The science of the 21st century is multidisciplinary for one, simple reason: It has to be. The grand challenges of science -- curing cancer, understanding climate change, and overcoming our dependence on fossil fuels, to name a few -- require nothing less than collaboration across disciplinary boundaries. This extraordinary book tackles the grand challenge par excellence of paleobiology: the "Cambrian Explosion," in which nearly all of the extant phyla of the animal kingdom seem to have appeared quite suddenly, at least in in geological terms (about 20 million years, from the middle to the upper Cambrian), without readily apparent precursors in the fossil record. And it does so with a commitment to multidisciplinarity that amounts to an education, in itself, in the principles of the advanced science that is now emerging.
The problem tackled by this book is by no means new. In his Origin of Species (1859), Charles Darwin noted that the sudden appearance of animal phyla at the base of the Cambrian raised questions about the veracity of his theory of evolution by means of natural selection -- a theory that predicts substantial prior development during the Neoproterozoic. Yet, so far as Darwin or anyone else knew at the time, animal fossils first appeared at the base of the Cambrian. Pointing to the deficiencies of the fossil record, Darwin believed that evidence of pre-Cambrian animals would one day come to light -- and he was correct.
Today, we know that a strange collection of ocean-floor-dwelling fauna appeared during the Ediacaran, which immediately preceded the Cambrian. There are trace fossils, too, which suggest that soft-bodied bilaterian animals had indeed developed prior to the Cambrian without leaving any trace in the fossil record. Moreover, "molecular clock" studies, which assume that certain components of DNA change at a fixed rate over time, suggest that animals began diverging even earlier, during the early- to mid-Neoproterozoic. Finally, studies of the changing chemistry of the Earth's oceans during pre-Cambrian times suggest a gradual, if fitful, oxygenation of continental shelf oceans during the Neoproterozoic -- a key development, since multicellular organisms cannot develop without available oxygen.
But these discoveries have not, in themselves, solved the problem. For one thing, the evidence is difficult to interpret. Most of the Ediacaran fauna, for example, appear to amount to brief-lived evolutionary experiments that had little, if any, impact on subsequent animal evolution. Distinguishing trace fossils from marks left by non-biological processes is difficult, at best. Molecular clocks turn out to keep time differently, depending on their underlying assumptions, as do chronologies of ocean oxygenation. There is evidence, in sum, of animal divergence prior to the Cambrian, but it took place at an exceedingly slow pace, at best. Against the slow pace of animal evolution prior to the Cambrian, the Cambrian explosion still appears, arguably, to be an explosion indeed -- a remarkably sudden event that cannot be easily explained by its mysterious antecedents.
Tackling this problem in a truly multidisciplinary spirit are the book's distinguished co-authors, Douglas R. Erwin of the U.S. National Museum of Natural History (Washington, D.C.) and James W. Valentine of the Department of Integrative Biology at University of California, Berkeley. The key to grasping the book's approach is the book's subtitle, a point that other reviewers have not as yet noted: "The Construction of Animal Diversity." As against studies that try to identify a single "cause" of the Cambrian explosion, usually from a disciplinary point of view, the authors emphasize the development of complex networks linking geochemical, environmental, genetic, evolutionary, and developmental processes. For example, although the timing and pace of the ocean's oxygenation is still poorly understood, there is no doubt that this process involved the cumulative network effects of organic activity -- activity that helped to transform the Earth into a stage, so to speak, for an apparent eruption of biological diversity. Their point is this: If the Cambrian fauna seem to have appeared on the stage quite suddenly, they did so only because the theatre had been built, the air conditioning installed, the various services (restrooms, stage, telephones etc.) provided, and the scripts were written -- and, from this point of view, it is unsurprising that a variety of characters would suddenly appear, spanning the gamut from Hamlet to Malvolio. The theater took a hell of a long time to build, to be sure, but once it was finished, the company could run through the repertoire in a season or two. And by no means were animals the only biota to take advantage of this new community resource: similar, explosive diversity occurred in the other kingdoms of life, as well.
To build their picture of network construction, the authors call on an impressive variety of disciplinary perspectives, including geology, evolutionary biology, paleoenvironmental and paleoclimate studies, the comparative biology of today's living animal phyla, genomics, molecular biology, and even economics. The picture that emerges is a series of steps involving positive feedback, in which new innovations created opportunities for the development of new species. For example, sponges helped set the stage by ventilating the water column in the Cryogenian, just as bioturbation of ocean sediment altered the microbial environment of the ocean floor in ways that facilitated metazoan development. And by the dawn of the Cambrian, it seems that the true metazoa had developed a shared and remarkably stable genetic toolkit that enabled body plan variations -- precisely the variations that were to occur, in short order, as nearly all of the animal phyla appeared. Here, too, feedback played a crucial role: the sudden emergence of animal diversity is clearly linked to the development of new networks of gene interaction and new methods of gene regulation, both of which enabled early Cambrian fauna to diversify rapidly in the face of unprecedentedly permissive ecological opportunities. The scripts had indeed been written, a fact that is strikingly confirmed by the surpassingly odd fact that very few new animal phyla have appeared since the Cambrian explosion.
Ranging as far afield as it does, this book will sooner or later expose the weak spots in a reader's education -- and, as other reviewers have pointed out, it is by no means an easy read. It's worth noting that, as the authors recall, the book was originally intended for graduate students in molecular biology! Still, any reader with a solid undergraduate education should be able to piece through the book's various chapters without too much trouble. I found it helpful to have my laptop at hand, so that I could look up unfamiliar terminology. Certainly, this book isn't for the casual reader. Still, the effort involved produces a handsome payoff. By the time you've finished reading it, you'll have a good sense of the state of the art across a range of cutting-edge scientific disciplines -- and, most of all, an appreciation of the stunning progress that can occur when workers put them together.