Reflections

Thinking that my days are limited made me want to make sure that the things I have  to contribute to understanding the major features of the evolutionary tree of life are passed on to the next generation.  I think this blogsite has enough information to do the job if it is studied by a well-trained biologist.

THE BODY CAVITY

Others apparently have found the 2/17/15 post on the body cavity of interest since over half of the pageviews each month are to that post.  I suspect it is assigned reading in some classes on several continents because of the clustered nature of the country sources of the views.  The post was included almost as an afterthought to supplement coverage of evolutionary discussion of body systems.

Its major interest to me was the minor bit about formation of openings where layers of bare ectoderm and entoderm cells meet.  That can help one understand the new location of the mouth of deuterostomes when the connecting link, the pognophorans, emerged upside down (compared to protostome ancestors) as they re-entered coastal waters following extinction events.  The resulting inversion of systems of deuterostomes, as compared to protostomes, was discussed in the post dated June 28, 2013.  This process enabled fusion of ganglia to a compact brain close to sensory inputs of a head in an anterior position enabling short and speedy neural connections as the owner probed the environment.  Perhaps this phenomenon is best illustrated by birds and their ability to fly through a sea of tree branches.

THE DEEP SEA

Structure of the ocean and its physical state under great pressure needs to be understood to see how the pogonophorans can survive the many generations virtually unchanged in the deep sea while their wandering descendants evolve to produce the deuterostomes in shallow water.  Several old and some recent posts address this issue.  The common ancestry of all deuterostomes with a close relative of the earliest annelids explains why nucleic acids produce odd results in some erroneous molecular phylogenies.  The results are only odd because they complicate phylogenies calculated with invalid assumptions of uniform rates of genetic changes in evolution.

THE POGONOPHORA

These tubeworms represent survivors of an evolutionary bottleneck, the deep-sea, where they developed a new type of embryonic development (or cleavage), lost blood pigments other than hemoglobin, and lost the ability to make chitin in the stem group leading to vertebrates.

The blood vascular system was needed to store and transmit oxygen to the portion of the worm embedded in anaerobic sediments.  A functional digestive system was reduced to near disappearance and it reformed in a way that did not penetrate the brain in descendants moving to shallow sea areas.

REGRESSIVE EVOLUTION

It is counter-intuitive to give importance to regressive evolution when we think about the grand scheme of evolution going from the first small cells to the diversity of size and complexity of the world of life today.  Simple to complex, or progressive evolution, is the explanation our intuition provides.

So the loss of a functional digestive system seems counter intuitive and hard to accept as a way forward to the vertebrate gut from the "degenerate" pogonophoran reduction or loss.  Paleontologists found the simple to complex markings on certain cephalopod shells actually went from complex to simple as the fossil record was better developed in their collections.

In regressive evolution, the regressing feature may be targeted for loss indirectly by the better survival of organisms that no longer need the feature; any mutations causing less nutrients to go to such a feature leave the organism more for better reproduction.  The speed of regressive evolution can be much faster than progressive evolution.  Both can be happening at the same time.

The largest animals are a remarkable example of regressive evolution in the whale's adaptation to aquatic life.  All that process of evolution through four-legged ancestors to some with remnants of leg bones no longer used is one example of loss or regressive evolution while other adaptations are evolving.

The annelid worms provided an important feature in our evolution by the segmentation or metamerism producing a series of duplicated structures that could produce different structures in different parts of the body.  The phenomenon is graphically illustrated by the appendages of the crayfish in the protostome line of animals.

MOLECULAR SIMILARITIES

We share many molecular features with other organisms.  Nucleic acids and biochemistry of energy production are even shared across kingdom boundaries.  More specialized biochemical functions often are shared by coelomate protostomes and deuterostomes having closely related compounds although simpler molecular versions may also be found in more ancient protostomes.

Neurosecretions, biochemistry of vision, and biochemistry of metameric processes have similarities that help make the pogonophoran link of deuterostomes to protostomes much more obvious than is generally recognized.

DO NOT FORGET

Pogonophorans are the only animals with structural features demonstrating the transition from annelid to deuterostomes.  They explain the importance of their deep-sea life in the regressive evolution leading to a new type of embryology in the deuterostomes, their survival during extinction events, their extremely low metabolic rate enabling long life and slow evolution in the impoverished environment of the deep sea.

Joseph G. Engemann      Emeritus Professor of Biology, Western Michigan University, Kalamazoo, Michigan            February 12, 2019    (Happy 128th birthday, Dad)