EVOLUTION: THE POGONOPHORA LINK

THE LINK

Pogonophora are the previously unknown link in the evolutionary sequence connecting annelid worms to vertebrates via the hemichordates.  Some items of evidence not mentioned in earlier posts on this topic include information compiled by Libbie Hyman and presented in her 1959 volume on The Invertebrates, volume V, 1959.  The link status demonstrates the error of the widely assumed evolutionary separation of deuterostomes from advanced protostomes.

The confusion of the features of pogonophorans blending features of protostomes and deuterostomes was easily dismissed as caused by lack of adequate availability of well preserved specimens or some such estimation of the recently discovered group.

On page 59, she noted that, prior to the recognition of the pogonophorans as a phylum, one was described as "a vermiform animal with a crown of tentacles and was considered by its describer to be a polychaete annelid of the family Sabellidae".

Also, on page 59 she says "These outstanding studies by Ivanov have shown that the pogonophores are closely related to the hemichordates and belong among the Deuterostomia."

Webb's (1964) discovery of the segmented posterior of a pogonophoran helped endorse their annelid origin.  Later, the contrary view was demonstrated by  Gans and Northcutt (1983) affirming their connection to the deuterostomes despite the obvious annelid connection demonstrated by the generally unknown work of Webb.

So, Gould's (2002) contention that the evidence for elimination of the annelid theory should be dismissed means that the annelid theory should be reinstated as a leading explanation for the inversion of annelid features shown in the chordates.  [  http://evolutioninsights.blogspot.com/2013/06/evolution-protostome-deuterstome-link.html  and  http://evolutioninsights.blogspot.com/2015/05/evolution-annelids-to-chordates-middle.html  ]  The second of the two posts just mentioned illustrates the larval similarity of pogonophoran larvae to solitary hemichordates.  Hyman, on page 219, notes Caullery's 1944 observation that one pogonophoran's developing young "strikingly resemble young stages of the buds of Cephalodiscus", a colonial hemichordate.

I have tried to clarify the protostome-deuterostome connection demonstrated by the pogonophorans in numerous posts on this blog.  By the time I realized the connection existed when provoked by the 1983 assertions of Gans and Northcutt, I had already published two revisions of Hegner's invertebrate zoology text, one in 1968 the other in 1981.

THE EMBRYOLOGICAL SHIFT FROM PROTOSTOME TO DEUTEROSTOME

The stumbling block for accepting the link is the drastic change in embryology that occurs.  I might not have realized how it occurred if I had not revised Hegner's Invertebrate Zoology for its second edition (1968).  In the process I had to deal with the proper classification of pogonophorans and realized the affinity that seemed impossible.

I tend to think there are answers for everything.  I did not have the answer, but the needed facts were already in my background.  As a result of the book preparation I had come to the conclusion that pogonophorans were extremely long-lived. [ http://evolutioninsights.blogspot.com/2015/05/abyssal-ocean-environment-and-extreme.html ]  I did publish that conclusion in a 1978 abstract (Engemann, J. G., Indirect evidence shows deep-sea benthos may reach extreme ages as individuals.  Am. Zoologist, 18:666).

My 1963 doctoral thesis at Michigan State University made me aware of the extreme affects environment can have on selection and evolution.  I had not made much attempt to publish bits of the thesis because they were not greatly different from many other studies.  But it made me aware of some necessary bits of information that helped explain the way embryological shift had been expedited.

The peculiar egg appendage of the isopod Asellus had been illustrated in Sar's work on the crustacea of Norway in the 1800's.  K. H. Barnard had studied phreatoicid isopods in South Africa in the early 1900's and noted a bulge in embryos in a homologous position with the egg appendage that persisted briefly after the embryo hatched.  My sections of phreatoicid eggs showed a thin-walled, yolk-filled homologue of the appendage existed before hatching as described in - http://evolutioninsights.blogspot.com/2014/06/egg-appendages-of-michigan-isopod.html .

The big thing to take from the above is that embryological features can evolve separately from the rate of adult features.  Thus the old presumption that the earliest features in the evolution of the embryology of an organism correspond to the older ancestral stages in its evolution is not always true.

The thesis comparison of Tasmanian and Michigan isopods from comparable latitudes but quite different ecological circumstances in temporary pools showed drastically different rates of development and how r- and K- selection can work before the theory was described (MacArthur, R. H., and E. O. Wilson.  1967.  The Theory of Island Biogeography.  Princeton Univ. Press, Princeton, N.J.  203 pp.).  It made it possible for me to understand the extreme age due to the abyssal conditions caused loss of some features enabling the development of the inverted deuterostomes.  Some of those ideas are in other posts.

