Rus Hoelzel at the University of Durham, UK and his colleagues looked at the mitochondrial DNA from human remains at 4th and 11th century archaeological sites in England, and compared them to samples from the modern population stored on DNA databases such as GenBank. They found there was more variation in the ancient mitochondrial DNA sequences than in modern sequences.
Hoelzel thinks random genetic drift may have lowered genetic diversity naturally. But the large unexpected drop in diversity was more likely to have been caused by population crashes following major outbreaks of the Black Death in England during the 1340s and the 1660s.
"The main factors in support of a role for plague are the timing and the fact that it affected different families [to a differing degree]," says Hoelzel.
Comment: Note that, 600 years ago, they CLOSED BORDERS!
This is something only a handful of countries seem prepared to do with the current Coronavirus epidemic. Russia did so wrt China early on, and it has still only recorded two cases, both of them Chinese nationals, and both now recovered.
But the 'Black Death' of the mid-14th century, and its similar recurrences over the next two centuries, were in a whole other league. The astrobiologists are likely correct: truly civilization-decimating viruses come from outer space...
New Light on the Black Death: The Viral and Cosmic Connection