of those platforms are considerable and you’d be punishing a lot of innocent people.”

No one is denying that social media companies should do as much as they can to tackle terrorism. Bartlett thinks that platforms can do more to remove information under warrant or hand over data when the police require it, and making online policing 24/7 is an important development “because terrorists do not work 9 to 5”. At the end of the day, however, it’s important for the government to accept technological limitations.

“Censorship of the internet is only going to get harder and harder,” he says. “Our best hope is that people are critical and discerning and that is where I would like the effort to be.” 

Amelia Tait is a technology and digital culture writer at the New Statesman.