Although this book is partly about the great end-Permian mass extinction it is also a historical account of how ideas about about catastrophism in palaeontology have changed in the last 200 years. Benton traces how views of mass extinctions have developed since the early nineteenth century, when Charles Lyell advocated 'uniformity'—the view that the processes shaping the earth in the past were those we can still see in action today. Arguments about this have continued down to our own time, though a decisive shift occurred in the twentieth century with the widespread acceptance of impact as an explanation for the demise of the dinosaurs. As a student Benton himself was taught by anti-catastrophists, but now he preaches asteroids and mass extinctions to his students.
The modern view is that there have been several mass extinctions in the past—at least five. They were characterised by the loss of 20%–65% of families and 50%–95% of species. Whether mass extinctions are different from 'normal' extinctions is uncertain, though they may be. During a mass extinction survival is largely a matter of luck, although geographically separate groups may be more likely to survive. Some mass extinctions were virtually instantaneous though others lasted for up to 10 million years. Recovery has usually taken about 10 million years although after the end-Permian event it took ten times as long.
The date of the end-Permian event is now fairly well established at about 251 million years ago, with a margin of about 550,000 years. At least 90% of the species alive at the time were lost. One of the principal survivors was a plant-eating reptile called Lystrosaurus. It survived probably because it was so widespread; Benton says it was one of the most successful reptiles of all time. It found itself in an almost empty world after the catastrophe (and feeling pretty hungry, one imagines).
The cause of the end-Permian extinction is still unknown and some pretty wild ideas have been proposed. Benton thinks the most likely explanation is a complex series of interactions triggered by the prolonged volcanic eruption known as the Siberian Traps. Worldwide devastation was caused by the production of various gases, including carbon dioxide, sulphur dioxide. and chlorine. The result was a runaway greenhouse effect and global warming.
The book concludes with a consideration of the so-called Sixth Extinction, which we are causing by global warming. Benton points to numerous uncertainties concerning biodiversity and the real rate of species loss and says that, paradoxically, we know more about past extinctions than we do about the present one. The book ends with questions, not answers. "How diverse is life? How does the world react to human intervention? What will happen in the next 100 years? Where did life come from? How resilient is life to crisis?"
There is a lot of information in this book though I think it is rather over-long, with some digressions that might have been pruned with advantage.
See also Extinction, by Douglas H. Erwin.
24 April 2008