Time and chance, as the Good Book says, come to us all. We all know that each of us will soon disappear from the Earth. David Raup’s book compounds our pessimism by pointing out that – if humans are anything like other animals – the fate awaiting our species as a whole is also an almost certain annihilation. Very few creatures persist for long in evolutionary time. There are a few hardy survivors, like the cockroach (which has remained unchanged for tens of millions of years), but for most, extinction follows quite soon after origin. This sad fact has been neglected by biologists, who, being – in general – optimists, are far more interested in how new forms of life appear than in how they depart: a philosophy which, as Raup says, is rather like a demographer concentrating on births and forgetting about deaths. Raup is an obituarist. His book, like all good obituaries, tells us more about its subjects after their demise than could safely be revealed when they were alive. He asks a deeply theological question: is it their fault that people (or species) are damned – they have bad genes – or do they perish at random because of simple bad luck?
The facts of death are remarkable enough. About one in twenty of all the people who have ever existed are alive today: but only one in a thousand of the different kinds of animal and plant. The rest have gone for ever, usually leaving no trace of their passing. Over the three thousand million years since life began, there has been a constant erosion, with thousands of species disappearing each year. The dismal picture is further darkened by five mass extinctions during which up to 70 per cent of all species vanished. At the end of the Cretaceous period, about seventy million years ago, the marine reptiles and the ammonites died out completely, and the dinosaurs suffered a fatal blow. There was a massive shift all over the world from flowering plants to ferns, which is just the sort of thing that happens after a forest fire today. Disaster may even have a pattern, with mass extinctions happening about once every 26 million years.
What causes extinction is not clear. There has long been a vague feeling among evolutionists that animals have a sort of planned obsolescence. They simply give up the ghost through biological ennui when their days are up: to quote an American palaeontologist, ‘the Age of Reptiles ended because it had gone on long enough, and it was all a mistake in the first place.’ An alternative view is that creatures disappear because they change into something else: the dinosaurs, for example, just grew wings and flew away. Yet another sees evolution as a metaphor for capitalism – a species goes bankrupt because it fails in the marketplace when a new competitor comes along. Although, no doubt, each idea may explain why particular creatures vanish, none illuminates why extinction has gone on unceasingly throughout evolution, and worse, why there have been simultaneous holocausts of thousands of unrelated organisms.
Raup gives a balanced account of ideas about extinction. However, he obviously likes catastrophes, the more dramatic the better; in his book things go out with bangs rather than whimpers. His account is full of vivid images – the Kill Curve, the Field of Bullets theory and the mathematics of Gambler’s Ruin. Gamblers are ruined because, if they start with a certain sum, and place bet after bet on the roulette wheel, sooner or later they are bound to hit what mathematicians call an absorbing boundary: they have no money left and cannot go on. In principle, exactly the same could happen to the casino – the gambler could break the bank. However, because most casinos start with more money than most gamblers it’s usually the punter whose fortune goes extinct when the laws of probability take their toll.
In exactly the same way, any species liable to accidents which reduce its numbers is constantly at risk of extinction. There is an analogy with the fate of human surnames. When not worrying about population outstripping resources, Thomas Malthus was concerned with the history of the burghers of Berne. He made lists of the names of prominent families from 1583 to 1783. To his surprise, three-quarters of the names present at the beginning of his study were gone at the end, although the numbers of bourgeoisie stayed about the same. Exactly the same is true for other well-documented groups, such as the royal families of Europe. Francis Galton, Charles Darwin’s cousin and the father of human genetics, pointed out why. Each generation, there is a chance that any family will have no sons, and their name will die out. Once lost, it is gone for ever and will be replaced by an existing name. In theory, and given enough time, everyone will one day have the same surname. The mathematics of the process has been worked out and produces a characteristic pattern of decay. David Raup was among the first to show that the theory applies to the rate of extinction in the fossil record. As he points out, there is a background level of extinction throughout history which is due, not to the merits or deficiencies of the species which go bust, but to the simple fact that all creatures are gambling with limited capital for their survival in the great casino of life. Sooner or later some are bound to strike unlucky, and will have no resources – in the form of individuals – left.
