The proposed theory suggests a fundamental model for ontogeny, a basic pattern underlying embryonic development. The protovertebrate consists of a mass of connected identical organisms, each having the same full complement of organ systems. Once the protovertebrate complex is formed, subsequent evolution in this lineage is a matter of the reduction and distortion of the growth or phenotypic expression of this original complex.
The early stages of the vertebrate embryo show axial segmentation in the train of structures called somites. Under the proposed model each somite was primitively a complete organism. But under this model the ribs are formed from other trains of segments, each of which was also a complete organism like those ancestral to the axial segments or somites.
Limbs are primitively identical to a longitudinal half of the axial skeleton. In the evolved ontogeny of vertebrates the limb appears as a mere bud in the early embryo; but if its ontogeny were less suppressed, it would show segmentation just as the axial somites do at this stage.
Differentiation of the early embryo into segments or somites is referred to as a process of subdivision; this is certainly what it looks like. But 'subdivision' may be a misleading term in this context. Each segment or somite results from a distinct mechanism; its differentiation is extant before (and whether or not) any level of morphological visibility is attained by that segment. The apparent subdividing and increase in number of segments is only one aspect of ontogeny, one level of observation. It is not an illustration of an evolutionary history during which organisms gradually acquire more segments. Differentiation is the revelation--the partial revelation--of a hidden structure, a plan or program that is pre-existent.
The problem is that ontogeny appears to be a gradual process of increasing complexity in every sense, and this fits all too easily with a conception of evolution as gradually increasing complexity in every sense. Under the proposed model ontogeny is a matter of the suppression of the ontogeny of a fundamental complex structure--the structure which is represented in Fig.2. Archetypes are anathema to modern evolutionists; evolutionary constraints are an important subject, but to be deeply and seriously bound to some primeval pattern--that smacks of pre-Darwinian thinking. Yet if constraints are to be discussed, there must be discussion of what is being constrained.
Within a limited evolutionary context, one in which all important changes are reductions and distortions from previous generations, it is reasonable to think in terms of archetypes. The evolution of the vertebrates over the last half billion years is such a context.
As an illustration of the difference between recapitulation theory and the proposed model, consider the evolution of bone and cartilaginous skeletons among vertebrates. It was long assumed that skeletons of cartilage were the evolutionary precursors to bony skeletons, because bone exists first as cartilage in its embryonic development. Sharks were presumed to be primitive, because sharks have skeletons of cartilage. Under the proposed model, cartilaginous skeletons are evolved through mutations that arrest the development of skeletal bone while it is still in the embryonic cartilaginous stage. Thus bony fishes cannot have evolved from forms with cartilaginous skeletons. Evolution proceeds through truncation of ontogeny.
The most blatant phylogenetic assumption in traditional evolutionary biology--and the point of clearest conflict with the proposed model--is the view that the ancestors of the vertebrates were simple chordates like lancelets or larval tunicates. This is recapitulation theory, inference based on the fact that such animals resemble the vertebrate embryo.
There are no fossil series illustrating gradual elaboration in early vertebrates which would support such a phylogenetic claim; some of the earliest vertebrates known were quite complex. Under the proposed model, the simple chordates represent the extremes of reduction and simplification within the phylum. Their evolutionary potential is minimal, as they have lost the unspecialized parts which could have been adapted to new functions.
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