The black death is dead

Evolution tells us a lot about death. Of course it's about life too, but it's really about survival, which involves both life and death.

As most people know, the Black Death was a horrible plague that swept through Europe, Asia, and Africa in the 1300's, killing tens of millions of people at a time when there weren't so many people to begin with. The world's population prior to the plague, about 450 million, dropped to 350 million. About one-third of the entire population of Europe, and half the population of China, may have died. Centuries earlier, the Plague of Justinian in 541-542 C.E. may have killed even more, up to half of Europe and untold millions elsewhere around the world. In ancient and medieval times, people thought the plague was caused by rats, but the true cause wasn't discovered until 1894, when Alexandre Yersin of France and Kitasato Shibasaburo of Japan finally traced it to a bacterium now called Yersinia pestis, which is transmitted by fleas, which in turn are carried around by rats.

The plague kills all of its hosts, even the fleas:

"The bacteria multiply inside the flea, sticking together to form a plug that blocks its stomach and causes it to starve. The flea then bites a host and continues to feed, even though it cannot quell its hunger, and consequently the flea vomits blood tainted with the bacteria back into the bite wound. The bubonic plague bacterium then infects a new victim, and the flea eventually dies from starvation. " Source: Wikipedia

Gross, I know. But the original plague, the Black Death, has never returned. Why not? A study last year and another one published just this week provide the answer.

Last year, Barbara Bramanti and colleagues collected DNA from mass graves dating to the Black Death, and showed conclusively that the victims were infected with Yersinia pestis. Until this study, some scientists were uncertain about whether Yersinia pestis was the true cause, but Bramanti's research should settle that question once and for all. They also showed that at least two distinct strains of plague bacteria infected Europe, each arriving via a different route.

Further evidence appears in a remarkable new study published this week by Hendrik Poinar and colleagues. They exhumed over 100 skeletal remains from victims of the Black Death, collected from a ancient London cemetery, East Smithfield, which has been conclusively dated to the plague years, 1348-1350. Using the latest DNA sequencing methods, they identified Yersinia pestis DNA in 20 of the 109 victims.

Both studies collected enough DNA to show that the strain of Yersinia pestis from 1350 C.E. is unlike any modern strain. In other words, the original plague died out, probably long ago. The likely explanation is just this: the Black Death was simply too deadly to persist. Evolutionary theory tells us that a pathogen that kills all its victims will eventually run out of victims, leading to its own extinction. The plague bacteria needed to evolve into something less virulent, and that seems to be what happened. A bug that doesn't kill its host is far more successful evolutionarily. (Just look at the common cold, which we can't seem to get rid of.)

The same thing happened to the "Spanish" flu virus, the one that cause the terrible 1918 flu pandemic. It too evolved into a milder pathogen, and it is still with us today - the 2009 influenza pandemic was caused by a direct descendant of the 1918 virus.

The Black Death was so widespread that it even affected human evolution. In 1998, Stephen O'Brien and colleagues showed that a mutation that confers resistance to HIV first appeared in the human population in the 1300's. They concluded that this mutation can best be explained by "a widespread fatal epidemic"; in other words, the Black Death. I should be careful to explain that the plague didn't actually cause the mutation: the mutation occurred naturally. The Black Death selectively killed more people without the mutation, leaving us with a population of humans that tended to have the mutation.

In light of these new results about evolution, I can't help pointing out that, finally, that evolution has been in the news recently for another reason. Several U.S. politicians, some campaigning for President, have been attacking evolution, saying that it has "got some gaps in it" and even supporting the teaching of creationism. Scientific facts aren't affected by political statements, of course, but the future of the U.S. is. Politicians who attack evolution, whether from ignorance or from some political or religious agenda, only hurt our future potential as a technology leader. I can only hope that the public won't support these anti-science positions.