Science & technology | Biology’s plague journal Ancient DNA is rewriting the history of plague Dense cities do not seem to have been necessary for outbreaks of the disease June 18th 2026 IN 2011 A team of geneticists managed to recover centuries-old DNA from the teeth of bodies that had been buried in East Smithfield, a medieval cemetery in London. Besides human DNA, they were able to isolate genetic material from Yersinia pestis, the species of bacterium that causes plague. This let them confirm what historians had long suspected but had never quite been able to prove: that the Black Death, which killed perhaps half of Europe’s population between 1346 and 1353, was indeed an outbreak of plague. For a field more used to pottery, ancient ruins and often unclear written records, such “ancient DNA” has been revolutionary. It has allowed

scientists to reconstruct the history of plague in unprecedented detail. Genomics has likewise confirmed that the Plague of Justinian, which struck the Byzantine empire between the years 541 and 549, was also caused by Y. pestis—although, interestingly, the strain responsible was not ancestral to the one that devastated Europe 800 years later. The Plague of Justinian is the first big outbreak mentioned in the historical record. But DNA has proved that the disease goes far back into prehistory, too. Now, in a paper published in Nature, Eske Willerslev, a geneticist at the University of Copenhagen and the University of Cambridge, and his colleagues describe the earliest evidence of plague to date: two lethal outbreaks of the disease around 5,500 years ago among hunter-gatherers living near Lake Baikal, in what is these days called Siberia. Intriguingly, that is long before the appearance of the sorts of densely populated, squalid cities—such as medieval London or ancient Constantinople—that had been thought to be necessary for big outbreaks of the disease. As with the bodies from East Smithfield, Dr Willerslev and his colleagues were interested in the hunter-gatherers’ teeth. Being tough and hard-wearing, teeth help protect DNA from the ravages of time. And because the pulp inside them is connected to the circulatory system, DNA from pathogens in a person’s blood can be preserved as well. Teeth are a “bio-archive of a person’s life”, says Christina Warinner, an anthropologist at Harvard University. Hundreds of ancient Y. pestis genomes have now been sequenced. The origins of the Black Death have been traced to Lake Issyk-Kul in modern- day Kyrgyzstan, near the trade routes that linked Central Asia to Europe. The source of the Justinian plague is still debated. But the genetic evidence suggests that it too emerged in Central Asia, possibly in the second century. Researchers now know that Y. pestis was circulating among humans as early as the Late Neolithic period. They have discovered strains that are now extinct. Others have adapted and survived, mutating into the strains that killed millions of people thousands of years later. When Dr Willerslev and his colleagues sequenced the DNA from Lake Baikal, they found evidence of two distinct outbreaks, occurring about 5,050 and 5,520 years ago. Almost 40% of the people buried in four different

cemeteries during the Late Neolithic had been infected, a rate higher than some of London’s plague pits. And the infections appear to have been deadly. That is significant, because whether early outbreaks of plague were as lethal as later ones has been a subject of much academic debate. Radiocarbon dating suggests the skeletons were buried at similar times. There are several instances of siblings, or parents and children, being buried together, indicating that the disease spread among close relatives, perhaps as they cared for one another. One particularly striking feature, says Dr Willerslev, is the number of children and teenagers. Genetic analysis showed that the Baikal strain of Y. pestis carried genes for a toxin found today in Y. pseudotuberculosis, a closely related bug that causes a disease called yersiniosis. The toxin is a “superantigen”, which can provoke a violent and sometimes fatal over- reaction from the immune system. Such complications occur more commonly in children, rather than adults, infected with Y. pseudotuberculosis. If the same was true of Y. pestis 5,500 years ago, that could explain the high death rate among the young. The findings also challenge the idea that densely packed populations and proximity to livestock set the stage for big outbreaks of the disease. Y. pestis is mostly found in rodents, such as marmots, mice and rats, and is often spread by flea bites. The large rat populations in medieval towns and cities —and the fleas they hosted—are thought to have been vital in spreading the Black Death. But the victims at Baikal were not sedentary urbanites. They were hunter- gatherers living in small, mobile communities. Intriguingly, many prehistoric strains of Y. pestis, including those found in Dr Willerslev’s studies, lack a mutation that helps the bug to survive inside fleas. Besides flea bites, plague can spread through contact with bodily fluids, or as droplets in the air. Dr Willerslev’s results suggest that those alternative transmission methods may have been enough to cause calamitous outbreaks by themselves.

