This flea died 20 million years ago, but the bacteria on its proboscis look familiar. Scientists believe that these bacteria, preserved in amber along with their host, may be a very early version of the Black Death.
We know what Yersinia pestis, the bacteria that modern scientists believe caused the Black Death, does to humans. What does it do to the fleas that carry it? First, it makes them hungry. The fleas suck up bacteria along with the blood of an infected animal, but the bacteria make the blood form a sticky, clotted mass before it can hit the flea’s stomach. The hungry flea searches out another host and often regurgitates some of the sticky mass before drinking from something new. That’s part of how the bacteria spread from one animal to the next.
The dried blood and bacteria stick to the flea’s proboscis. Meanwhile, the bacteria still moving through the flea’s system often form a mass in the flea’s rectum. This trait is what tipped the scientists who found this 20-million-year-old flea preserved in amber that they might have an ancestor of the plague on their hands. A closer examination of the bacteria on the proboscis allowed scientists to confirm that the bacterial fossils had the size and shape of modern plague bacteria.
But they aren’t modern plague bacteria. Different strains of Yersinia pestis have been around for a long time, and not all of them sicken humans. In fact, genetic studies show that strain that is thought to have caused the Black Death evolved only 20,000 years ago. This strain, if it is related to the modern plague, is a distant ancestor and probably extinct by now. But we may be looking at the first glimmer of a pestilence that changed the world.