In the formation of Christendom, and therefore of Europe, many factors played their part: the barbarian invasions, the rise of Islam, the collapse of city life, the emergence of the Papacy as a political force. While, however, we jumped-up ex-simian bipeds - parvenus, after all, on the world scene - were plotting and warring, one of the earth's oldest inhabitants was preparing a new mutation. This mutation had the unfortunate, for our ancestors in the sixth century, side effect of killing 25 million people. Bubonic plague had arrived. An ingenious bacterium entered a swift flea which bit a slow-moving black rat. This alliance of the disregarded could only be interpreted at the time as a divine scourge. But, as William Rosen argues in Justinian's Flea, the story does not stop with a dreadful human tragedy: the plague changed Europe utterly. It weakened the Roman Empire at a time when it was already overstretched, thus softening it up for the armies of Islam; it ensured, paradoxically, the survival and success of the Franks and other Germanic tribes; it created the great cultural hiatus we know as the Middle Ages.
On first consideration, Justinian's Flea seems like just another account of the reign of Justinian (last Roman Emperor or first Byzantine emperor, as you prefer), however detailed and engrossing. But the reader's pilgrimage through Rosen's presentation of civil law, architecture, theology, evolutionary biology, demographics, philosophy, literature (there really is nothing he doesn't touch upon and gild) runs through a forbidding landscape indeed, for Rosen's avowed aim is to remind us of our true place in the cosmos: that for bacteria, who've been around for almost three billion years longer than we, the human race is food and housing.
I wasn't expecting to enjoy a lengthy explanation of the nature of bacteria (and I won't pretend I understood it all), but Rosen's gift for the apt metaphor saw me through. Explaining the essential difference between a bacterium's adaptive habits and everything else's, he writes: "Consider the challenge of providing for scientists at an arctic weather station: Instead of hiring a mechanical engineer to construct a heating and ventilation system, you would formulate a pill that changes internal body temperature to cope with subzero temperatures. Bacteria are lousy mechanics, but are chemists par excellence."
He's at his best when bringing into sharp focus what the reader knew only vaguely. Thus, it's fairly well known that Justinian's great legal code had a huge impact on European jurisprudence. Rosen goes into the detail, and, more significantly, the consequence. …