THE MAJOR EVOLUTIONARY TRANSITION

Annelid worms are ancestral to all of the advanced major phyla.  Arthropods and mollusks retained annelid features as one major grouping of advanced phyla.  Chordates and echinoderms are the other major grouping.  The two groups share some common feature as a result of their annelid ancestries.

There are eight posts in this blog-site from 6/17/13 to 6/30/13 that are a more extended description of the basis for accepting the annelid theory of chordate origin.  The annelid theory eliminates the need to (1) assume a second early origin of segmentation and/or metamerism from acoelomate protostomes, (2) explain numerous bits of molecular similarity between coelomate protostomes and deuterostomes, (3) and, with the incorporation of information about the pogonophoran bottleneck connection provided in this blog, provides a logical rationale for the embryological differences as well as the mechanism of inversion of systems in the transition of protostomes to deuterostomes.

THE ANNELID PHASE

The pogonophora are fairly well established as close relatives of some marine polychaete annelids.  The fossil record provide very little evidence of events, but some annelid descendants clearly  existed during the pre-Cambrian.

Step one.  Tube-dwelling worms such as Sabella, started the rotation as they reoriented their posterior to a position in the sediments and left their plumose tentacles up in the water.

Step two.  Those becoming pogonophorans had multiple changes selected by the rigor of life at constantly increasing depths as they struggled to survive asteroid bombardments in the pre-Cambrian.  In particular, low metabolic rates due to abyssal pressure and temperature, tubes extending deep into the sediments for absorption of fossil nutrients, as well as retention of a blood vascular system to transfer oxygen from the water to the embedded end of the worm deep in the sediments, and loss a many non-essential features.  The thinning of the eggshell enabled dropping the spiral development of annelids and the ability of eggs to develop even if some cells were lost.  Loss of obvious segmentation left only a remnant of segments at the  posterior to anchor the worm in its tube.

Part  of step two was the reduction of the digestive system, particularly the mouth and esophagus.  This allowed the later fusion of the ganglia of the head into a brain blocking the mouth formation on the old ventral side.

Step three.  As asteroid caused extinctions in the surface areas of the oceans eased, progeny of the abyssal worms survived as they moved into shallow water and benefited by mouths developing where gut remnants touched epidermis on  the old dorsal side.  As they extending from their tube they now would use their relocated mouth to feed on particulate matter in their vicinity.

Step four.  As they increased their activities they were doing it with the old dorsal side now the new ventral side.  SO THE WORMS DID NOT ROLL OVER, THEY DID A BACK-FLIP as they made their transition from annelid to pre-Chordate organisms during the pre-Cambrian.

Referring to the February 27, 2015 post on "Evolution: the body cavity" may help you understand the bit about relocation of the mouth above.  The March 2, 2015 post on "Abandoned theories and Libbie Hyman"  has a brief discussion of the annelid theory and an associated figure.

THE IMPORTANCE OF THE REGRESSIVE  POGONOPHORAN STEP

The loss of features as pogonophorans adapted to life in the deep sea were essential factors enabling the climb to the branch of the animal kingdom known as the deuterostomes, with the vertebrates members dominating life on earth.  It enabled rearrangement of the head with fusion of ganglia into a brain and mouth formation on the former dorsal surface, simplified embryology of radial cleavage and delayed determination of first cells of the embryo.  The loss of ability to form chitin in deuterostomes meant other structural materials became more important.  Hemoglobin is the only blood pigment surviving in deuterostomes although a diversity of blood pigments are found in annelids prior to the pogonophorans.

OTHER PHYLA

There are other major and minor animal groups that are side shoots at many positions along the groups of the tree of life leading to as well as following the significant deuterostome branch.  They continued, some becoming extinct, others diversifying into forms still present.  One major group of arthropods, the trilobites, dominated paleozoic seas before becoming extinct.  The sponges of today no longer include the group giving rise to early protostomes.

The above figure shows the central sequence of protostomes leading to annelids from which all above them trace their ancestry and have coelomate body cavities.  The sequence follows a time sequence of origin of groups having living representatives (except for the hypothetical protonemerteans).  The pogonophorans provide a transition from coelomate protostomes to the vertebrate line that clarifies the transition without the mystery of many unknown ancestral groups coming from flatworms.

Many invertebrate groups such as ctenophorans, chaetognathans, lophophorates, echinoderms, sipunculids, extinct groups, and many others are not represented in the diagram.

Joseph Engemann   Emeritus Professor of Biology, Western Michigan University, Kalamazoo, Michigan      July 16, 2018

EVOLUTION: OFFBEAT IDEAS

Evolution: Offbeat Observations

Georg Lichtenberg said “It is impossible to carry the torch of truth through a crowd without singeing someone’s beard”, according to James Geary (2005, The World in a Phrase, Bloombury Publishing, New York, 229 pp.).  It somewhat comforts me as I write about the multiple errors earnest and intelligent researchers of evolution have made, errors I am trying to replace with a more accurate depiction of the tree of life.

