EVOLUTION

ORIGIN OF DEUTEROSTOMES

Abandoned views

Once natural selection was accepted as the way evolution of species produced the variety of life on earth, it became a goal to determine the ancestral line of intermediate forms leading to the major groups of animals.  A series of successive creations suggested by the drastic changes in the geologic fossil record had earlier been suggested but abandoned.  Likewise the concept of inheritance of acquired characteristics was seldom considered after an understanding of genetic inheritance developed.

Features of some value

Radial symmetry versus bilateral symmetry was given some emphasis for a while.  The grades of body complexity were, and still are, given considerable significance.  Grades went from cellular level, to tissue level, to organ system level.  The most primitive of those with organ systems had only a mouth opening, whereas the more advanced had both mouth and anus.  Blood vascular systems represented a greater advance.  Skeletal systems, segmentation, and metamerism complicated the picture as different branches of the ancestral tree diverged.

Invertebrates and vertebrates were treated as two vastly different groups in some ways and may have been a major factor in the annelid theory of chordate origin never getting full acceptance.  The recognition of embryological differences of protostomes and deuterostomes made the presumption of the deuterostome line separating from the protostome line at about the time of the early flatworms a generally accepted view; the speed and ancestry involved in the shift will be shown to differ from the recent views as well as indicating the error of recently accepted proposals.

Time for return of the annelid theory

The annelid theory of origin was a result of the comparison of annelid worms and vertebrates when one was inverted.  When one is inverted and then compared, the arrangement of nervous system and blood vessels and directions of flow are similar.  But the embryological differences seemed insurmountable.  Biochemistry and genetics, as well as greater understanding of embryology and biology made it evident that embryonic and larval features were not a sure guide to determining ancestral paths.  But, as Stephen J. Gould noted in his book (2002.  The Structure of Evolutionary Theory.  The Belknap Press of Harvard University Press, Cambridge, Massachusetts.  1433 pp.), the annelid theory was not revived when the developmental grounds for its dismissal were eliminated.  The developmental grounds do not need to be completely discounted;  they can still be helpful if used with some flexibility.

The inversion the annelid theory encompasses still troubles the investigators below who imply other explanations are needed for the inversion.

Arendt, D., and K. Nübler-Jung.  1994.  Inversion of dorsoventral axis?  Nature, 371:26. 

De Robertis, E. M. , and Yoshiki Sasai.  1996.  A common plan for dorsoventral patterning in Bilateria.  Nature, 380:37-40.

Their efforts are commendable, but unnecessary if the annelid theory is reinstated.  Numerous other reasons to reinstate the annelid theory of chordate origin exist.

Others have not yet shared in my 1983 awakening to the validity of the annelid theory.  By that time it was evident that the Pogonophora were near relatives of polychaete annelids.  But a conflicting paper revived the notion that they were deuterostomes based on embryology.  The article, quote, and notes from my reference file are as follows:

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.  “Both  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 is the article that triggered my (1983 eureka event) awareness of the pogonophorans as the protostome-deuterostome link after initial disgust of their inclusion in the deuterostomes. 

Other factors lend support to the need for reinstatement of the annelid theory with the addition of the Pogonophora as a formerly missing link.  A hint of the overwhelming evidence will be provided in blogs on embryology, inversion, anatomical, and other evidence.

Joseph G. Engemann     June 23, 2013