Douglas H. Erwin

EXTINCTION

How life on earth nearly ended 250 million years ago

There have been many extinction events since life began on Earth but the event that occurred at the end of the Permian was by far the biggest, far more disastrous than the famous impact that is thought to have put an end to the dinosaurs 65 million years ago. Erwin provides an excellent account of the catastrophe, written in a relaxed accessible style but including plenty of detailed discussion about what the evidence does, and does not, tell us.

The book is written in the detective story mode, but in the end no final conclusion is reached. Erwin says he does not know what caused the extinction, although he doesn't shirk the question and he offers his own best guess about what happened. As the book is avowedly written as a detective story I shall not give away Erwin's own conclusion, even though he is uncertain about it.

A very wide range of scenarios has been proposed, some more plausible than others. A huge impact, bigger than the one that put paid to the dinosaurs, is, perhaps inevitably, one popular theory; others include huge volcanic eruptions in Siberia, the oceans losing their oxygen content, and various combinations of these events. Erwin nicely describes the last possibility as the "Murder on the Orient Express" scenario, after the Agatha Christie novel in which the murder was committed by all the suspects. He admits to disliking the impact theory on aesthetic grounds: it would be boringly similar to the dinosaurs' extinction and he would prefer different extinctions to have different causes. However, he gives all the main theories fair play.

Quite apart from any interest one may have in the extinction itself, Erwin's treatment offers an excellent insight into the way science really functions as a human enterprise.

Like many other people some scientists are … sure that we are right and anyone who disagrees with us is wrong. This is not to say that scientists do not change their minds. We do, if only under duress. Some of the best scientists are most willing to change their minds. But research is often a battlefield between contending egos, uncomfortably constrained by reality.

The evidence for what happened comes from sites all over the world, including the USA, Italy, Iran, South Africa, and especially China, where Erwin himself has done field work. The story is inevitably complex. Although we speak of the end-Permian extinction, there were in fact two events, separated by about 10 million years: one at the end of the Middle Permian, the other at the very end of the Permian. We know little about the first of these extinctions; it was the second that produced the really catastrophic loss of life and it is this that Erwin mainly concentrates on.

One important question concerns the rate at which the extinction occurred: was it rapid or drawn out? On this, Erwin's ideas have changed. Ten years ago he thought it was a lengthy affair, but he says he was wrong. The extinction happened abruptly, although how abruptly is still a matter for debate.

In a very interesting coda, Erwin has a couple of chapters discussing how recovery occurred in the Triassic after the mass extinction. This raises nearly as many questions as does the extinction itself; indeed, Erwin thinks that it provides a far greater intellectual challenge. Two puzzles in particular stand out. One is the long duration of the post-catastrophe collapse, and the other (and bigger) is the fact that many genera disappear from the record for millions of years and then reappear, as if "resurrected" (the so-called Lazarus phenomenon).

There is also an important question about the relative lack of diversity in the recovery. The extinction eliminated over 90% of all marine species and left the world as barren as in the late precambrian, but there was no outburst of morphological innovation in the Triassic like that of the Cambrian. Erwin discusses various possible solutions to the paradox and suggests that there were simply not enough unexploited ecological opportunities in the Triassic to support major innovations.

Conservationists are fond of telling us that we are currently causing species loss on a scale that has not been seen since the great extinctions of the past. and speak of a "sixth extinction". Erwin regards this as emotional hyperbole. We do not have the data that would allow such comparisons. He points out, however, that if we are indeed in the midst of such a man-made extinction it may already be too late to do much about it, and we ourselves will probably disappear along with the rest. And recovery from such a disaster would take longer than the recorded history of Homo sapiens and perhaps even longer than the entire history of hominids.

August 2006