The frequency of animal life in the Universe must be some function of how often it arises and of how long it survives after evolving. We believe that both of these factors are significantly influenced by the frequency and intensity of what are termed mass extinctions, brief intervals when significant proportions of a planet's biota are killed off. There is no mystery about what kills organisms: too much heat or cold; not enough food (or other necessary nutrients); too little (or too much) water, oxygen, or carbon dioxide; excess radiation; incorrect acidity in the environment; environmental toxins; and other organisms. Mass extinctions occur when one or some combination of these factors kills a significant percentage of the planet's biota. There has been no shortage of them in the past.
Mass extinctions have the potential to end life on any planet where it has arisen. On Earth there have been about 15 such episodes during the last 500 million years, 5 of which eliminated more than half of all species then inhabiting our planet. These events significantly affected the evolutionary history of Earth's biota. For example, if the dinosaurs had not suddenly been killed off following a comet collision with Earth 65 million years ago, there probably would not have been an Age of Mammals, because the wholesale evolution of mammalian diversity took place only after the dinosaurs were swept from the scene. While dinosaurs existed, mammals were held in evolutionary check. Mass extinctions are thus both instigators of and impediments to evolution and innovation. Yet much of the research into mass extinctions suggests that their disruptive properties are far more important than their beneficial ones. If planets with life are gardens, then the mass extinctions are the pests and droughts as well as, perhaps, the fertilizer. Yet as any gardener knows, plants are most susceptible when they are young, and disasters are most pronounced early in the growing season. A late frost, a catastrophic hailstorm, the emergence of early spring pests, a lack of sun—all make the early growing season the most hazardous time. So, too, with animal life on any planet. We believe that the early period in the evolutionary history of complex metazoans is by far the most dangerous interval. In our view, planetary disasters (resulting in mass extinctions) that occur before the evolution of complex metazoans and those that occur after they are established through the process of species diversification are far less likely to end in the extinction of all life. The fossil record of life on Earth supports this prediction in that the Cambrian period, when complex animal life had recently evolved, shows the most significant losses of higher taxa.
Unlike animals, which are fragile and easily killed, microbes are less susceptible to mass extinction events. Once established as a deep microbial biosphere, the bacterial grade of life is probably very difficult to eradicate. Short of planetary sterilization through destruction of the planet by a supernova or collision with a very large asteroid, the deep microbial biosphere of any planet must act as an effective reserve of life, because the regions several kilometers beneath the surface are insulated from even prodigious disasters that affect the surface regions. Surface life, on the other hand (even bacterial surface life), is surely susceptible to major planetary catastrophes, such as the impact of truly large comets or asteroids. It may be that life on Earth's surface was repeatedly sterilized during the period of heavy bombardment about 4 billion years ago, only to be reseeded by the deep-earth microbes or by the return of rocks ejected by the collisions. But for animal life, quite the opposite is true. Animals are not capable of the safer subterranean existence or of hibernating in the vacuum of space. If they are wiped out by catastrophe, they cannot be immediately restocked from some underground reserve. They have to evolve again in a slow, step-by-step process that lasts hundreds of millions or even billions of years.
On every planet, sooner or later, a planetary catastrophe can be expected that either seriously threatens the existence of animal life or wipes it out altogether. Earth is constantly threatened by planetary catastrophe— mainly by impact from comets and asteroids crossing the Earth's orbit, but also from other hazards of space. Yet it is not only the hazards of outer space that threaten the diversity of life on this planet and on any others where it exists. There are Earth-borne causes of catastrophe as well as extraplanetary causes. Both types have brought about mass extinction on this planet in the past and would be likely to do so on other planets as well.
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