EVOLUTION

UNDERSTANDING EVOLUTION

The study of evolution encompasses all of biology.  So we face a problem in understanding evolution much as the ant faces in understanding the tree it is climbing.  It can climb all over it.  But much is hidden in the ground and under the bark.  Even the living part of the tree is mostly hidden by the dead outer layer of bark.  If all the tissues and functions that make up the tree were known, it would still be a monumental task to understand how it worked and how it was affected by its environment.

ENTRY POINTS TO UNDERSTANDING EVOLUTION

The fossil record, comparative anatomy, comparative embryology, comparative physiology, and nucleic acid analysis have been the major entries to understanding evolution used by past and present researchers.

1 - The fossil record has left a partially connected chain of organisms leading to the present living world.  It has not left an unambiguous source due to- gaps in fossil bearing sediments - soft bodied forms rarely leaving fossils - and our inability to always determine if a line of fossils was gaining or losing a feature.  Ancestral forms may appear in strata with later descendants as shown by an example in  [ http://evolutioninsights.blogspot.com/2014/05/living-fossils.html ].

2 - Comparative anatomy and related microscopical studies, especially embryology, did the heavy lifting in bringing evolution studies toward maturity.  It still is a major source of new understanding; for an unusual example connecting anatomy and psychology see [ http://evolutioninsights.blogspot.com/2014/07/evolution-intelligence-and-creativity.html ].  Many studies need a combination of factors other than anatomy considered, an example is shown in how the extreme longevity of pogonophorans was determined [ http://evolutioninsights.blogspot.com/2014/06/evolution-and-oldest-animal.html ] and [ http://evolutioninsights.blogspot.com/2013/06/evolution-in-deep-sea.html].  The anatomical similarity of pogonophorans and hemichordates is illustrated in [ http://evolutioninsights.blogspot.com/2015/05/evolution-annelids-to-chordates-middle.html ].

The sponge-cnidarian transition was first hinted at by a fossil comparison [ see http://evolutioninsights.blogspot.com/2013/07/acoelomate-evolution-1-sponges.html ] supported by a hypothetical argument based on spicules in both sponges and some cnidarian nematocysts [ http://evolutioninsights.blogspot.com/2013/07/acoelomate-evolution-2-cnidarians.html and illustrated in http://evolutioninsights.blogspot.com/2015/04/cnidaria-nematocyst-origin.html ].

The cnidarian-flatworm transition is further discussed in [  http://evolutioninsights.blogspot.com/2013/07/acoelomate-evolution-3-flatworms.html ] and illustrated in [ http://evolutioninsights.blogspot.com/2015/03/evolution-quiet-pre-cambrian-genes.html ].

3 - Comparative physiology provides many insights regarding relationships.  An example is noted in a comment about vitamin C in the previous post [ http://evolutioninsights.blogspot.com/2015/07/the-evolution-diet.html ].  The same post mentions the anatomical evolutionary connection of our appendix with the cecum of herbivores, noting that function lacking structures are often rapidly lost by lacking selection for their retention; the appendix may have value from the lymphatic tissue it contains as well as for possible retention of gut organisms for re-inoculation of a gut purged of them by diarrhea.

4 - Comparative biochemistry studies, especially DNA and RNA studies, are still the gold standard of determining close evolutionary relationships.  It is far less useful in determining relationships of phyla because rates of change can vary among the chromosomes as well as differing in different organisms.  This has been discussed in many posts, especially [ http://evolutioninsights.blogspot.com/2013/05/science-screw-up-no-1.html ]

SYSTEMATICS / TAXONOMY

Systematics indirectly confirms the reality of evolution.  When Linnaeus classified plants and animals he is thought to have assumed the different forms were created separately by special creation.  Consequently, the groups named were clustered in a hierarchy based primarily on anatomy.  When groups are determined by scientists using an evolutionary hypothesis, they approximate the relationships in the classification designed by Linnaeus.

COUNTER ARGUMENTS TO EVOLUTION

No one knows the mind of God.  Lacking direct revelation of the creation of diversity that is not clouded by inspired story, parable, and limitations of knowledge among inspired writers, we are left with only science to give us a clear look at the details of how God did it.  Even the best intentions of christian writers can go astray as noted in [ http://evolutioninsights.blogspot.com/2014/09/evolution-and-error-of-irreducible.html ].  My recent post [ http://evolutioninsights.blogspot.com/2015/07/creationevolution-debates.html ] might be helpful in providing greater understanding of other arguments.  The tools of science are adequate to prove the existence of evolution, but not the existence of God; they are inadequate to prove God does not exist.

BIOLOGICAL SCIENCE

The complexity of the interrelationships of biological sub-disciplines is most fully appreciated in the study of evolution and ecology.  Genetics is also an excellent organizing principle making sense of the diversity of life and evolutionary processes.  One or more of the first three entry points are often missing or insufficiently well known in the education of today's biological scientists.  For those well versed in genetics and molecular biology, just realizing the limitations of those areas are cause for not rejecting the older scheme of phylum relationships, prior to the invalid acceptance of Ecdysozoa and Lophotrochozoa as natural groups.  The post of 2013/05 listed at the end of 4 above tells why.

