The body-plan explosion

Abstract

poetry and ideas from the history of the longest continuous civilization on Earth. The relationship of that civilization with its fragile and often tortured surroundings contains lessons for others-particularly at a time when industrial society in China, as elsewhere, is pressing harder than ever on the environment. This will be a source book, elephants and all,for generations to come. ■ Crispin Tickell is a member of the China Council for International Cooperation on Environment and Development. the possibility of extinct phyla. From a cladistic point of view, it is easy to argue that two extant sister phyla arose at the point of branching from the last common ancestor, and that anything that branched off later should be included in the respective phylum. A distinction can then be made between the crown group (the descendants of the last common ancestor of all the living members of the phylum, including this ancestor) and the stem group (everything else). This avoids the question of how body plans arise and whether there may be others not represented by living forms. Defining a body plan isn't easy, however. Valentine's definition,for example,is dangerously circular: "an assemblage of morphological features shared among members of a phylum-level group". What does that mean, except that when we define a phylum we also define its body plan, or vice versa? Valentine proposes to define the origin of a phylum by the acquisition of a key apomor-phy-a unique derived trait. This may be more subjective and less convenient than letting the total (stem and crown) group or the crown group define the phylum, but it gives due priority to biological significance over methodological convenience. After all, we want to know how different kinds of organism evolve by natural selection, and how they interact with each other and with the environment. They do that with their phenotypes, not their pedigrees. A key question, then, is whether the body plans of the recognized phyla represent more or less the total number of possible solutions to the problem of being an animal, or whether there were numerous other possibilities that came into being but became extinct because of bad luck or bad design. Valentine argues that the Cambrian explosion initially produced great disparity in design, but that this was subsequently diminished by extinctions. The pattern of diminishing evolutionary novelty subsequent to this event, he says, may have been due less to developmental constraints than to a saturation effect (candidates for new adaptive radiations were already available among existing body plans). He also believes that the Cambrian explosion produced a lot more homoplasies (similar characters with independent origins) than most phylogenetic analyses suggest-in my view an extremely important point that calls for much more careful character evaluation than is commonly done. He is clearly not impressed, then, by some recent attempts to use fossils to bridge gaps between phyla. Valentine seems most happy with intrinsic biological mechanisms for the rapid appearance of phyla. Large parts of the book deal with developmental prerequisites (such as cell-type numbers and gene regulation) for the event. Ecological interactions, such as predation, are given more cursory treatment. As for the physical environment, he merely concludes, somewhat apologetically, that although physical environmental factors were "supremely important", he does not see any evidence that extraordinary environmental events were causally connected with the Cambrian explosion. Given that extraordinary environmental events did indeed occur shortly before the explosion, I would give the jury just a little more time to ponder the question. But first I would make sure they had read this magnificent book.