Extinction: How Life on Earth Nearly Ended 250 Million Years Ago
Douglas H. Erwin
Princeton University Press, 2006
320 pp., 24.95
Stephen O. Moshier
That Was a Close One
Rocks and fossils contain many lessons of life and death on ancient earth. A particularly impressive example is found in the Permian limestone of the Guadalupe Mountains in west Texas. Fossil sponges and algae embedded in the rocks formed perhaps the most spectacular barrier reef ever to exist in an ancient sea. Hikers on the trail up McKittrick Canyon follow the traverse of an imaginary paleo-scuba diver swimming up from deep water to the reef crest and then landward across a wide shallow lagoon.
All of the fossil species in the Permian reef are long extinct. This is true of most fossils in the geologic record of life. Paleontologists reckon that most fossil species survived no more a than a few hundred thousand to a few million years before extinction (often to be replaced in overlying strata by similar species with modified features). Overlapping ranges of species generally appear to have kept the ecosystems of ancient earth filled and functioning like an exceedingly long baseball game in which generations of players gradually replace their predecessors. But the Permian reef teaches us another lesson about earth history, for the lords and tenants of this reef were doomed to a mass extinction that would leave few descendants in the overlying strata. Game called on account of global holocaust.
The mother of all extinctions happened at the end of the Permian Period, some 251,400,000 years ago. Ninety-five percent of all marine species were vanquished, with profound effects on emergent terrestrial life. An asteroid that hit the earth more recently, a mere 65 million years ago, managed to take out only 50 percent of marine species while wiping out that hearty group of terrestrial beasts we call dinosaurs. There is substantial evidence for that end-Cretaceous scenario, including a crater buried beneath the Yucatan coast and a thin layer of clay around the world containing abnormal concentrations of iridium, a rare element that is found in meteorites and "cosmic dust." A satisfactory explanation for the end-Permian extinction remains up for grabs. Solving this ultimate murder mystery is the preoccupation of an international cast of earth scientists using all available technology for wringing history out of rocks.
Paleontologist Douglas Erwin has hiked up McKittrick Canyon and other trails around the world searching for clues. In Extinction: How Life on Earth Nearly Ended 250 Million Years Ago, Erwin offers a thorough overview of one of the most interesting problems in earth history. A curator of fossils at the Museum of Natural History in New York City and author of numerous professional papers related to the Permian extinction, Erwin takes the reader on an insider's journey that includes adventures in the field, tedious hours in the laboratory, and stimulating but sometimes contentious exchanges among colleagues at scientific meetings. He gives rigorous consideration to every reasonable hypothesis, which means the reader is presented with data and interpretations from paleontology, paleoecology, sedimentology (the deposition of strata), geochronology (radiometric dating), and geochemistry (chemical tracers in rocks that relate to environmental conditions).
Erwin interweaves the history of paleontological work on the Permian mass extinction with an account of contemporary work, including his own multidisciplinary and international investigations. He describes travels to China with MIT geochronologist Sam Bowring and Nanjing Institute paleontologist Jin Yagan to look at marine deposits spanning the Permian-Triassic boundary. Their detailed sampling and dating of volcanic ash deposits in the strata narrowed the duration of the end-Permian extinction event to less than a few hundred thousand years, a mere instant in geologic time. Erwin was guided by South African paleontologists to study the impact of the extinction on terrestrial animals, plants, and fungi (yes, fungi) preserved in the strata of the Great Karoo Desert. But it is not enough to collect ash and bones; even the isotopes of carbon in sediments contain information related to the health of the Permian ecosystem. Erwin not only reports the results of scores of scientific studies but also describes the investigators with collegial grace. More than eighty contemporary scientists are mentioned by name in the text, and even more in footnotes and references. This is a mystery with more detectives than suspects.