Joseph G. Engemann    Kalamazoo, Michigan    August 1, 2015

ACOELOMATE EVOLUTION, 3, FLATWORMS

Flatworms: planarians, flukes, tapeworms

The above sounds less pretentious than Platyhelminthes: Turbellaria, Trematoda, and Cestoda.  For purposes of showing the ancestral links from protozoa through sponges, corals, jellyfish, flatworms and an unknown intermediate (probably a ribbon worm) link to annelids then deuterostomes, the list is more comprehensible.  Side shoots from the ancestral line to chordates leave far more passed than are listed. 

Planaria

Planarians are the turbellarians most like the ancestral link of interest.  Flukes and tapeworms are of interest too, but are all parasites and neither is a link to any other major group.  The common planarian has been an example of flatworms for many students because it is abundant world-wide. 

Marie Jenkins (1963) made an observation of planarians that may be instructive.  She found ones that were cultured in a slippery container could not divide by pulling themselves apart to regenerate two new planarians.  They just kept growing longer until they eventually formed a second head at the distant tail end.  Apparently, the head releases some chemical messenger that inhibits head formation until it is too far away to be in an effective concentration.

Other turbellarians much smaller than planaria have an anterior mouth, are not flattened, and have new mouths and a fission plane develop before separation.  The preparations for division can produce a chain of connected potential individuals up to 16 in number.

Possible descendent groups

Small flatworms may have given rise to Aschelminthes, including a branch becoming the nematodes and another branch, the rotifers.  An intermediate group, the Gnathostomulida, have some flatworm characteristics, and like most aschelminths are adapted to life in sediments of the sea and freshwater. 

The central selective action shaping the aschelminths was their adaptation to the interstitial water (water filling the spaces between sand grains and other small particles of the bottom of aquatic habitats and beaches) where small size enabled their movement while excluding slightly larger predators.  Besides their minute size similar to large protozoans, they often have a forked posterior with each short branch having adhesive glands.  In the nematodes the posterior toes are missing but some have a pair of gland cells and can attach temporarily to the substrate.  Many parasitic nematode species are much larger than their microscopic free-living relatives.

A feature of aschelminths that make it very unlikely they were ancestral to any mainline animals is the fact that they lack the ability to regenerate, probable because they adapted to miniaturization by reduction of chromosomal material as cells of the embryo differentiate into the adult.  A specific number of cells and or nuclei are found in adults of some smaller aschelminths.  Nematodes are unusual in lacking cilia; having only longitudinal muscles in the body wall; and having those muscles enervated by muscle cell processes (the processes lack muscle fibers) that reach either the dorsal or ventral nerve to receive the nerve impulses.  Rotifers are very numerous in lakes and their sediments and beaches.  Pennak (1978) describes in his introductory material the importance of the interstitial habitat as a route for some smaller organisms as they adapt and invade fresh water.

And the likely link

The nemerteans are thought to be descendants of flatworms also because some of them have rhabdites in their ciliated epidermal cells.  But nemerteans, like most aschelminths, have added an anal opening to the digestive system.  Nemerteans also have a blood vascular system so it is possible they were part of an ancient complex derived from flatworms that served as intermediates on the way to annelids at an early stage in the evolution of higher animals.  The living nemerteans do not have clear evidence of an ancestral role, but they are most representative of living animals approximating an intermediate form.  In my unpublished 2010 manuscript, Evolution Insights, I refer to the putative ancestor as a protonemertean.

The hypothetical protonemertean may have used a central branch of the turbellarian gastrovascular system to complete the digestive system with the posterior opening never disappearing after the individuals divided.  Lateral branches of the gastrovascular system may have lost their connection to the gut and become modified into blood vessels.  The benefit of a long body may have been the selective force keeping individuals attached as they evolved coordination as one organism, becoming the annelid worm central to the remaining major phyla evolution.

The soft body of this step in evolution may not have left a fossil record.  The steps along the way may not have anatomically instructive living descendants.  The answer may be in carefully targeted molecular phylogeny studies.  I don’t expect to have another eureka event like the one that made me see the pogonophorans were the missing link between annelids and the deuterostomes.

References

Jenkins, M. M.  1963.  Bipolar planarians in a stock culture.  Science, 142:1187.

Pennak, R. W.  1978.  Fresh-Water Invertebrates of the United States. 2^nd Ed.  Wiley, New York.

Joseph G. Engemann    July 22, 2013      minor editing November 7, 2014; also this note that the features noted in the post about nematodes, almost certainly precludes nematodes from an ancestral role in the Ecdysozoa, as also noted in other posts. 

ANIMAL KINGDOM EVOLUTION

THE MAJOR GROUPS

The roots of the animal kingdom and other kingdoms are closely intertwined prior to the origin of multi-cellular plants and animals.  We think the earliest organisms are still represented today by the bacteria and other forms lacking a nucleus in their membrane-enclosed selves.  During this stage, perhaps the first billion years of evolution, the basic biochemistry of life evolved.  The RNA, DNA, and much of the basic materials still found in subsequent organisms evolved.