THE MOLECULAR DATA

The exquisite work molecular biologists can do is remarkable.  But it is limited and not easily applied to discovering ancestral relationships at the phylum level.  Those limitations have been discussed in the post - http://evolutioninsights.blogspot.com/2013/05/science-screw-up-no-1.html  .

My understanding of the slow rate of DNA changes in the pogonophora was confirmed by seeing how they showed up in phylogenetic studies among clusters of disparate groups due to little change in the over half-billion year old group almost in suspended animation in the abyssal sediments.  Unfortunately, the generation time error affecting molecular clocks has not been considered in most work on molecular phylogeny.

MISSING LINKS

What do you call a missing link, such as the pogonophorans, once they are discovered?

CREDITS

I never expected to have work on an obscure invertebrate lead to the above understanding.  But, in retrospect, I realize that much credit goes to unknown biologists and scientists of the past as well as many known ones.  Thanks to Dr. V. Hickman of the University of Tasmania for calling the phreatoicid isopod to my attention, Dr. T. W. Porter of Michigan State University and the rest of my doctoral committee for keeping me focused on the comparative thesis study, the U. S. Fulbright Agency for the award for study at the University of Tasmania, Western Michigan University for allowing me to teach a ridiculous range of classes whose subject matter provided some needed insights, and the Macmillan Publishing Company for contracting with me to revise their texts.  A book would be needed to explain how students, family, friends and strangers contributed to my development under the guidance of the same Creator that made evolution a reality.


Joseph Engemann   Kalamazoo, Michigan      October 24, 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

CREATIVITY and COGNITIVE DISSONANCE

Creativity and Cognitive Dissonance

COGNITIVE DISSONANCE, the protostome-deuterostome missing link found

The insoluble clash of mutually exclusive ideas may trigger creative solutions.  This post was not edited from a previously written project as I intended to do for posts.  So it is subject to being posted with lack of checking on details.  But the term cognitive dissonance was one I learned of a long time ago in a workshop on teaching provided colleagues at Western Michigan University by Dr. Robert Travers and Dr. Greg Fisk.  They were emphasizing creative approaches to teaching.

The previous post on creativity was one I was pleasantly surprised at getting posted since I had been having problems doing so.  The morning paper inspired me.  In it were two columns using the term cognitive dissonance; a term I have only found in the newspaper a few times before.  One, on the first page of The Wall Street Journal Sunday inclusion within the business section written by Brett Arends (WSJ.com/Sunday), even used a cartoon illustration to show stock prices and interest rates make the bull and bear riders an instance of being on the same financial roller coaster, hence a case of cognitive dissonance.  Stock prices and interest rates typically go in opposite directions.

My most enlightening case of cognitive dissonance was finding a worm that was put in two disparate parts of the animal kingdom.  The conventional view put them in the annelid line of animals.  A new view (Gans and Northcutt, 1983) put them in the opposite cluster of animals that humans are in based on their common embryology.  I knew the existing data meant they had to be annelids, the new study noted they were in the vertebrate line based on embryology.  I was somewhat irritated and distressed by what I knew, the new study had to be wrong.

As I thought how honest scientists have to be and how careful they are with their data I somehow thought the only way it could be true would be if the worms were an intermediate connecting link.  The eureka moment came almost instantly, blending knowledge of the old annelid theory that had been rejected with the new embryological similarity to the vertebrate line.  Differences were so great it had previously been thought to be necessary to go back to the flatworms to have a likely common ancestor for the annelid line and the vertebrate line.

The many other bits of evidence for the vertebrate line originating from the annelid level of animal development rapidly became apparent thanks to merger of disparate facts gained from others and from my research.

Gans, Carl, and R. Glenn Northcutt.  1983.  Neural crest and the origin of vertebrates: a new head.  Science, 220:268-274.  (15 April 1983)  Includes Pogonophora in the deuterostomes.  “. . .  the neural crest and the epidermal placodes for special sense organs and other neural structures.  These structures may be homologous to portions of the epidermal nerve plexus of protochordates.  The transition to vertebrates apparently was associated with a shift from a passive to an active mode of predation, so that many of the features occurring only in vertebrates became concentrated in the head.”  This article triggered my (1983 eureka event) awareness of the pogonophorans as the protostome-deuterostome link after initial disgust at their inclusion in the deuterostomes.

           Joseph G. Engemann   May 19, 2013