Friday, June 28, 2013

EVOLUTION of systems inversion


Several major steps were involved in the inversion of systems as polychaete annelids gave rise to the chordate line of deuterostomes. 

First a branch of tube-dwelling, bottom-dwelling polychaetes evolved with some reduction of clear segmentation; development of a plume of tentacles for feeding and/or respiration probably was occurring as well.  A variety of marine species having a similar intermediate condition still exist.

The second major step involved the complete adaptation to life in abyssal sediments as pogonophoran worms.  This included (1) a loss of dorso-ventral distinctions common to species ancestral to groups with radial symmetry, (2) reduction of development of the gut, (3) increased reliance on passive absorption of nutrients, and (3) retention of the circulatory system to provide oxygen to the portion of the worm embedded in low oxygen sediments. 

The embryological changes noted in the previous post were occurring simultaneously with these changes.  They now are the pogonophorans, well-adapted to survive the extinction events of pre-Cambrian times.  The major feature identifying them as annelid descendants was the extreme lower segmented and setae bearing portion that was not noticed in specimens of early collections made by dredges that did not retrieve whole worms; the deeply embedded part presumably was left in the ocean bottom.

The third major step in the protostome-deuterostome journey was the sequence of changes as descendants moved to shallower seas during the period following the intense asteroid bombardment (see May 11 post).  Those moving from their tubes to reach particulate food more abundant on shallower sea sediments found it less jarring to the nervous system to emerge with the previous ventral nervous system of the annelids positioned so it was nearer the upper surface.  In such a position, a remnant of the gut, perhaps more substantial because of the greater abundance of food, put endodermal and ectodermal tissue closer together to induce the mouth formation associated with such an event.

A further consequence of this new position of the mouth enabled fusion of ganglia and connectives to form a brain without encircling the esophagus.  These and other changes above were facilitated by natural selection of those with genetic modifications better serving the processes.

The culmination of this process needed very little fine tuning to make a hemichordate, probably the closest annelid derivative to the chordate line of deuterostomes.  The larval stages of pogonophorans and hemichordates are very similar.  Correspondence of the anterior of a hemichordate and the upper portion of a certain pogonophorans is quite similar.  The lowest segmented portion of the pogonophoran degenerated to leave three body regions some think are characteristic of chordates.  Previous discussions indicated the speed of loss of features not contributing to survival by the greater energy left over for reproduction is a common feature in evolutionary events.

Clearly, pogonophorans are excellent candidates for the missing link clarifying the inversion of systems suggested by the annelid theory of chordate origin.  


Joseph G. Engemann    June 28, 2013

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