A consequence of the development of photosynthetic organisms in the world, then lacking oxygen in the atmosphere, was the production of oxygen as a toxic waste product that accumulated and changed the biosphere for the remaining time on earth.  Some of the early organisms developed the ability to utilize oxygen to oxidize organic material for their energy.  They could then remain active in the absence of light while extracting more energy from food than was possible by anaerobic process alone.  

Organisms that protected their genetic material from the oxygen with a nuclear membrane could better survive as oxygen reached higher levels.  Some developed a symbiotic relationship with other organisms.  Details of these early steps are discussed by Lynn Margulis (1981, Symbiosis in Cell Evolution, W. H. Freeman and Co., New York).  The evidence that mitochondria of our cells are a result of symbiosis is very strong; perhaps cilia are derived from flagella that also came from a similar symbiotic origin.

At this stage of evolution the Animal Kingdom or its one-celled progenitors, the Protozoa, had representatives so overlapping with plants and fungi that many biologists prefer to put them in a separate kingdom, the Protista.  These early steps were developing during the second billion years of life on earth.

By the beginning of the third billion years on earth a protozoan that could change back and forth from one with a flagellum to one with pseudopodia had evolved.  Sometime the pseudopodia would develop into a collar around the flagellum.  Eventually some of these dual potential cells stuck together and developed small colonies that eventually specialized into sponges.  The single cell with the capacity for diverse structure and a mechanism for controlling it needed a few control changes in a few different cells of the colony to provide the basic material for evolution of many of the features of all animals.

The Porifera were the first phylum of animals to develop.  They diversified into many different sponge types until one group gave rise to coral-like animals as indicated by the similarity to a Middle Devonian anthozoan (Kazmierczak, Jozef. 1984.  Favositid tabulates: evidence for poriferan affinity.  Science, 225:835-837.).  

Recognition of this evidence of anthozoans as the first cnidarians provides a basis for a simple continuity of phyla in the early stem of animals leading to the next phylum, the Platyhelminthes which may be considered the earliest protostomes.  A simple but unconventional view is that anthozoan polyps gave rise to jellyfish ancestral to triclad planarians.  The complexity of the simple process is why I needed to write my manuscript, Evolution Insights, to make it evident.

The sponges have less well-defined tissues than phyla that follow.   But the main mass of sponge is jelly-like with a few amoeboid cells and a tangle of collagen-like fibers and is much like loose connective tissue in our own bodies.  The jelly-like mass is mostly covered with flattened cells and is perforated by many pores leading to canals and or cavities lined with choanocytes.  Choanocytes are cells with a flagellum surrounded at the base with a collar that collects microscopic food items to nourish the sponge.  Water is passed out one or a few large openings.  Most sponges have spicules.  Spicules are mineralized (calcareous and/or siliceous), often needle-like, or three-pointed and other shapes often specific to the class of the sponge.

The protostomes included all the animals above the cnidarians until the deuterostomes evolved.  The seemingly hidden origin of deuterostomes becomes simple and clear when the role of the Pogonophora is known.  The next several blogs are expected to deal with the origin of the deuterostomes.  Then it will be time to clarify the Porifera-Cnidaria-Platyhelminthes links.  Later, the origin of mollusks and arthropods from annelids will be covered.  The foggiest portion of animal evolution, Platyhelminthes to Annelida, is obscure because the intermediate steps left neither a fossil nor living close relative to my knowledge. 

The annelids seem to be the living representatives of the most ancient animals with a true coelom, a body cavity with body wall lined with a cellular layer of flattened cells.  Organs enclosed in the coelom are also covered with a similar cellular layer; the two layers often connect to form a double layered mesentery.  The mesenteries may help keep organs in position, including blood vessels and nerves servicing them.  Of the simple animals, more complex than flatworms, but still lacking both a true coelom and segmentation (or its derivative, metamerism), although having characteristics more in common with advanced animals, we find only the nemerteans.

The protostomes including flatworms, nemerteans, annelids, mollusks, and arthropods get their name from the embryonic origin of the mouth from the blastopore.  The first (proto-) opening becomes the mouth (-stome), thus their name Protostomia.  In deuterostomes a second embryonic opening or region becomes the mouth.  The deuterostomes include hemichordates, chordates, and echinoderms.

Besides mouth origin, major contrasts between major phyla of the two groups (advanced protostomes and deuterostomes) include spiral versus radial cleavage, determinate versus indeterminate cleavage, presence or absence of chitin.  A minor phylum, the Pogonophora, blurs these and other distinctions and gives good reason to be the link between the two branches of higher animals.  To me, the evidence is so good any other proposals lack standing.  

An earlier post (SCIENCE SCREW-UP NO. 1) provides reasons the currently popular view of phylum relationships is incorrect.  Most of my immediately following posts will address various aspects of the origin of deuterostomes.  

Joseph G. Engemann, Emeritus Professor of Biology, WMU, Kalamazoo.  6/20/2013