That all changed following a meeting I had in 1996 in the Wyndham Hotel near Los Angeles International Airport, with Professor James A. Lake of UCLA, who proposed a new molecular-based phylogeny of metazoa in which annelids and arthropods were fundamentally separated. Arthropods would be united with the determinedly
What this means for segmentation is this: that each major animal group has one phylum within it that is habitually segmented. In the Lophotrochozoa, it is the annelids, standing amid a squishy morass of unsegmented molluscs and brachiopods. In the Ecdysozoa, it is the arthropods, rising above the unsegmented nematodes and a load of other stuff you've probably never heard of, and even if you did, you'd probably want to forget about it immediately afterwards. And among the Deuterostomes, it is the chordates - including we vertebrates - with our mesoderm divided into somites, each with its own vertebra, innervation and muscle block. If you don't believe me, just ask a salmon steak.
Now, this poses a nice conundrum - which my father says is different from two elephants sitting on a bagel, who have a bunundrum - and this conundrum is this: did the habit of segmentation originate in each phylum independently, in which case the common ancestor of all these animals was unsegmented, or was it a more general feature of animals that most creatures have lost, all except for the annelids, arthropods and chordates? This, I discovered, is a matter still ripe for debate, and what made the symposium at Euro Evo Devo so exciting.
To posit an independent origin for all three instances of segmentation would be by far the easiest option, given that the three phyla concerned are widely sundered by evolution, and the alternative is to suppose a wholesale loss of segmentation in virtually every other kind of animal. The ugly fact that spoils this easy idea, though, is that in all three cases, the process of segmentation utilizes pretty much the same network of genes - cognates of genes isolated long ago in the fruit fly and bearing names such as engrailed and wingless. These days it is fashionable to invoke the concept of deep homology in which structures in widely separated animal groups, although they look very different from each other and might have indeed appeared independently, are based for their engenderment on a similar cassette or module of interacting genes. This idea allows the invocation of segmentation in each of the three phyla to appear separate, even though it is based on fundamentally the same genetic substructure.
This, however, raises the question of why segmentation has not happened in all the other phyla, given that they all must share the same module or cassette of genes, and, to follow this line of thinking, why the ancestor of all three major animal groups - which would have also shared this cassette or modeule of genes - might not also have been segmented.
Truly, a question that would cross a rabbi's eyes, noch?
The solution, it seems, is to loosen what we think of as segmentation. For it is not the case that all animals other than annelids, arthropods and chordates are utterly without any sign of segmentation. True, their bodies might not be so rigidly divided into compartments in accordance with our usual requirements for segmentation - but they do, very often, show a repetition of parts along the body axis. Primitive molluscs, such as chitons and some other forms, show repetitive arrangements of shells, gills and so on, even though they are not usually thought of as segmented. Non-chordate deuterostomes such as acorn-worms show repeated gill slits. Even humble flatworms show repeated arrangements of gonads, gut diverticulae and so on. In the widest sense, therefore, there is a tendency for creatures to divide their bodies, to a greater or lesser extent, into a series of repeated structures, forming a continuum from the completely unsegmented to the fully segmented.
Perhaps Bateson had it right all along, if only figuratively. Perhaps there is a tendency within animal bodies to create distinct domains by means of wavelike morphogenetic gradients, whose results are not distinguished by a simple division into those animals that are segmented against those that are not - but simply by those animals in which the wave crests of such oscillators are higher or lower, more distinct or less. The common ancestor of Ecdysozoa, Lophotrochozoa and Deterostomes might not have been strictly segmented, or strictly unsegmented - it is probably impossible to know - but it would have had an oscillatory system of body partitioning that would have fallen out naturally from the interaction of cassettes or modules of genes. Like many revelations in science, it is not the data that change - but the way you look at them in the light of new evidence.