Where did the death occur? When did it show up?

It's a question that has plagued scientists and historians for over 700 years, as it was the worst epidemic in recorded history, killing 50 million people in Europe and the Mediterranean.

Researchers say they've found the genetic descendant of the Black Death, which still causes thousands of deaths each year. According to new research published this month in the journal Nature, the origin of Black Death in Central Asia can be found in what is now modern-day Kyrgyz.

Phil Slavin, co-author on the paper and a historian at the University of Stirling in Scotland, said that the strain from this region gave rise to the majority of modern plague strains.

This one is not easy to solve.

A tantalizing lead

Black Death is a type of plague. The name frightened people because they developed gangrenous, blackened wounds all over their bodies. The disease is caused by the bacterium Yesenia pestis which is spread by rodents.

A single plague strain evolved into four different strains. The strain that caused Black Death evolved from one of those lines. It has been a mystery where and when this occurred.

It has been suspected for a long time that the Y. pestis may have diversified in the TienShan Mountains.

Researchers discovered a clue in 1885. About 8 years before the Black Death epidemic began in Europe, there was a high number of tombstones in the area.

The cause of death was mentioned in the tombstones. It was a sign that a plague might have swept through the area.

The inscriptions on the tombstone weren't enough to prove that the people there died from plague. Slavin and the team would need to know their genetics.

An ancient DNA test

The team turned to experts for help. Maria Spyrou is a geneticist at the University of Tbingen in Germany.

The Peter the Great Museum of Anthropology and Ethnography in Saint Peter's, Russia housed hundreds of bodies that had been removed from the cemeteries.

The tooth samples were obtained from some of the bodies. The teeth have a lot of blood vessels and are the most likely place to find evidence of plague.

The team recovered traces of Y. pestis in 3 of the samples. The people who died in the valley were confirmed to have died from plague.

Tracing the strain's lineage

The next step was for the scientists to find out if the strain was related to other plague strains.

In order to do that, the team obtained samples from modern-day plague and historical plague from other sources. To map out the relationships between the strains and compare them to the strain from Chy Valley, they used these sequence.

The tree showed that the Black Death strain was different from the chy valley strain. They were able to confirm that the Black Death strain was older than the Chy Valley strain because of the inscriptions on the tombstones.

The evolutionary tree showed that the Chy Valley strain was the progenitor of most other plagues. Plague diversified into four major lineages from this ancestral strain. Scientists don't know where or when the "big bang" strain came from, but they have evidence that it could be from the region.

Adding to the plague 'origin story'

The origin of Black Death has been a mystery.

The director of the Ancient DNA Center in Ontario, Canada, who was not involved in the study, said he would be cautious about stretching it that far. Identifying a date and location for emergence is not easy.

He says that Y. pestis has about one change every 10 to 15 years. He thinks that the strain could have come from another part of the region.

People in the valley were traders and moved around the world. They might have picked up the strain on their travels. Because the strain is slow to change, it's hard to tell when and where the strain came from.

The study helps answer questions that plague researchers have spent years trying to find out, despite the critique. He says that there was plague at that site 10 years prior to the strains that were circulating in western Europe.

Max is a graduate student at the University of Georgia. He is working for NPR this summer.