Extinction: How Life on Earth Nearly Ended 250 Million Years Ago [ペーパーバック]

In Kentucky, there's a museum with a lifesize model of a dinosaur with a saddle on it. This is a hymn in fiberglass to young Earth creationism, the idea that the Universe was created about 6,000 years ago.

It costs $1,500 to become a charter member (family rate) of this museum. A much better investment would be $24.95 for Douglas Erwin's thriller about the Permian extinction.

More than nine-tenths of all species died out 251 million years ago. Erwin, a researcher with the Smithsonian Institution's National Museum of Natural History and the Santa Fe Institute, finds the end-Permian "enigma far more compelling than the end of the dinosaurs," a relatively minor event from 65 million years ago.

For an event that Kentuckians think never happened, the end-Permian event left a lot of debris, of which the most interesting is in China. Until 20 years ago, the paleontological record there was unknown to the outside world.

What the evidence is telling us is difficult to say. Erwin says "Extinction" was "frankly written as a mystery story." In this one, the clever detective does not wrap up all the loose ends on the last page.

Instead, we learn that there are at least seven major theories of what might have happened. These range from a big meteorite to gigantic volcanic eruptions in Siberia to a climatic or biological or geological change that drove oxygen out of the oceans.

The first chapters set the stage. Life was very different in the Permian. There were reefs in warm oceans, and they contained corals, but the corals were only distantly related to those of today and they were not as important as crinoids and lampshells, animals that still exist in out-of-the-way places.

On land, flowering plants had not yet evolved, nor mammals, dinosaurs or saddles. In South Africa's Karoo basin, fossils remain of a fabulous, lost fauna.

There were widespread extinctions on land as well as in the sea during the end-Permian event, but it is hard to say whether the land extinction was as complete as in the sea, where 94 percent of species disappeared in a short time. Erwin's team and their Chinese collaborators have found evidence that it all happened in less than 160,000 years -- maybe a lot less.

It is also not proved that the big land extinction exactly coincided with the sea kill, but it seems likely. The land kill was a whopper, too. This was apparently the only time in history when a mass extinction had any real impact on insects.

Whatever the cause, it did set up the modern world. "Mass extinction is a powerful creative force," says Erwin.

Or did it? As they learn more and more of the details, scientists are also learning to question the easy assumptions of more innocent decades.

Evolutionary biologists are vigorously debating whether the animals and plants that dominated the Permian were already being outcompeted by the early forerunners of modern flora and fauna, or whether they would have maintained their control of resources.

Erwin, splendidly agnostic about this and other debates, lays out the questions but leaves the resolution for some other time. Perhaps not too far in the future. He notes that his 1993 book on the Permian extinction already is out of date in many ways.

In fact, after decades researching the extinction itself, he has now concluded that "understanding the recovery from the extinction poses a far greater intellectual challenge."

Any scientist who opens [and closes!] a book by saying "We [I] don't know!" is worthy of your attention and respect. Too many others have taken up a theme and defended against all comers. Erwin's examination of the catastrophic close of the Permian Age is complete, admirably researched and exquisitely written. Within its pages, this work examines the various ideas on the massive loss of life 250 million years ago. These days, not to have heard of an meteor's killing off the dinosaurs 65 million years ago suggests you've lived hidden in a cave for a generation. Erwin opens with a brief overview of that event, reminding us that extinctions, particularly "impact events", have loomed large in discussions of the history of life ever since Walter and Luis Alvarez proposed the idea.

It's easy to rattle off the numbers: when the dinosaurs "went West", perhaps 75% of life was also extinguished. When the Permian ended, over 95% of living things disappeared. Erwin asks: "How do we know this? What life forms disappeared? Did they all go at the same time? How long did it take to recover?" Most important, of course, "What killed them off?" Instead of dull statistics, Erwin asks the important questions. Acknowledging that "Triassic rocks are boring", he explains why this is so. Fossils are scarce is the obvious answer, but why they are missing is his quest. With most of his attention focussed on ocean life, he details what causes shifts in benthic populations. The seas rise and fall - for a variety of reasons. Glaciation takes up sea water and leaves continental shelves high and dry. Oceans need to "turn over" an oxygen supply. What is the result of that failing? Carbon, with its various isotopes, passes through life selectively. Tracing that path provides insights into where it's been - and where not. When did the Siberian "traps" form? How much lava spewed from that rift, and what other products did it bring along or destroy? Finally, is there evidence that Earth was pelted by another bolide to provide an easy answer to all those questions? That reply is almost surely negative.

Erwin would like to couch this narrative as a detective story, but it doesn't really work. There are too many victims - unless you count life as one entity. There is also a phalanx of detectives all trying assiduously to solve the case. If you thought there were too many cooks spoiling the broth, wait until you meet this mob. Nearly all of them have an agenda and they have a disturbing tendency to trumpet a single tune. Erwin should have portrayed them as an orchestra, with himself as conductor. Van Kariajan would go emerald with envy. Each investigator supplies a theme, striving for a solo performance. Erwin cautiously assesses the tune, fits it nicely into a grander theme and produces a symphony instead of a cacophony. It's quite a performance. To keep himself from the sin of hubris, he points out his own flaws in a previous effort. The strain wasn't discordant, but the composition needed refinement.

