EVOLUTION: ANNELIDS TO CHORDATES, THE MIDDLE STEP

THE MIDDLE STEP

The pogonophora to hemichordate transition is the most difficult step to see because pogonophorans are most easily seen as specialized polychaete annelids; and hemichordates are generally accepted as being in the chordate line.  So the big step from the first (the protostome part of the lineage) to the last (the deuterostome part of the lineage) faces the greatest difficulty in seeing for those not familiar with what I have tried to show in earlier posts about this topic.

An obstacle is the residue left from past objections when the annelid theory was thought to be discredited by embryological and molecular evidence.  Why those objections are irrelevant has been treated in earlier posts.

SOME GRAPHIC EVIDENCE OF THE MIDDLE STEP

A, B, and C illustrate the progression of stages from early larva to adult anterior of the hemichordate, Saccoglossus.
D, E, and G illustrate the progression of comparable stages of the pogonophoran, Siboglinum.
F illustrates the segmented posterior portion of an adult Siboglinum, and G- its anterior.
H illustrates the anterior of a giant pogonophoran from a thermal vent community.  (A, B, and C, after Hyman and others; D, E, F, and G, adapted fromEngemann and Hegner after Webb; H, from a specimen in the U. S. National Museum.)  [Fig. 12-5 in my unpublished 2010 manuscript, Evolution Insights.]

F, above, illustrates the posterior segmentation not reported until done so by Webb [Webb, M.  1964.  The posterior extremity of Siboglinum fiordicum (Pogonophora).  Sarsia, 15:33-36].  Possibly the segmented end of some species lost this region, or an intermediate species did before becoming hemichordates.
 G and H show some of the variability of the anteriors of pogonophorans that resemble the anterior of C, a hemichordate.

A and D are not as precisely alike as one might like.  But this was near the evolutionary step where the early embryology was changing drastically from protostome to deuterostome embryology.

REFERENCES to many of the research articles influential in helping me see the protostome-deuterostome connection, especially the molecular evidence showing the error of current studies were posted in

http://evolutioninsights.blogspot.com/2014/11/evolution-molecular-clocks.html

which you can get to by using the Blog Archive at the right and clicking on the 2014 November arrows.  Or send the address above to a friend you think might find it interesting.  Most of the references are annotated with my comments to myself or quotations from the article.

Joseph G. Engemann    Emeritus Professor of Biology, Western Michigan University  May 4, 2015

EVOLUTION of systems inversion

EVOLUTION – ORIGIN OF DEUTEROSTOME SYSTEM 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.  

BRING BACK THE ANNELID THEORY.

Joseph G. Engemann    June 28, 2013