Exactly how the outbreak began remains unclear. But it seems to have happened on several occasions. Similar strains of plague have been detected in individuals dated to only a few centuries later, but found thousands of kilometres away in Latvia and Sweden. That could reflect a big rodent reservoir of Y. pestis that spanned the Eurasian continent. Another option, speculates Dr Willerslev, is transmission through some other, as-yet- unknown insect, perhaps carried on animal skins and spread through trade. Plague has not gone away, but these days it is not the threat it once was. Around 540 cases a year were reported worldwide between 2010 and 2015, the most recent years for which figures are available. Treatment with antibiotics can cut the death rate to 20% or so. But tracing the lineage of bugs like Y. pestis is not of interest only to academic historians. By tracking how pathogens emerge, thrive and disappear across thousands of years, researchers are building a record of which genetic mutations were and were not successful for the spread of disease. Dr Willerslev hopes that will provide data for future vaccine development and disease monitoring. In a world still suffering from the lingering effects of the covid-19 pandemic, the wisdom of such research should be obvious. ■ Curious about the world? To enjoy our mind-expanding science coverage, sign up to Simply Science, our weekly subscriber-only newsletter. This article was downloaded by zlibrary from https://www.economist.com//science-and-technology/2026/06/17/ancient-dna-is- rewriting-the-history-of-plague

Science & technology | Boy problems The coming El Niño could be the strongest ever recorded That is bad news for a warming world June 18th 2026 HUNDREDS OF YEARS ago Peruvian fishermen noticed that, every few years, the anchovies in the equatorial Pacific Ocean would vanish. Since the disappearances happened around Christmas, they named the event after el niño Jesus—“the Christ child”. These days the phenomenon, known simply as El Niño, is recognised as a recurring climatic pattern that alters the weather all over the world. It causes droughts in some places, heavy rain in others, heatwaves, wildfires and a general warming of the planet. On June 11th the National Oceanic and Atmospheric Administration, an arm of the American government, announced that a new El Niño had begun. And this one could be a whopper.

El Niño is driven by a shift in the winds above the eastern Pacific, which draws a band of warmer-than-average surface water into the “Niño 3.4” region (see map). The strength of any particular El Niño is measured by how much warmer the water gets. Anything above a 2°C rise over the long-run average is considered strong. Forecasts from most of the world’s modellers for the rest of this year and the first couple of months of 2027 suggest a rise in sea-surface temperatures of more than 2.5°C—and perhaps even 3°C. That would be unprecedented in the 75 years for which scientists have been keeping records. (The current record is held by the 1982-83 El Niño, when water temperatures rose 2.5°C.) Anchovies prefer cooler water, which is why they decamp southwards during an El Niño. But the effects are not felt just by Peruvian fishermen. The sheer size of the Pacific, and the interconnectedness of the world’s weather systems, means that each El Niño causes a vast redistribution of heat and moisture across the planet. One result is to make the world warmer. El Niño is not caused by climate change. But the two phenomena amplify each other’s effects (see chart). A strong El Niño in 1997-1998 made 1998 the hottest year ever at the time, with average temperatures nearly 1°C above pre-industrial levels. Following another strong El Niño in 2015-16, the record was broken again, with

temperatures in 2016 up more than 1°C. The current record holder is 2024, when temperatures were 1.55°C higher than the pre-industrial average. Climate modellers think 2027 could be hotter still. When it comes to countries and continents, El Niño’s effects are more variable (see map). The El Niños of 1997-98 and 2015-16 caused havoc in eastern and southern Africa, Central America and Oceania. Droughts parched crops and pasturelands, leaving millions hungry and many forced to migrate in search of food. Similarly baleful effects are possible this time, too. On June 9th the Food and Agriculture Organisation (FAO), an arm of the UN, warned that southern Africa and the Sahel—a semi-arid ribbon of land along the southern borders of the Sahara desert—are particularly at risk. The most recent El Niño in 2023-24, when water temperatures peaked at 1.5°C above normal, was associated with the worst drought in more than a century in southern Africa. In east Africa, the FAO warned that Somalia could be hit by a double- whammy of a drought until October followed by heavy rain until December. That is less reassuring than it sounds: instead of providing relief, heavy rainfall after a long drought can cause floods, because water cannot sink fast enough into parched soil. Central America, the Caribbean and parts of Asia are also at risk of drought.