Books: Gerbils stalked the earth

If history is "the register of the crimes, follies and misfortunes of mankind", this book emphasises the latter. Nevertheless, there are plenty of crimes and follies in David Keys' lively narrative. A Byzantine emperor attempts to murder the guests at his daughter's wedding on the unreasonable grounds that the happy couple are upstaging him. At about the same time in Mexico another emperor has his back broken and becomes a living hoop rolled around by his enemies. These two monstrous events can be linked, according to Keys, and even comprehended, once they are fitted into the bigger pattern of a disastrous weather system which engulfed the world in the middle of the sixth century AD.

This in turn can be traced back to one catastrophic event. In the best tradition of the whodunnit, Keys does not identify the culprit until the end, but it will spoil nobody's enjoyment to reveal that the likely candidate seems to be a volcanic eruption so powerful that it blew apart the islands of Java and Sumatra, which had been one land mass until then. Anybody within 150 miles of the event would have been instantly incinerated, so no eyewitness accounts have come down to us, but the bang was loud enough to be heard in Nanjing, and noted at the time. This was a global disaster launched on a world full of scholars, poets, philosophers and engineers, and they were as helpless before it as naked savages. From Rome to Japan, the only benefit of civilisation was the agonising comprehension that something was dreadfully amiss.

At first the sun turned "pale as the moon" behind a stratospheric cloud of ash, sulphuric acid and water vapour. Other disasters manifested themselves more slowly - a decade of drought followed by famine and disease and war. Although this was a global event, the effects were uneven. Cattle economies did well, because cattle, unlike horses, can live off dried grass, and herdsmen, unlike farmers, do not keep barns, a significant detail in time of plague because barns attract rats, and rats attract fleas , the vehicle of the disease. Similarly, trading communities are more vulnerable to plague than their isolationist neighbours. As some populations declined their more robust enemies naturally felt this was an opportune moment for realigning the frontier.

The political fallout from the big bang of 535AD or thereabouts is the most contentious part of Keys' hypothesis. The natural consequences are easy to trace: climate and rainfall can be estimated by the study of tree rings, and where gaps occur educated guesses can be made. It seems likely that the bubonic plague which swept the Mediterranean at the end of the sixth century can be traced to an expansion of the gerbil population in East Africa. An astonishing thought, but one built on quite logical steps. It is only when we consider human affairs that we have to take the irrational into consideration.

Keys acknowledges that determinism, the idea that one event inevitably causes another, like a row of dominoes knocking each other down, has become discredited in the history faculties. But even if he does stretch credulity on occasion, it must be said that to ignore his evidence altogether would be more deterministic still. For if we say that natural disasters have no effect on the outcome of history whatsoever, we are going some way to claiming that the outcome was inevitable come what may, and that the present could be no different than it is. Over the past 20 years, under the influence of palaeontologists, catastrophes have become recognised as having an influence on evolutionary biology, much to the disgust of the Neo- Darwinists who prefer to see evolution as a smooth and even progression. Historical time is much shorter than evolutionary time, so much more vulnerable to sudden upsets. Beneath Keys' rather spectacular tableau of events, prepared with television in mind as much as the printed page, is a quieter plea for historians to take account of archaeologists' work.

But different people react in different ways in times of crisis, even if certain patterns can be seen if we consider events globally. At the time people were more inclined to consider things cosmically. As the fields dried out and were then washed away in the floods, many people began to revise their theological opinions. At opposite ends of the earth Ireland and Korea finally abandoned paganism for Christianity and Buddhism respectively. In Teotihuacan, Mexico's "Jerusalem Athens and Rome all rolled into one", years of drought caused the people to revolt against heaven because the temple complex there seems to have been burned down by the town's own citizens before they abandoned the city for ever.

Further south in the Andes the people of Tiwanaku were more pragmatic. When the rains failed for years on end they switched to an agricultural system based on irrigation. By a series of happy accidents this proved a far more efficient system. For example the irrigation channels grew weeds, which the farmers plucked out and dumped on their fields. The weeds were rich in nitrogen, a lucky stroke for a culture which never developed crop rotation. Everyone benefited, including the undeserving Rain God who despite his under-performance received a massive pyramid in his honour, down which flowed cascades of precious water day and night, fed from a cistern which had to be filled by a procession of water carriers. The parallels with our own modern management culture are too obvious to point out.

So even in the dreadful sixth century, nature was not all bad. The scale of Catastrophe and its scope stretch the imagination, especially as it cannot help but remind us what a weak purchase the human race has on a volatile universe. As one unknown Chinese bureaucrat lamented in a poem which recounted the natural and political disasters that had plagued his generation "How could God have been so drunk?"