The Course of Evolutionary History

Life Pulse: Episodes from the Story of the Fossil Record. Niles Eldredge. Facts on File Publications, New York, 1987. 246 pp. $19.95. Reading Life Pulse has a certain element of déjà vu for me. A decade ago a committee of paleontologists from the National Museum of Natural History began the complete thematic reorganization of our four major paleontology halls. To avoid the traditional "Hall of Fossil Invertebrates," "Hall of..." approach, we developed a comprehensive theme statement—a plan to highlight the major episodes of life history as shown by the integrated fossil records. Our list of "episodes" could have been the table of contents for Life Pulse. This concurrence might seem remarkable, but to a paleontologist it is not unexpected. It demonstrates the overall agreement in interpreting the general course of evolutionary history. Beyond the headings, the contrast between book and exhibit is striking, though both, in attempting to cover the whole history of life, are subject to severe limitations. My recent experience gives some basis for seeing the advantages and disadvantages of each in telling the same story. Eldredge's subthemes, after the obligatory introductory chapter explaining geologic time, the genetic basis for evolutionary change, ecological interaction, extinction, etc., include: (1) the earliest fossil evidence for life and the Precambrian records, (2) the appearance of trilobites and other hard-shelled groups in the Cambrian, (3) life in the Paleozoic seas, (4) invasion of the land, (5) life's middle ages, the Mesozoic, (6) dinosaurs, (7) the origin of flight, (8) parallelism, particularly as shown by the reptiles that returned to marine life from a land ancestry, (9) extinction, as it affects the fossil record and as evolutionary stimulant, and (10) Cenozoic life, featuring mammals and our own beginnings. Life Pulse continually reminded me of what I wanted to say in our exhibit, before compromises with specialists who had to consider what the typical museum visitor can be expected to read and understand. In the exhibit, attention is focused on the fossils themselves, and the condensed text simply helps explain what is not readily apparent in the fossils, or cannot be conveyed by their arrangement. But in the book, text conveys the information, and the infrequent illustrations are simply supporting examples. I often wish it were possible to use more written information in exhibits. And I'll wager that Niles Eldredge wishes his readers could see the actual fossil material and feel the excitement he obviously feels when the fossils reveal their own story to him. The well-written text of Life Pulse is enlivened by anecdotal accounts of the author's experiences, both in the field and laboratory; we can't doubt his enthusiasm and commitment for his work. The proofreader, if the book had one, was less enthusiastic. I wouldn't mention mechanical errors, except that this book just might set a record: certainly I've never read a book with so many typos. Pervading the text, beginning with its suggestive title and subtitle, is the hypothesis that evolution is episodic, with brief intervals of very rapid change followed by long intervals of no perceptible change. It was Eldredge, along with Stephen Jay Gould, who coined the oxymoron "punctuated equilibrium" for this evolutionary mode, and who in this book seems to be its overly persistent advocate. I am convinced, as I believe most paleontologists are, that the fossil record supports this as one of the modes of evolution, but I am equally convinced that it does not apply universally. Clearly, different lineages evolve at different rates; otherwise bacteria, representative of the simplicity of all life forms through much of Precambrian time, would not now coexist with extremely complex life forms. And surely the fossil record shows that single lineages evolve at different rates at different times, often apparently accelerated by extinctions that reduce competition. But the part of the fossil record I know best convinces me that gradual environmental change can also result in gradual evolutionary change. My quibbles do not mean Life Pulse is wrong, or necessarily even controversial. It is a readable account of the major chapters of life history, informative to the nonspecialist, while still stimulating to the specialist. Emry is curator of fossil mammals at the Smithsonian Institution's National Museum of Natural History, Washington, DC 20560.

The Course of Evolutionary History

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