When Life Nearly Died: The Greatest Mass Extinction of All Time

  • Michael Benton
Thames and Hudson: 2003. 336 pp. £16.95, $29.95
Exit stage right — even though Lystrosaurus survived the extinction at the end of the Permian.

Whatever hit the Earth at the end of the Permian period certainly struck hard, killing 90% of living species. Compared with this, the extinction at the end of the Cretaceous period was comparatively minor, with only a 50% death rate. Yet the latter event is much better known, because among that 50% were the last of the dinosaurs. Partly for this reason, Michael Benton uses the event at the end of the Cretaceous as an introduction to his account of the Permian extinction — he wants us to realize how limited it was in comparison with what he intends to describe.

But there is a deeper reason for linking the two episodes: Benton wants to show us how the catastrophist perspective has re-emerged in modern geology and palaeontology. He argues that the theory of catastrophic mass extinctions was widely accepted in the early nineteenth century, but was then driven underground by the gradualist perspective of Charles Lyell's uniformitarian geology and Darwin's theory of evolution. Only in the 1970s was catastrophism revived, through the claim that the dinosaurs were wiped out when an asteroid hit the Earth. Benton shows us how in the 1990s the evidence began to emerge that the species replacements marking the Permian–Triassic transition were also sudden, and hence were probably caused by some environmental trauma. He is describing both a geologically sudden event and a rapid transformation in our ideas about the Earth's past.

As a result, the book is partly historical in nature. It describes how the British geologist R. I. Murchison (himself a catastrophist) defined the Permian rocks of Russia in about 1840, and how Lyell and Darwin challenged the idea of mass extinctions by arguing that apparently sudden transitions in the fossil record were the result of gaps in the evidence, which created illusory jumps between one system of rocks and the next.

The triumph of darwinism ensured that catastrophist explanations were marginalized until they were revived by the asteroid-impact theory for the end of the Cretaceous. Even then, many palaeontologists resisted, arguing that the dinosaurs were declining anyway, so the impact only finished a job that had already been started by gradual environmental changes. At the time, knowledge of the Permian–Triassic transition was so limited that gradualism still seemed plausible here, too. Benton provides a graphic account of how more recent evidence has piled up, including his own experiences fossil hunting in Russia, making a catastrophic explanation inescapable.

There is one important twist in the story, however: Benton finds little support for the possibility that the Permian extinction was caused by an extraterrestrial agent. Wild theories about periodic bombardments by asteroids have not stood the test of time: the Permian event was probably triggered by massive volcanism, which injected poisonous gases into the atmosphere, both directly and by triggering the release of methane from deep-sea hydrates. Some geologists think that volcanism also played a role at the end of the Cretaceous. Significantly, Benton concludes by considering the implications of the latest, man-made mass extinction, asking what light the earlier events can throw on the potential for survival of modern species.

The historical aspect of Benton's book raises some intriguing questions. Many early catastrophists postulated the involvement of extraterrestrial agents — a comet was sometimes invoked as the cause of Noah's flood. But such ideas went out of fashion in the mid-nineteenth century, and later catastrophists, including Murchison, favoured explanations based on the supposedly more intense geological activity in the young Earth. The asteroid-impact theory of dinosaur extinctions seems to parallel some of the earliest speculations, but Benton has redressed the balance by favouring internal causes.

My one criticism of his account is that he accepts too readily the assumption that Lyell and Darwin marginalized all support for discontinuity in the Earth's history. There were few outright catastrophists left by around 1900, but many still believed that the history of life had been punctuated by environmental transitions far more rapid than anything observed in the recent past.

The real triumph of gradualism came with the modern darwinian synthesis of the mid-twentieth century, and even then it was confined to the English-speaking world. Benton notes that British and US palaeontologists of the 1950s ignored the catastrophism of Otto Schindewolf. But we need to recognize that German palaeontologists such as Schindewolf were continuing a long-standing tradition that had proved far more robust than our modern, Darwin- centred histories acknowledge. The fact that modern catastrophists do not see a link back to that tradition tells us about the effectiveness of the neo-lyellian interlude of the mid-twentieth century.