Readers will be thankful that Erwin frequently reviews major points or lines of evidence presented in earlier chapters (it gets pretty complicated). The text is animated with personal stories, and he is careful to explain the jargon and technicalities of modern science. Witty geo-jargon betrays well-lubricated campfire discussions after a hard day's work in the field (geologists are unrepentant beer-drinkers). For example, extinct fossils that mysteriously reappear in rocks above the Permian upper boundary are called "Lazarus species." Closer examination recently revealed that many Lazarus fossils are actually different, but nearly indistinguishable descendents of the extinct species. What to call these fossil imitators? "Elvis species."
The book begins and ends with a list of possible causes for the extinction. The suddenness of the event would make a dramatic extraterrestrial impact a reasonable culprit. The problem with that, Erwin argues, is the lack of consistent and convincing evidence in the end-Permian rocks that is associated with the end-Cretaceous extinction. Other ideas involve the collapse of ecosystems following sudden global climate change, rapid changes in sea level or ocean circulation patterns, or sudden anoxia (oxygen deficit) in ocean water. Anomalous carbon isotope compositions in shells and preserved organic matter deposited at the end of the Permian certainly are consistent with anoxia in shallow seas, but does this chemistry reflect the cause or a response to the extinction? Erwin elaborates on the significant coincidence of the extinction and the most extensive episode of volcanic activity in the past 600 million years, eruptions that spread basaltic lava across much of Siberia (an area roughly equivalent to the continental United States) to a depth of 6,000 meters. Heat from the massive extrusions would have burned coal deposits in the region, releasing atmosphere-warming carbon dioxide. Erwin humbly admits that he does not know exactly what happened. He even suggests a Murder on the Orient Express hypothesis, in which all the suspects have a hand in the disaster.
Erwin avoids deep thinking on the philosophical implications of the end-Permian extinction. Paradoxically, he suggests that the event really may not have made that much difference in the development of life (contrary to the declarations of typical museum exhibits and other popular literature on the topic). A comprehensive survey of some 35,000 fossil genera by the late University of Chicago paleontologist Jack Sepkoski indeed shows the dramatic dip in numbers at the end-Permian. However, so-called modern faunas that proliferated after the mass extinction were already emergent and growing in ecological stature before the event. Given this perspective, Erwin avoids the temptation to get too gloomy about alarming rates of contemporary extinctions.
Of course, the brute fact of species extinction—like the notion of geological time—challenged traditional interpretations of Genesis when early natural philosophers like Robert Hooke (1635-1703) began to come to terms with the mutability of living things. Many Christian philosophers of that period accepted the concept of a "Great Chain of Being," including the doctrine of plenitude, an assumption that all that had been created continues to exist. Such would be the case in a perfect and complete creation. In contrast, modern biblical scholars have attempted to replace Greek presuppositions with the ancient Hebrew presuppositions underlying biblical accounts of origins. Good creation does not mean perfect creation. Mass extinctions are not necessarily inconsistent with the way God appears to work in creation and even in human history. The geological record is a gift from our creator that contains lessons about the durability or vulnerability of earth systems, helping us to be better stewards of creation.
Erwin's short course is a professional service for geologists (like me) who have read only some of the primary literature on the end-Permian extinction. I wonder if the complexity of the problem and the depth in which Erwin covers the research might discourage all but the most determined readers who lack professional training in geology or the equivalent thereof. Certainly Erwin has worked hard to hold willing readers without dumbing the story down, and as an advocate of geology, I applaud his effort. This scientific mystery story should be read and appreciated by thoughtful Christians. The popularity of creation science and flood geology in our subculture has produced an unfortunate and unreasonable skepticism toward geology. Erwin's elaborate presentation of our effort to understand an important milestone in earth history is but one example of the kind of remarkable work being done in the geosciences today. The story should also give Christians pause to again ponder some of the old questions about the implications of natural history for theology and how creation, past and present, reveals the character and provision of God in strange and wonderful ways.
Stephen O. Moshier is associate professor of geology at Wheaton College.
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