On Fri, 2 May 2003 15:29:43 +0000 (UTC), ragland37{at}webtv.net (Michael
Ragland) wrote:

>
>On Thu, 1 May 2003 15:04:48 +0000 (UTC), ragland37{at}webtv.net (Michael
>Ragland) wrote: 
>http://www.emc.maricopa.edu/faculty/farabee/ BIOBK/BioBookENDOCR.html 
>
>I was looking for information on the human endocrine system and I came
>across the above website. What I find interesting about the ordering of
>the cells which make up human embryonic development is the seemingly
>evolutionary sequence. For example, the endoderm is the inner layer of
>cells in embryonic development that gives rise to organs and tissues
>associated with digestion and respiration. These would be the most
>primitive cells. The mesoderm is the middle layer of cells in embryonic
>development that gives rise to muscles, bones, and structures associated
>with reproduction. Finally, there is the ectoderm which is that outer
>layer of cells in embryonic development that gives rise to the skin,
>brain, and nervous system. So if you think of a layered cake the
>ectoderm is the most recent evolutionary layer of embryonic cells. 
>
>Perhaps biologists are aware of this but I was kind of excited when I
>learned about it. Is there an evolutionary ladder relationship between
>endoderm=mesoderm=ectoderm? There certainly seems to be. Since Darwinian
>evolution is so slow how do we know when endoderm cells evolved into
>mesodermic cells and when mesodermic cells evolved into ectodermic
>cells? Through fossils and comparative embryology? 
>
>Obviously, these three embryonic cell layers work in conjuction with
>each other but it is clear we don't understand fully how they relate to
>each other. I'm particularly interested in the relationship between the
>mesoderm and the ectoderm. 
>
>I'm also puzzled by hormones. The hypothalamus, pineal gland, adrenal
>glands, gonads, etc. produce hormones. I've never seen hormones refered
>to as genes. Hormones actually influence genes. Still, I would think
>there would be genes which are responsible for these hormones which are
>produced by the aforementioned. Is this too simplistic or is it that we
>just haven't discovered the genes for these hormones yet? 
>
>Michael Ragland 
>
>
>
>You have an interesting idea, but unfortunately it is quite incorrect.
>In fact, it is the ectoderm and endoderm which are evolutionarily the
>oldest and the mesoderm which is more recent. That is, all members of
>the animal kingdom above the level of sponges have a particular form of
>embryonic development which involves the production of ectoderm and
>endoderm. The Cnidaria (jellyfish, anemones, and Hydra) retain this
>level of development. Starting with the flatworms, all the remaining
>animals also develop a mesoderm. Check with any intro biology textbook. 
>
>Response:
>You state, "All members of the animal kingdom above the level of sponges
>have a particular form of embryonic development which involves the
>production of ectoderm and endoderm. The Cnidaria (Jellyfish, anemones,
>and Hydra) retain this level of development." Is it determined through
>fossil evidence all members of the animal kingdom above the level of
>sponges have a particular form of embryonic development which involves
>the production of ectoderm and endoterm? I ask this because you state
>the Cnidaria retain this level of development. You write, "Starting with
>the flatworms, all the remaining animals also develop a mesoderm.
>Obviously, the dichotimization of the endoderm, mesoderm, and ectoderm
>is over simplified. If the mesoderm is the most evolutionary recent and
>is "associated" with structures of reproduction then the preceding
>ectoderm and endoderm must, in some ways, be also associated with
>structures of reproduction. Is there evidence of this being the case? 
>
>
>Richard Norman:
>The endoderm is only associated with respiration in some animals. Our
>lungs are develop from swim bladders of fish, sacs that branch off the
>gut. Fish gills and the respiratory systems of other animals are not
>endodermal in origin. 
>
>Response:
>Do we really know what the oldest embryonic form is? I'm receiving the
>distinct impression the evolution of embryology is so complicated the
>terms endoderm, mesoderm, and ectoderm are largely useless and need to
>be updated with additional knowledge. You state, "Fish gills and the
>respiratory systems of other animals are not endodermal in origin. Is
>there evidence our respiratory system in mesodermal or ectodermal in
>origin? How useful is it to study embryonic evolution using the
>endoderm, mesoderm, and ectoderm as guidelines. My impression is that it
>is relatively useless unless you are a biologist who has indepth
>knowledge of the subject matter.  
>
>Richard Norman:
>Your question about hormones is sort of strange. We certainly do know
>all the genes involved in producing the protein hormones and that are
>involved in producing the enzymes that make the non-protein hormones.
>Hormones are simply chemicals secreted by some cells that act as
>signaling agents carried in the blood. They are closely related to other
>signaling chemicals -- neurotransmitters and local hormones which also
>carry signals from one cell to another but are not transported in the
>blood. Signaling systems can, indeed, influence gene transcription and
>signaling systems can also influence hormone release. Hormones are not
>referred to as "genes" because the two concepts are entirely separate.
>
>Response:
>We haven't discovered all the hormones have we?
>
>Stephen Hawking quotes from Larry King LIve Weekend December 25, 1999,
>9:00 ET
>
>"I think the biggest challenge we face is from our aggressive instincts.
>In caveman -- or caveperson days, these gave definite survival
>advantages and were imprinted in our genetic code by Darwinian natural
>selection. But with nuclear weapons, they threaten our destruction. We
>don't have time for Darwinian evolution to remove our aggression. We
>will have to use genetic engineering."
>
>"I think genetic engineering with humans is going to occur whether we
>like it or not. It will change our standard of what is human but it will
>be a gradual change because there's so much we don't know and because
>humans take time to grow up. We won't change much in the next 100 years
>but we might after that."
>
I am not sure just what you are really asking.  The notion of
ectoderm, endoderm, and mesoderm is really quite well developed (and
it is certainly not a "dichotimization" -- a dichotomy is a division
into two parts).  Early embryos (at those of triploblastic animals)
first produce the three germ layers: endoderm, ectoderm, and mesoderm.
Then, from these develop the tissues and organs.  The notion of these
cell types is not useless nor does it need to be updated with more
modern information. 

The information I described about different types of animals is not
derived from studies of fossils, but from studies of modern, living
animals; their structure and their embryonic development.  However,
these embryonic characters are considered so fundamental that they are
used as an important foundation for evolutionary thinking.  That is,
the distinct separation, first, of sponges (and some other phyla) who
do not have this type of development at all, then the diploblastic
(ectoderm and endoderm only)  animals with radial symmetry (like
Cnidaria) and then the triploblastic with mesoderm is generally
assumed to be the evolutionary sequence.  In any event, just because
triploblastic animals form reproductive structures and muscle from
mesoderm doesn't mean that jellyfish can't reproduce and can't move.
Just that their reproductive and motile cells form in a different way.

No, we certainly have not discovered all the hormones.  Still, I
wouldn't cite Steven Hawkins as an authority on evolution or biology.
And, in any event, his comments on genetic engineering have nothing at
all to do with whether we have discovered all the hormones, nor does
our knowledge of all the hormones have anything to do with your
original quesitons: "I've never seen hormones refered
to as genes....I would think there would be genes which are
responsible for these hormones".  We certainly know an awful lot about
many of the genes that are responsible for the known hormones.
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