Erwin fastidiously acknowledges his contributors. Jack Sepkowski comes in for deserved accolades, as do Bruce Rubridge, Yugan Jin and many others. Their methods, results and further work - including that incomplete but "promised" - are given a full hearing. Even those whose suggestions are highly suspicious, such as Luann Becker's Bedout "crater" are given a respectful hearing. Nobody's work is chastised or rejected. "We need more investigation" is the running theme of Erwin's account. The reason the ongoing search is important lies in understanding what is happening around us today. Are we, in Dave Sepkowski's words a "Dead Clade Walking"? Or can we glean enough information from the rocks to find the means to succeed through the extinction we seem to be part of - and likely creating? The "95%" means life had to restart the clock after the Permian. There were a few "Lazarus species" that re-emerged after the cataclysm. Will the human species manage to revive itself when so much life around us has been decimated? No more pertinent question confronts us. [stephen a. haines - Ottawa, Canada]

Even kids now can tell you about the mass extinction that wiped out the dinosaurs. When I was a kid, the dinosaur extinction was a big mystery, but there has been good evidence, now broadly accepted, that 65 million years ago a meteor as big as a mountain smashed into the Yucatan, turning everything for miles around into ash, wrapping the world in a cloud, and blocking the sunlight that runs all life. Everything all over the world changed, and we mammals got our try at reproductive success. The horrendous extinction that ended the Cretaceous age, however, wasn't the worst our old Earth had seen. 250 million years ago, there was an extinction that ended the Permian and began the Triassic periods (which is also the border between the larger, more general Paleozoic and Mesozoic eras). This Permo-Triassic event extinguished around 95 percent of all living species, and was as close as we have ever come to having all life wiped out. In fact, in the 19th century, geologists thought that life had been wiped out and a separate creation had occurred to start the Triassic. What really happened, and how, are the subjects of _Extinction: How Life on Earth Nearly Ended 250 Million Years Ago_ (Princeton University Press) by Douglas H. Erwin, Senior Scientist and Curator of the Department of Paleobiology at the Smithsonian. He has made the end-Permian mass extinction his research interest for the past twenty years, and has traveled all over the world to the fossil beds and geologic boundary layers remaining from around the time of the catastrophe. Looking back so many millions of years ago is not easy, and the picture is not as clear as that of the dinosaur extinction. Erwin's book, however, is a fine demonstration of how geologists and paleobiologists have come to some admittedly limited understanding of what happened.

There are many factors that have been suspects in the great killing, and Erwin likes to think of himself as a detective out of Agatha Christie set to finger the actual culprit. It's not that easy, of course. Catastrophic explanations for the end-Permian abound, and Erwin's book is an examination of the more likely causes, about six of them. Of course, a main one, borrowing from the success of the impact explanation of 65 million years ago, is an extraterrestrial impact. It is certainly a plausible explanation, since it is accepted as the cause of the more recent extinction. There are problems, however. The impact that wiped out the dinosaurs left clues like an iridium layer in geological strata (there is lots of iridium in meteorites, not so much on Earth) and "shocked quartz" impact crystals, but such clues are lacking for the earlier event. Another explanation might be volcanism, resulting in dust and acidic chemicals and basalts that cover a countryside "much as honey fills in the roughness of an English muffin." Yet another is that continental drift (plate tectonics) was causing collisions at the time, forcing species that had not previously met each other to compete, and changing the global climate by newly formed landmasses. Vast glaciation may have caused cooling and a decrease in sea level. Perhaps there was a drop in oxygen levels of the oceans. Maybe sea levels dropped and caused a huge release of methane from sediments.

He has not, however, wrapped everything up, as, say, Hercule Poirot might. He does, indeed, call his own proposal at explanation the _Murder on the Orient Express_ hypothesis, based on Agatha Christie's book which is a who-didn't-do-it rather than a whodunit. The explanation calls on aspects of many of the other explanations, but Erwin admits this makes it hard to test. One of his colleagues had dubbed it "Erwin's kitchen sink hypothesis", and, as Erwin says, was not being complimentary. There are implications for our own times in this story, since we are now in another period of great extinctions and our climate may be changing irrevocably, but Erwin does not stress these. His book is a fine summary of current thinking on the extinction. Readers will come up against sentences like, "Some surviving ammonoids with extreme morphology died out in the Griesbachian, but new Dienerian ammonoids were more similar to the norm..." (all the terms are well explained, but will be new to most readers), but then a few pages later readers will learn that the "quality of many Early Triassic fossils is really pretty lousy." There may be fewer hard answers here than in a murder mystery, but the explanations about how scientists came up with ideas about the extinction make this a fascinating look at experts confronting profound and distant mysteries.