If one accepts the pattern of reduction and distortion of segments or skeletal parts as a rule rather than an exception, it is inescapable thatt the progenitors of segmented organisms consisted of many symmetrical parts; the ultimate outcome of this extrapolation is an organism consisting of a grouping of identical segments, a condition of ideal homology among skeletal parts. If the parts are linked in a chain, the homology is serial; radial and bilateral patterns are also possibilities.
William Bateson raised the general question of whether evolution is proceeding from the simple to the complex, or the other way around (Bateson 1922), but he rejected the view that the progenitors of segmented organisms consisted of identical segments, a condition of ideal serial homology (Bateson 1913). Bateson maintained that a degree of antero-posterior differentiation was an absolute necessity (Bateson 1913). I argue that Bateson was not justified in denying this possibility.
If each segment of the theoretical progenitor were a complete organism, each with its own complement of anatomical systems, there is no compelling reason why the concatenation could not be viable. It could have functioned like a colonial organism. In a variation with the segments arranged in an orderly linear orientation, there could be a lead segment with greater perceptual and manipulative freedom. This would lead to the evolution of specialized behavior and anatomy in the anterior segments.
Specialization would proceed among segments. While the original organism had a full complement of organs in each segment, its descendants would have segments which had lost most of their organs. Organs from certain segments would evolve the capacity to serve the whole organism, so that those organs need not be grown and maintained within every segment.
In standard texts on comparative anatomy the evolutionary trend toward distortion and loss of segments is explicitly recognized, but the conflict with the general dictum of evolving complexity is not addressed. Reduction is viewed as a secondary phenomenon, despite its ubiquity in the fossil evidence and its reasonableness as a mechanism, and regardless of the lack of evidence for increase in number of parts. It is as if a pattern of decreasing complexity were unthinkable in any broad view of evolution. The answer lies in legitimising loss of parts as a sufficient mechanism for the evolution of certain segmented organisms within a specific time frame.
How could segmented organisms arise in early evolution, and why should they succeed? The "how" question is answered by one simple mechanism: parabiosis or Siamese-twinning. The "why" question is also easily answered: increase in the number of parts allows the rapid evolution of greater size, greater locomotive and manipulative capabilities, and a collection of generalized structures, capable of specialization through distortion. To evolve greater size within a given organismic structure is problematic; all kinds of adjustments must be made. But parabiosis can instantly create a much larger organism, simply by agglutination. In the proterozoic ecosystem, such an agglutination might have had an advantage over its simpler relatives, especially after a few generations of reductive/distortive evolution shaped the mass of segments into a more efficient structure.
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