The Development of Animal Form: Ontogeny, Morphology, and Evolution. Alessandro Minelli. Cambridge University Press, Cambridge, United Kingdom, 2003. 323 pp., illus. $75.00 (ISBN 052180851 cloth).

In the preface to his engaging and groundbreaking book, Alessandro Minelli points out that “most of the steam pushing the new engine [of evo-devo] is coming from developmental biology rather than from evolutionary biology.” He continues: “Developmental biology is rapidly transferring to evolutionary biology a wealth of precious data and concepts, which are revolutionizing our current views on homology, body plans, and the origin of evolutionary novelties” (p. xv). With his book, the evo-devo engine switches tracks; abundant facts, concepts, and problems from comparative morphology and the study of postembryonic development generate powerful new “steam” for future research.

Minelli proposes nothing less than a paradigm shift in the field of evo-devo, one that moves the field of developmental biology from finalism (what he terms an “adultocentric” view) to a truly developmental view. Finalism is thinking about development as the program by which an egg becomes an adult, and it is this outlook that Minelli seeks to remove from developmental biology (evolutionary biology, he argues, has already expunged finalism). Some examples are necessary to understand adultocentrism, and he weaves these throughout the book. I found two of them particularly compelling.

In the first, Minelli posits that the evolutionary origin of the skeleton and cuticle (chapter 2) in vertebrates and ecdysozoans was for a developmental purpose: the control of mitosis in the epidermis (in the absence of cilia to control it) and hence the control of growth and thus of body size. Because ciliated cells in metazoans cannot divide, they are removed from the lineage of proliferating cells and take on a morphostatic role—that of maintaining the animal's shape. In animals in which cilia are lost (vertebrates and ecdysozoans), Minelli proposes that other cell types/tissues (bone/cartilage and cuticle) evolve to maintain body shape. The adultocentric view is that the skeleton and cuticle evolved for locomotion and protection in the adult; the developmental view is that they evolved to make the developing animal more predictable, stable, and robust.

In the second compelling example, Minelli argues against the adultocentric concept of “provisional scaffolding” (chapter 6), the view that the patterning of early development is a scaffold for later development. Cartilage precursors of long bones, for example, typically are viewed as necessary scaffolds for subsequent ossification. Minelli argues that we must reverse this perspective: What is interpreted as a developmental scaffold is actually a structure that is in congruence with a particular developmental stage. Thus, cartilaginous skeletal elements would be interpreted as present in early development because they are congruent with the developmental dynamics of that stage and not because they will be useful for later ossification. The criteria by which such congruence can be recognized, however, are unclear.

Biologists remain fascinated by the magic numbers of animal body segments and regions (tagmata), and Minelli boils down the problematic issues surrounding their comparisons and homology assessments in chapters 4 and 5. He considers at length the very low upper limit (approximately six) of differentiated parts in any given series or body dimension (e.g., the number of tagmata in an animal's body, the number of kinds of body segments in a polychaete or fingers in a tetrapod limb). This discussion will be of considerable interest to developmental biologists whose work focuses on a segmented model such as Drosophila. In chapter 10, the final one, he weighs in on the side of partial homology (that a feature may be homologous to more than one other feature) and stresses the relative nature of this concept.

One of Minelli's goals was to “inject… into the lively arena of evo-devo biology a number of facts, concepts, and problems that have failed, until now, to find the place they deserve in today's debates and research agenda.” These are a few of his thought-provoking ideas:

Minelli views the cell as the basic unit of development. Understanding cell properties and functions, such as cell size, cell cycle length, and cell number, is key to understanding the emergence, complexity, and patterning of multicellular systems (chapters 6 and 7). Development is thus defined as the “complex networking of cellular behaviors and mechanisms influenced by the expression of genes.” Instead of regarding development as a global property of the organism, Minelli emphasizes the high degree of local autonomy that cells and other subsystems (modules) hold within an organism as it develops. Minelli's considerations of the robustness of the network of interactions existing among cells and subsystems, and the degeneracy (as opposed to redundancy) of these interactions, are up-to-date from a molecular standpoint.

Perhaps the most important chapter in The Development of Animal Form is chapter 8, in which Minelli addresses questions of axes and symmetry in a new context. He first asks what the main body axis is, and he demonstrates, by comparing a planarian with a ventral mouth and no anus to polypoid bilaterians and gastropods, that it is not easy to define. This brings him to a discussion of an idea generalized from A. S. Romer: the “dual animal,” composed of two segmental systems that are largely patterned independently. Most of us find it easy to distinguish between the body axis of an animal and its appendages, but Minelli's view may be a surprise: The vertebrate tail, he argues, is an appendage and not part of the main anterior-posterior body axis. His arguments rest on the fact that the tail, like paired limbs, has only ectodermal and mesodermal derivatives. Minelli builds the concept of axis paramorphism, that is, the idea that appendages are duplicates of the main body axis (without endoderm). Structural correspondence between the main body axis and appendages is demonstrated by two observations: (1) that the degree of segmentation in the body axis is frequently mirrored in the appendages and (2) that many trends simultaneously affect the anterior-posterior axis of the body and the proximal-distal axis of the appendages. He thus extends an answer to the question of why fishes have not evolved paired fins posterior to the anus: The tail is an appendage, like the fins themselves.

Minelli's well-edited, well-referenced, and nicely illustrated volume is the first evo-devo book in recent decades to be written by a comparative evolutionary morphologist. His appealing style of prose is illustrated by this example: “We must dispose of the idea that housekeeping genes evolved once and forever, in a remote aeon, and are now continuing to perform their job ‘at the service’ of a separate and still evolving company of ‘higher level’ developmental genes. The whole system and all its components are evolving without rest” (pp. 24–25). This important book should be read by every graduate student whose work touches either or both of the fields of developmental and evolutionary biology. It is not simply a rich collection of comparative data (though I did pull various pearls of factual information into course notes); most important, it is a rich source of fresh ways of viewing animal body plans, development, and developmental genomics. I think that the ideas in this book will, as the author hoped, stimulate interest in research on a broader assemblage of taxa, their body “syntax,” and a host of overlooked features, and perhaps begin to shift our perspective from an adultocentric to a developmental view of biology. To understand the puzzles of the diversity of animal forms and development, Minelli points out that we need not only molecular developmental genetics but also the theoretical tools of updated comparative morphology. As a comparative developmental morphologist, I could not agree more.