Plague-re­lated mi­crobe found in an­cient hu­man re­mains

The News & Observer - - News - BY DEB­O­RAH NETBURN

In an an­cient grave in Swe­den, sci­en­tists have un­earthed the old­est known strain of a deadly bac­te­ria that has killed mil­lions of peo­ple over thou­sands of years. They call it Yersinia pestis. You may know it as the plague.

The new dis­cov­ery sug­gests that the mi­cro­scopic bac­te­ria has been wip­ing out great swaths of the hu­man pop­u­la­tion for more than 5,000 years – de­stroy­ing em­pires, spurring po­lit­i­cal up­ris­ings and leav­ing a per­ma­nent mark on re­gional gene pools.

“What we found in the Swedish grave site is not only the old­est sam­ple of the Y. pestis genome but also the old­est ver­sion of the genome,” said Si­mon Ras­mussen, a metage­nomics re­searcher at the Tech­ni­cal Univer­sity of Den­mark, who led the work. “Think of it as the root of the tree.”

The old­est recorded plague pan­demic, known as Jus­tinian’s Plague, dates to 541 AD. Over the course of 200 years, it killed more than 25 mil­lion peo­ple across the Byzan­tine Em­pire, hit­ting the cap­i­tal city of Con­stantino­ple es­pe­cially hard.

The next ma­jor plague pan­demic, known as the Black Death or the Great Plague, started in China in 1334 and spread along trade routes to Con­stantino­ple be­fore reach­ing Europe in the 1340s. It also claimed the lives of an es­ti­mated 25 mil­lion peo­ple, in­clud­ing about half the pop­u­la­tion of Europe, ac­cord­ing to re­searchers at the Cen­ters for Dis­ease Con­trol and Pre­ven­tion. Some towns weren’t left with enough sur­vivors to bury the dead.

The third ma­jor out­break, known as the Mod­ern Plague, took root in China in the 1860s.

The new study, pub­lished Thurs­day in the jour­nal Cell, re­veals that the re­la­tion­ship be­tween hu­mans and plague goes back even fur­ther than sci­en­tists had re­al­ized. The bac­te­ria iden­ti­fied by Ras­mussen and his col­leagues may rep­re­sent a pre­vi­ously un­known out­break of plague that struck Europe as much as 5,700 years ago.

The re­searchers al­ready knew that the pop­u­la­tion of Europe plum­meted 5,000 to 6,000 years ago, af­ter it had grown for thou­sands of years. This sud­den plunge is known as the Ne­olithic de­cline, and its cause is still up for de­bate.

To see if the plague was in Europe at the end of the Ne­olithic pe­riod, the group turned to data­bases of DNA ex­tracted from an­cient hu­man re­mains – specif­i­cally, an­cient teeth.

Be­cause blood cir­cu­lates through the cen­ter of our teeth, Ras­mussen said, it is pos­si­ble to de­tect the DNA of pathogens that were present in a per­son’s blood­stream at their time of death by ex­am­in­ing a tooth sam­ple.

“If you die from it and it’s in your blood,” he said, “then we can find it.”

Af­ter scan­ning for ge­netic se­quences re­sem­bling mod­ern-day Y. pestis, the group even­tu­ally found a match. It was in DNA ex­tracted from the tooth of a 20-year-old woman who died in west­ern Swe­den be­tween 5,040 and 4,867 years ago.

“This re­ally sur­prised us,” Ras­mussen said. “It was the old­est plague sam­ple ever found.”

Next, the au­thors com­pared the newly dis­cov­ered Y. pestis genome to 150 other plague sam­ples that spanned thou­sand of years, go­ing back all the way to the Bronze Age. This anal­y­sis re­vealed that the strain from the Swedish woman was closer to the ori­gin of Y. pestis than any other, and there­fore could in­form sci­en­tists about the first plague an­ces­tor from which all sub­se­quent strains evolved, Ras­mussen said.

Us­ing the new find­ings as a guide, the re­searchers pro­pose that the plague first evolved in megaset­tle­ments, mor­ph­ing from a rel­a­tively be­nign stom­ach bug to a deadly mi­cro­scopic killer around 5,700 years ago, around the time when the Swedish strain di­verged from all oth­ers then in ex­is­tence. This could ex­plain the pe­ri­odic burn­ing of the build­ings – per­haps they were set aflame to erad­i­cate the dis­ease.

GETTY IM­AGES TNS

A bubonic plague smear demon­strates the pres­ence of the Yersinia pestis bac­te­ria that causes the dis­ease. New ev­i­dence sug­gests a strain of Y. pestis might have emerged in hu­mans about 5,700 years ago.

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