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The Long-Term Evolution of Life

Dale Russell

Deep Time: Paleobiology's Perspective. Douglas H. Erwin and Scott L. Wing, eds. vi + 373 pp. The Paleontological Society (printed by Allen Press, distributed by University of Chicago Press), 2000. $25.

The distinction between living and nonliving things is a fundamental one. Life is transient and cannot exist beyond the thin edge of the present; its record has been reduced to nonliving vestiges strewn along an enormous unidirectional axis of time. Deep Time, originally published as a supplement to Paleobiology (volume 26, number 4), commemorates the 25th anniversary of the journal, which was founded to provide a forum for the integration of nonliving fossils with living biology?an obviously formidable but fascinating task. The 15 invited papers, chosen to reflect the current state of knowledge of the long-term evolution of life, represent the collaborations of 26 authors. The result is a marvelous monument to how knowledge itself is a creature of the present, its dynamics recorded within a human lifetime in the unchanging printed word.

The editors, in selecting contributors who are respectful of each other's thought, have created a work that is consensual rather than discursive. The papers are carefully written and more demonstrative than speculative, with a tone that is analytic, encyclopedic, quantitative and synthetic in approximately equal parts. Much consideration is given to mathematical procedures and models in order to compensate for various factors in the fossil record that distort the dynamics of evolution. There are references to databases, software and listservs on the Web. The target audience thus appears to be university-level students and scholars. The volume is both exciting and thought-provoking. Origin of novelty. Major evolutionary changes may be the result not of the number and structure of genes but of changes in gene regulation. Regulatory genes were responsible for the radiation of body plans at the beginning of Phanerozoic time (about 543 million years ago). Organisms with patterned genes outcompeted those without patterning, and with the extinction of the latter the capacity for innovation at the phylum level was lost. Phylogeny. Evolutionary stasis is generally underlain by nondirectional change rather than absolute invariance. Origination and extinction rates vary in different groups of organisms and probably declined through Phanerozoic time. Phylogenies based on parsimony are improbable because they are unrealistically simplistic. Biodiversity. Diversity remains a central theme of paleobiological investigation, and global biodiversity increased through geologic time. Marine organisms are much less diverse than terrestrial organisms but contain more cryptic genetic variation. Effects of mass extinctions. Dominant taxa are removed in mass extinctions through factors that do not operate during intervals of lower extinction intensity. Mass extinctions do not reset the evolutionary clock; survivors demonstrate continuity and often undergo dramatic diversifications following the removal of dominant taxa. Sequence. Development is characterized by sequence?cells arise before tissues, patterns of cleavage before embryos, patterns of symmetry and segmentation before body plans. The history of life is characterized by a similar sequence?reproduction precedes metabolism, symbiosis, multicellularity, organ differentiation and intelligence. The evolutionary sequence is paralleled by increasing organismal size, complexity and use of ecospace.

In their preface, the editors state that the long-term effects of the interactions of organisms with each other and with their inorganic environment are uniquely revealed by the perspective of "deep" time. They thus clearly distinguish between organic and inorganic processes and imply the presence of pattern in evolution. This position contrasts with that of the 23 participants in a colloquium held 20 years ago on "The Evolution of Complex and Higher Organisms," who reached a consensus that evolution is predominantly driven by changes in the physical environment and that there was no straightforward progression of effects through deep time (a summary of the proceedings, edited by David Milne and others, was published in 1985 by NASA [NASA-SP 478]). The contrast suggests that the past 20 years have seen a watershed in how the history of life is regarded. The 15 articles in this volume provide ample support for this possibility, support that can be summarized under the following headings: Fitness. Plant form is determined by bioengineering function, and fitness increases with natural selection. Form diversifies when natural selection acts on the performance of two or more tasks (for example, water conservation, reproductive efficiency, mechanical stability and light interception) rather than one. Form analysis in animals focuses on whether a structure could perform a function. How an animal's neuromuscular system interacts with a structure to produce movement is less predictable than whether or not a structure could perform a function. Natural selection allows those animal forms that are best able to perform various tasks to survive. Effects of physical environments. Physical environments sort associations of species on timescales shorter than those on which changes occur within species lineages. Effects of organismal interactions. The evolution of biodiversity is unrelated to changes in the availability of physical habitats. However, with physical factors controlled, the relative importance of physical and biological factors in evolution is clarified. In general, evolutionary change may be the result of trophic (nutrient-based) relationships, competitive superiority and "arms race" escalations. There is no correlation between climatic change and broad patterns in the evolution of land mammals; biologic factors?such as competition, key adaptations, diversity dynamics and within-lineage evolutionary trends?have been far more important. Contingency. Some patterns of genetic and developmental change (and evolutionary pathways) are more probable than others. Evolution is directional, and a general progression is probably typical of life anywhere in the cosmos.

The contributors should be encouraged to consider further whether activity levels (metabolism) and behavioral complexity (central nervous system) augment the effects of diversity in driving the complexity of animal interrelationships; these two factors, like biodiversity, increased in importance through Phanerozoic time. Another issue remaining to be addressed is the cause of the regular pattern of sea-level fluctuations that has imposed episodicity on the fossil record. And as Anna K. Behrensmeyer and colleagues note in their article "Taphonomy and Paleobiology," there should be "much more attention to synthesis, at the local, regional and global levels," for therein lies an area of unusual conceptual opportunity.

A recurring theme is that evolution is patterned, and the contributors are engaged in an exciting chase of discovery in elucidating those patterns that are evident in perspective of deep time. These essays thereby imply that the pervasiveness of order in the nonliving world also characterizes the living world. I thank the contributors, and the editors, for this excellent volume.?Dale A. Russell, North Carolina State University and the North Carolina Museum of Natural Sciences, Raleigh

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