One, of the two worst research reports establishing a faulty superphylum, Ecdysozoa, had a lady as the primary author; it made me realize the sexist nature of the aphorism quoted above.  I also do not know if the gentleman first author of the publication setting up the other faulty superphylum, Lophotrochozoa, had a beard to be singed.

The most influential zoologist of the last hundred years could well be Libbie Hyman.  See http://evolutioninsights.blogspot.com/2015/03/abandoned-theories-and-libbie-hyman.html  She deserves the respect given her, mistakes in her work are minimal, and I also do not want to detract from other work by the two authors whose work is criticized in http://evolutioninsights.blogspot.com/2013/05/science-screw-up-no-1.html , after all, “to err is human”.  I guess that proves I am human too.

When offbeat becomes main-stream evolution

Three of the last four blog posts have had bits about the abyssal ocean.  There are several facts about the abyss that can help us understand major consequences for evolution shaping life today in unappreciated but interconnected ways.

One, the stability of the abyssal region offered refuge from numerous early extinction events.

The extreme pressure, lack of light for photosynthesis, low input of surface debris reaching the abyss, near-freezing temperature, and sparse populations were ecological factors leading to the long-life, low reproductive rates, and emphasis on survival adaptations characteristic of K-selected life styles.  They contrast to r-selected life styles of organisms where abundant food and high predation lead to short lives, high reproductive rates, rapid growth, and perhaps higher evolutionary rates of most organisms in lighted, warmer surface habitats.

Two, the abyssal affects on embryology and metabolic rates

The low reproductive potential in the deep sea put such a high priority on survival that it selected for delayed specification of embryonic fate of cells so loss of a cell from an early embryo would not prevent normal development.  This led the transition from protostomes to deuterostomes that have the ability to have an early embryo divide and produce two individuals instead of dying like a protostome embryo would.  Pogonophorans are at the junction where this happened and they have a mix of protostome and deuterostome features.

As I have noted elsewhere, the extreme pressure is probably a factor slowing metabolic rates and extending life-spans in the abyss.  Several studies have shown respiration is slowed greatly beyond what colder temperatures alone would depress rates.  One of the most enlightening clues was that a brown bag lunch contained a sandwich and an apple, that sank to the ocean bottom many months before they were retrieved with the sunken research vessel, Alvin, and were both in fresh condition.  Similar food in cold seawater decayed within a few days.

The ocean layer and circulation patterns described in a recent blog show that oxygen levels below the oxygen minimum layer would be impossible to exist if respiratory rates at abyssal pressures were anywhere near rates normal in shallow water below the photic zone.

Three, it’s a bit complicated, 

but the things above help show the role of the pogonophorans as an intermediate that also accounts for some features of our development and structure that were first accounted for by the annelid theory of chordate origin, an abandoned theory that is correct when adjusted for the role of the pogonophorans.

Joseph Engemann    Emeritus Professor of Biology, Western Michigan University, Kalamazoo, Michigan    September 30, 2017

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

EVOLUTION IN THE DEEP SEA

ORIGIN OF DEUTEROSTOMES BACKGROUND

EXTREME LONGEVITY IN THE DEEP SEA  

There are deep-sea worms that can possibly live longer than a hundred thousand years.  I became aware of this because of my interest in factors responsible for extreme longevity.  I developed that interest from my (1956-1963) doctoral research on a Tasmanian isopod crustacean that takes three years to reach maturity.  A Michigan species from a related isopod suborder can do so in three months.

 

There are several categories of factors that are responsible for extreme longevity and/or life cycle stage duration differences.  Factors making the deep-sea worm live for thousands of years include.

                Genetics.  This factor is unknown for the deep-sea worm, but is certainly important.  We know insects such as the aphid can produce a generation in a few weeks or less, whereas some cicadas require seventeen years.  Conclusion: the genetic factor can be responsible for great longevity differences.

                Temperature.  In the deep sea temperatures are close to four degrees centigrade year around.  At sea level, temperatures can be more than twenty degrees centigrade higher year around in the tropics.  Many biochemical reactions used by living creatures double in speed for every ten degrees increase in temperature.  The bullfrog may take three years to mature in the northern part of its range, but only need a year in the southern part.  Conclusion: Temperature can be responsible for over two doublings of longevity in the deep sea, over a four-fold difference.

                Extreme pressure.  For partially unknown reasons life processes in the deep sea are greatly reduced.  Depths of 6,000 feet are associated with a 99% reduction of metabolism of bottom communities.  At the pressure of depths beyond 21,000 feet a deep sea bacterium showed a dramatic drop in respiratory rates although other environmental factors were the same.  Conclusion:  abyssal depths could be responsible for a thousand-fold difference in longevity.

