The Origin of Animal Body Plans:

A study in evolutionary developmental biology.

Wallace Arthur

Cambridge University Press. Cambridge. 1997.

Ó 1999 Art Chadwick

Perhaps the fact that I filled twenty pages of my yellow pad with notes from this book will convey something of the impact this work has had on me.

This remarkable book, following on the train of a series of papers and books on related subjects by the same author, sets out to lay cornerposts for a developmental approach to evolution. Evolutionists have long overlooked the message of developmental biologists and are still tending to ignore the significance of the recently unfolding story of molecular developmental biology. This discrepancy, Arthur seeks to redress.

The developmental journey begins with a single fertilized cell that continues to divide and differentiate into an adult form with many cell types and many cells. However, if the life history is dissected into morphogenetic and allometric phases, the vast majority of the ontogenetic history is devoted to the allometric expansion of early stage morphogenetic fixation. Thus the events happening early in the life of an embryo are the events most critical to the ultimate outcome. Arthur suggests this history roughly parallels the evolutionary history of organisms, in which all of the crucial directing events occur early and since that time we have seen only "allometric" expansion of forms. Of course he offers no evidence that this is more than fanciful speculation. The body types of all modern organisms are already present in Cambrian strata. Why is this so, and what does this tell us about the evolutionary history of life. These are the issues this insightful book addresses.

Arthur freely acknowledges the anachronistic problem of the Cambrian-Precambrian boundary. How life forms could have simultaneously achieved multicellularity and hard body parts in over 35 phyla simultaneously defies explanation. Why no additional major body plans have arisen in the animal phyla since, likewise challenges the evolutionary interpretation of the history of life. But these were things we already knew. Arthur goes further in suggesting the Neodarwinian theory is incapable of explaining any type of process that can produce a body plan change. His reasoning is as follows: Neodarwinian changes are all developmentally late. That is they must (at least in Dawkins incantation of Neodarwinism) be small gradual changes that are the kinds of mutations that manifest themselves late in ontogeny. On the other hand, any mutation that is going to result in a different body plan must be a change in the early morphogenetic phase, early in development. Such changes can result in the development of a new set of wings, or with legs developing instead of antenna in Drosophila, etc. Now, no matter how many developmentally late mutations you may accumulate, they can never add up to a development early change required to produce a new body plan. Thus Neodarwinian evolution, which may suffice for describing changes in the color of eyes or moth's wings, cannot account for the kind of change we call macroevolution - from one body plan to another.

The book makes an important contribution to our understanding of many issues having to do with the origin and development of life. Arthur considers the matter of cladistics and the problem presented by using cladograms to defend evolution, when they assume evolutionary relationships. Some cladists have sought to develop cladistics apart from the evolutionary presuppositions to avoid this circularity. The real problem with cladistic analysis is that cladists are forever having to propose convergences to explain how some characters appear to be linked by lineage whereas others do not appear to follow along. The principle of "parsimony," choosing lineages with the least difficult convergences, is often invoked. But one such convergence, shown to be evolutionarily impossible, falsifies the whole evolutionary enterprise.

With respect to "colinearity," the discovery of genes lying in the same order on the chromosome as the linear order of the processes they affect on the axis of the embryo, Arthur states "These observations are clearly telling us something even if it is not yet clear what. This statement is often echoed in molecular biology. Could it be that the answer is too obvious to need stating?

Arthur often laments the absence of a unifying theory of developmental biology. He attempts to develop at least the ground rules for such a theory, a Herculean task gives that at this point "…we cannot be certain that one is possible…"(p94).

Arthur describes the developmental process as working like a computer with at least three instructions: Do, stop, and wait. The programs however, "…are highly complex, involving an interplay between multiple types of signals in a network that includes a diversity of feedback loops."

This of course begs the question of the origin of all of this development complexity. It was clear that it was all present in the first preserved organisms of the Cambrian. With precious little prior history to go on, one can only stand in amazement at the kind of blind faith that leads evolutionary biologists to continue to hold tenaciously to a theory of origins which offers no solution to the most fundamental question: "Where did we come from?" This book is remarkably candid and informative and should be read by all students of molecular biology with an interest in origins.