On Sat, 3 May 2003 23:59:43 +0000 (UTC), Count_Blastula{at}webtv.net
(Michael Ragland) wrote:
>
>I found the below information helpful. It states,


Actually, if you want to learn about biology with questions about the
role of ectoderm, endoderm, and mesoderm in organ formation and then
want to know about the evolutionary origin of these germ layers and
the relationships between animals without all three layers
(diploblasts) and those with all three (triploblasts), you really do
have to work through a basic intro biology course and learn all the
stuff you cite.  Professional biologists (I teach Intro Biology,
Animal Physiology, and Neurobiology) are happy to answer questions
like you ones you raise up to a certain point.  The time does arise,
though, when we tend to respond testily "read the damn book!".
Happily, after a few back-and-forths, you took the initiative to look
up all the relevant information which you found very enlightening.  A
most happy outcome for teacher and student alike!

There are several points to make.

The early evolution (preCambrian) of animals is indeed shrouded in
mystery for the reasons you cite -- the lack of fossil evidence in
very small soft-bodied animals.  Even with some fossil evidence, what
you can see is gross morphology (or, more usual, just hints of gross
morphology).  What you cannot see even with good fossils is the
important details of developmental patterns.  We do try to reconstruct
possible evolutionary sequences from an examination of modern, extant,
animals.  We have many tools: developmental patterns supplemented by
an increasing understanding of how developmental genetics works plus
an increasing understanding of genetic sequence homologies.  All this
is the newly developing field of "evo-devo", evolutionary
developmental biology.

In another posting, you lament my dogmatism in declaring that we
already know all we need about things like ecto/endo/mesoderm and
don't need to re-evaluate our thinking with updated information.
There are different levels of understanding.  Your questions seemed to
be at the introductory biology level.  At that level, the specifics
you seemed to be interested in (which tissues and organ systems
develop from which germ layer) are indeed "well known".  All of the
information (the 500+ lines) you cite is "well known".  Of course new
developments are going to change our thinking.  Still, pretty much all
of the "basic biology" about development and the relationships between
animal phyla has remained unchanged for fifty or even a hundred or
more years.  Yes, we have reorganized the world of biology, moving
from two kingdoms (plants vs animals) through five kingdoms, into
three domains and six kingdoms. We have sequenced genomes and
discovered homeobox genes and unearthed all sorts of preCambrian
creatures and revised our thinking about many aspects of biology.
Still, the old notions of ecto/endo/mesoderm and of
blastulation/gastrulation and of diploblastic/triploblastic have held
fast.  

You raise questions (in another post -- I am combining my responses to
save newsgroup bandwidth) such as:  "how the endoderm and ectoderm are
involved in Cnidaria reproduction ", "how the endoderm and ectoderm
contributed to the mesoderm...specifically in the area of
reproduction", "of whether endoderm preceded ectoderm or ectoderm
preceded endoderm or something else" and "the question about our
respiratory system and that of other animals. What cell type is
responsible for them."  These indicate you are trying to put all the
ideas you are learning about into some context, but you are probably
looking in the wrong direction.  It is not really the case that, say,
first endoderm evolved and then ectoderm, and finally endoderm.  What
seems to have happened is that, in those early days (eons, actually)
of early eukaryote evolution, is that all sorts of different ways of
organizing a genome and controlling cell differentiation were
explored.  Finally a scheme of control genes evolved allowing complex
multicellular organisms with differentiated cell types.  This led to
the relatively rapid proliferation of the three major multicellular
lines -- plants, animals, and fungi.  Within the animal line, another
exploration of control genes led to the preCambrian radiation of basic
animal types.  However, even this proliferation of types was
constrained by the enormously difficult problem that, while a larval
animal is still growing and developing, it still has to solve all of
the problems of living.  Once you find a way of doing that, you tend
to stick with it -- any deviation and the embryo/larva either fails to
develop properly or can't make a living and dies.  No doubt almost all
of the attempts to explore different ways of producing an animal body
failed and became extinct.  What survived was basically the pattern of
blastula/gastrula with three germ layers that produces virtually all
of the animal phyla we see today.  There are a few survivors of
alternate systems and these are found in the sponges and the Cnidaria.

The sponges found a way of producing several differentiated cell types
without going through the three germ layer method.  However, these
cells are only loosely integrated so don't really count as "true"
muticellular animals (eumetazoa).  The Cnidaria found a way of
producing an embryo with a blastula which results in two cell lineages
and yet results in cells that can perform all the basic life functions
-- neuron-like and muscle-like etc.  However, these do not produce the
really well-differentiated tissues of more complex animals and the
differrentiated cells do not form into complex organs, each composed
of different tissues.  That is, the Cnidaria (and relatives) are
"tissue" grade but not "organ" grade. All the other
animals develop
from a common embryonic theme resulting in the three basic cell types
you originally asked about -- ecto/meso/endoderm and produce true
organs.  Most organs are combinations of tissues from different cell
lines.  The nervous system contains not just neurons (from ectoderm)
but also connective tissue and blood vessels (from mesoderm).  Skin is
not just ectodermal, but contains epithelium and muscle and fat and
blood and  nerve and a wide range of other connective tissues -- it is
a complex organ made of many cell types.  Similarly gut and lung are
complex organs with some tissues of endodermal origin, some of
mesodermal, and some of ectodermal. The tissue layer of origin of any
particular cell is of medical and developmental and evolutionary
interest, but all the cells interact to produce complex organs.

Each animal and each organ has its own particular form of development
and its own evolutionary history.  So you really can't look for
general, overarching theories -- as far as we know, much of this boils
down to particular details in each case which, once fixed into the
animal's devlopment, tend to get fixed into our evolutionary heritage.
So it is not really useful to enquire "why" there is ectoderm and
mesoderm and which came first and how.  It is useful to understand
that someone way back when worked out a scheme to build a complex
animal body this way that was so successful that virtually all animals
today use that system.
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