                Ecological factors.  These act on the genes through natural selection to make great differences in longevity adaptive to the environment of the organism.  In the comparison I made of Tasmanian and Michigan isopods the magnitude was more than ten-fold within a similar temperature regime.  Conclusion: ecological factors of low food supply, low predation, and stable environment could select for genetics leading to a more than ten-fold increase in longevity.  The pressure and temperature differences noted previously can be multiplied and the result multiplied by this ten-fold increase to make an enormous potential difference for increased longevity of abyssal organisms.

DISCUSSION

Evolution could be so slow in deep sea organisms that ancestral forms could survive relatively unchanged while descendants migrate to surface waters and change greatly into new groups.  Known examples will be discussed eventually, if I live long enough.  But the major one making revision of the tree of life, as envisioned by my peers, necessary is one of the themes of my 2010 unpublished book manuscript (Evolution Insights).  Parts of it may be condensed in future postings.  The next evolution posting is expected to explain how major errors have been made in proposed evolution of major groups because my peers were not aware of the longevity impact suggested above.

EXTREME LONGEVITY IN THE DEEP SEA, was first implied in my 1968 paper (see references below.  It was later treated on pages 717-732, Chapter 14, of Engemann and Hegner, 1981, Invertebrate Zoology, 3^rd edition, Macmillan Publishing Co., New York.  My peers tend to ignore things that are not in major journals, their specialty journals, or monographs.]

Some starting point references, for those reluctant to take my word for it, are:

Brooks, William Keith.  1915.  The Foundations of Biology.  Columbia Univ. Press, New York.  339 pp.   Comments on - the unchanging nature of Lingula (page 219), and p. 217 “the diversity of the Lower Cambrian fauna and of its intimate relation to the fauna on the bottom of the modern ocean”.  See Jablonski et al. below.

Engemann, Joseph G.  1968.  Pogonophora: the oldest living animals?  Pap. Mich. Acad. Sci., Arts, and Letters, 53:105-108.  Extreme age of individuals inferred from published data of others about tube length, probable depth in sediments, and sediment rates of accumulation in abyssal environments.

Ericsson, D. B., M. Ewing, and G. Wollin.  1963.  Pliocene-Pleistocene boundary in deep-sea sediments.  Science, 139:727-737.  Slow rates of accumulation for marine sediments.

Gadgil, Madhav, and William H. Bossert.  1970.  Life historical consequences of natural selection.  The American Naturalist, 104(935):1-24.  P. 12 “thereproductive effort increases with age”; p. 20 “the age for reproduction will tend to increase as the degree of satisfaction or the availability of resources decreases.”

Ivanov, A. V.  1963.  Pogonophora. Consultants Bureau, New York.  479 pp.  This major monograph on the pogonophorans was published prior to the discovery of their giant tubeworm relatives at thermal vents.

Jablonski, David, J. John Sepkoski, Jr., David J. Bottjer, and Peter M. Sheehan.  Onshore-offshore patterns in the evolution of Phanerozoic shelf communities.  1983.  Science, 222:1123-1125.  Fig. 1 shows older groups from shore area are now found in deeper water, older Ordovician inner shelf forms now on outer shelf, Cambrian shore forms now on slope and deeper. Consistent with comment of Brooks, 1915.

Jannasch, H. W., et al.  1971.  Microbial degradation of organic matter in the deep sea.  Science, 171:672-675.

Smith, K. C., and R. R. Hessler.  1974.  Respiration of benthopelagic fishes: in situ measurements at 1850 meters.  Science, 184:72-73.

Webb, M.  1964a.   The posterior extremity of Siboglinum fiordicum(Pogonophora).  Sarsia, 15:33-36.  Fig. 1, page 34, shows “anchor” with 17 setae bearing annulations.  This seldom recovered portion of the worm may be so because the tube portion it is in is deep in the sediments consistent with vertical orientation of the tube.

Webb, M.  1964b.  Tube abnormality in Siboglinum ekmani, S. fiordicum andSclerolinum brattstromi (Pogonophora).  Sarsia, 15:69-70.  When worm posterior protrudes through break in tube it secretes a new posterior tube portion with no annulations but is continuous with anterior portion of the tube and sealed off from old posterior portion; Fig. 1, page 69 shows it for three species. 

Yayanos, A. A., A. S. Dietz, and R. VanBoxtel.  1979.  Isolation of a deep-sea barophilic bacterium and some of  its growth characteristics.  Science, 205:808-810.  It showed rapid depression of growth beginning at depths with pressures exceeding 725 atmospheres. 

This post may have been lost or not published.  I just relocated it and as it is important to the topics I was about to address, I thought I should try to re-post it.  
Joseph G. Engemann, June 22, 2013