However, such probabilistic damnation is not enough to explain mass extinctions. In Raup’s view, the history of the persistence of species and their names in evolution is closer to that of the Russian aristocracy than the British. In Britain, grand family names disappear for the arithmetic reasons pointed to by Galton. Russia has oligarchy mitigated by assassination; families disappear en masse because of historical disasters. In the same way, Raup suggests, most evolutionary extinctions are due to boredom interrupted by panic. Species do not just go extinct because of the unbending rules of arithmetic. Instead, they are sometimes destroyed by external agents.
The assassins are, according to Raup, comets and meteorites – rocks falling out of the sky. He supports the idea that the mass killings happen at the same time as the impact of large meteors with the Earth. These do not occur very often, but when they do they can have a dramatic effect. A small one landed in Tunguska in Siberia in 1908. The explosion was heard by the only people anywhere in the vicinity – the passengers on a Trans-Siberian express train 350 miles away. All the trees over thousands of square miles were knocked down by the shock wave. Remarkably enough, no one was killed – but had the meteorite landed a few hours later as the Earth turned it would have hit the city of Lisbon and we would now take the theory of mass extinction by asteroid impact a lot more seriously than we do.
Meteorites of this size land every few thousand years. Others big enough to leave impressive holes in the ground – such as Meteor Crater in Arizona – hit about once in ten thousand years. Meteor Crater was made by a mere speck of astronomical dirt, a lump of rock as big as a football field. Raup estimates that an object one kilometre in diameter hits the Earth about once in 100,000 years, and one 100 kilometres across around once in 50 million years. The gaps between hits may seem comfortingly long for those, like ourselves, who count their own survival in mere tens of years. However, Raup calculates that an asteroid big enough to destroy civilisation – one equivalent to eight million Hiroshimas – is likely to arrive every three hundred thousand years. This means that each of us has a greater chance of being killed by such an event than we do of dying in an air crash. One theory has it that the Sun may be accompanied by a companion star whose orbit passes close enough to the Earth to cause the rain of asteroids to return at regular intervals. This might produce a 26-million-year rhythm to extinction. No evidence of such a star has been found, but it deserves to be remembered if only for its name – Nemesis.
Asteroids can do surprising amounts of damage. The entire Caribbean basin may be the remnant of a gigantic impact crater. Any creature unfortunate enough to be underneath such a rock when it lands is likely to become extinct rather soon. However, the destruction can spread far beyond the point of collision. There will certainly be shock waves, forest fires and tidal waves big enough to kill millions of animals. More important are the effects on climate. The Sun might be blocked out for years by the clouds of dust from the explosion. Whether this leads to freezing or boiling is not certain, and depends on how much the dust clouds act to turn the atmosphere into a green-house. Either way, the global effects are likely to be at the least uncomfortable.
Perhaps this is why so many creatures disappeared at the end of the Cretaceous. The rocks laid down at that time contain large amounts of an unusual element, iridium, which might have come from an asteroid. Two minerals of the same age (which glory in the names of stishovite and coesite) are twisted and fused by high pressure. Most of the damage was done to the inhabitants of shallow seas – which can warm up quickly – or to land plants susceptible to burning. The Cretaceous did indeed seem to end with a considerable bang, and David Raup makes an impressive case that the noise was the sound of a falling meteorite.
It seems, then, that perhaps Darwin was wrong. His passionate view, upon which the whole argument of the Origin of Species is based, was that evolution did not take place because of rare catastrophes in the distant past. Instead, patterns of change in the fossil record were driven by exactly the same slow and almost imperceptible changes we can see in the world today. Because the time available for evolution is so long, small changes can have enormous effects. He would not have liked Raup’s book, but he might well have been convinced by it.
The conflict between gradualism and catastrophism pervades human experience. There are proponents of each view among those who attempt to explain the origin of the Universe, the origin of species, and even the political origins of societies. In the evolutionary world the argument has turned nasty: the idea of evolution by rare disasters is sometimes referred to by its critics as ‘evolution by jerks’. In defence, its proponents call the model of gradual change ‘evolution by creeps’. Raup makes a good case that both the jerks and the creeps might be right about extinction.
If the world is indeed to end with a disaster, it is as well to plan for it. On the last page we learn that Vice-President Danforth Quayle recently sat on a committee to explore methods for deflecting or destroying any asteroid daring enough to threaten to land on the United States. Whether we should be reassured or alarmed by this is far from clear, but – with luck – we will all be extinct before the committee has to implement its decisions.