Wallace Arthur sets himself the goal to convince a wide audience that science can explain with ease the rise of complex creatures from simple beginnings. Explanations based on faith in an intelligent creator are not needed. Thus, we have another book in the fast growing intelligent design (ID) versus evolutionary theory (ET) literature. Arthur approaches his (non-scientific) audience with appropriate means: the conversational style makes it a pleasure to read the book, the chapters are short yet highly informative,  technical jargon is kept to a minimum, and a well organized glossary identifies key terms. Further, the author was able to condense a tremendous amount of facts on less than 250 pages.

When we accept an evolutionary explanation for the diverse life forms on our planet, we have to accept that all life on Earth, no matter how complex, shares a common, very simple, ancestor. From this starting point organisms could either diversify sideways (generating the numerous groups and subgroups of modern prokaryotes) or upward a 'ladder' of increasing complexity. Arthur illustrates the different perspectives of diversification with an analogy to molehills (creatures of varying complexity) on a vast bacterial lawn (p.5). This analogy attempts to correct a common misconception: creatures do not purposefully climb up a straight ladder towards maximal complexity. Instead, most changes happen slowly over time within existing levels of complexity and increases in complexity occur rather accidentally. Nevertheless, the aim of Arthur is to focus on the ascents in complexity and to provide an explanation for "how complex life-forms like ourselves arose from very simple beginnings" (p. 26).

Living creatures reproduce and the reproduction of creatures is based on the reproduction of cells (p.37). Reproduction depends on the precise replication of genetic material (DNA) and, in the case of complex creatures, on an intricate developmental process that leads from fertilized egg via embryo to adult. Several chapters (Building a Castle of cells, Dances with Genes, The Embryo Wars) describe the interactions between genes and environment en route towards a new organism and provide some historical background about the progression of our knowledge about these processes. But the reader has to wait until chapter 12 (Duplicate and Diversify) to get to "the book's heart" (p.136). Here Arthur introduces the key feature of the evolutionary process (the unbreakability of the flow of life across the generations) and it's implications for the 'design' of organisms. Accidental mutations, that affect individuals (as opposed to species), sometimes result in an 'improved' organism and the mechanism of natural selection can spread such an advantageous mutation through an entire species (p.138). Complex creatures have numerous different cell types and body parts on which mutation can 'work'. Using the example of arthropod segments Arthur shows that individual body components can be duplicated. Superfluous copies can change over time and fulfill new functions (p.139ff). Thus, redundant parts can diverge in structure and function, allowing for increased complexity.

Duplication and diversification 'works' at the gene level as well. In the 1990s it was discovered that all bilaterally symmetrical animals (from worms to humans) share the same underlying Hox gene mechanism to pattern their bodies. And again, we encounter the possibility that mutations cause the accidental duplications of single genes, Hox genes or the entire genetic material. Redundant copies of genes can evolve over time to fulfill new functions and over evolutionary time the genetic material of some creatures becomes more complex. "So the diversification of replicated parts is a key process-perhaps the key process- in the ascent of life's ladder in both genetic and structural domains" (p.160). In the following four chapters (From Simple to Complex, From Complex to even more Complex, Acquiring your Head, Crossing the Threshold) Arthur applies the general principle of diversification of replicated parts to particular evolutionary events. The first steps upward in organismic complexity lead from simple single celled organisms (prokaryotes) to more complex single celled organisms (eukaryotes) to multi cellular creatures. The step from single- to multi-cellular organism required the evolution of sticky cell-membrane structures that 'glued' two or more cells together. Multicellular organisms in turn could evolve different types of cells for different purposes (e.g., different organs composed of different cell types). Further, from simple body plans (lacking any symmetry) evolved more complex body plans (radial and bilateral symmetry). The evolutionary shift from radial to bilateral symmetry allowed for subsequent steps up the ladder of complexity. Why? Most importantly bilateral symmetry allows for the evolution of a head at the 'front-end' of the creature. Head evolution is another instance of increasing complexity that leads externally from headless via protohead to proper head and internally from nerve-cell concentration via simple brain to complex brain.

The evolutionary story that has brought us from our single celled Urancestor to modern human being is complemented by two chapters (Dinosaur blues and Beyond Pluto) that address the importance of mass extinctions (opening ecological niches for radiation of already existing creatures) in the evolution of complexity and the question of the nature of possible extraterrestrial life. From this context somewhat surprisingly, the final chapter (Big Questions) is an attack on fundamentalism of every stripe: atheism and theism are equally irrational in Arthur's view. "I see no more reason for a rational scientist to be a committed atheist than to be a committed theist" (p.231). This conclusion seems to require some further argument; an agnostic world-view doesn't flow necessarily from the story told in the preceding chapters. But this is a rather small flaw in a book that can add greatly to the education of lay readers interested in evolution.

© 2007 Christina Behme

Christina Behme, MSc (1986, Biology, University Rostock, Germany), MA (2005, Philosophy, Dalhousie University) is currently a PhD student in the philosophy department at Dalhousie University, Halifax, Nova Scotia, Canada. Her research interests are philosophy of mind and psychology, cognitive science, and philosophy of language.

Categories: General, Genetics