From: bk186@cleveland.Freenet.Edu (Julie Thomas)
Subject: Re: Developing ideas on hearts.
Reply-To: bk186@cleveland.Freenet.Edu (Julie Thomas)
Organization: Case Western Reserve University, Cleveland, OH (USA)
Date: Thu, 16 Apr 92 23:44:10 GMT


In a previous article, salem@pangea.Stanford.EDU (Bruce Salem) says:

>bk186@cleveland.Freenet.Edu (Julie Thomas) writes:
>
>
>>Let's look at the development of the human heart in more detail to 
>>determine if it really helps the notion that "hearts evolved".  For my 
>>primary source, I'll use the text "The Developing Human" by Keith 
>>Moore.
>
>	Please give a full citation, publisher, date.

The Developing Human:Clinically Oriented Embryology. It's by Keith
Moore, Chairman of the Anatomy Dpt. at the University of Toronto.
Published by W.B. Saunders Co, 1977.


>>that the human heart begins functioning in a so-called primitive 
>>form.  This is usually seen as some kind of phylogenetic evidence.
>
>	Please define "primative".
>
>>I 
>>doubt this, as what it could also mean is that the embryo needs a 
>>pump as soon as possible, and it employs the "primitive" pump 
>>because it can't wait around for it to develop into the more complex 
>>pump.  Thus the "primitive" heart need not reflect phylogeny, it may 
>>very well reflect the developmental need for some kind of pump as 
>>soon as possible.  
>
>>Secondly, the heart forms by first forming a heart tube.  Ah, says the 
>>evolutionist.  This stage reflects that first "just so" story about a 
>>blood vessel becoming enlarged, incorporating valves, and becoming 
>>a primitive heart, that was brought up earlier.
>
>	From Arey, 1941 "Developmental Anatomy" pg. 316
>
>	"The heart is a blood vessel with a large lumen and
>especially thick muscular walls. In lower fishes and amphibians
>it forms in a simple direct manner. A tubular cavity appears
>within the ventral mesentary of the foregut [...] Nevertheless,
>the heart material is originally furnished by paired folds of the
>lateral mesoderm, each one of which can, on ocassion, differentiate
>into a separate heart.
>	"In boney fishes, reptiles, and birds the early stages of
>cardiac development are more complicated. This is the result of
>a flatened blastoderm, due to excessive yoke, and the consequent
>necessity for the heart to develop as as two lateral halves [...]
>[that] fuse in the midplane.
>	"The heart in mammals does not arrise by pure fusion. [but
>from] a cardiogenic plate. [...]"


The description of the fusing heart tubes in the human embyro is explained
and diagramed on pg 260.  But thanks for the xtra info.  It seems that
the heart can develop along two pathways and both can give rise to a 
two-chambered or three-chambered heart.  How did this change occur?
Why did it occur? I doubt that phylogenetic relationship would predict
this. 


>>Unfortunately for the 
>>tale, the details don't support the story line.  First it should be noted 
>>that the heart tube actually forms from the merging of TWO smaller 
>>heart tubes which are laterally opposed.  The two tubes fuse to 
>>actually create a tube within a tube.  Between the tubes is a 
>>gelatinous tissue called 'cardiac jelly'.  Now, given these details, the 
>>story about an evolving blood vessel becomes quite a long shot.  If 
>>the story was true, the bipartite structure is not explained.  What is 
>>even more significant is that in some fish and amphibians, the heart 
>>does start as a single tube.  But in all other vertebrates, the heart 
>>starts with the fusion of two regions.  Thus at the beginning, the 
>>hearts of fish/amphibia and other vertebrates differ radically in 
>>development.  How and why did this state of affairs evolve?  We are 
>>not talking about modifying later states of development, but we are 
>>talking about starting off from a different subroutine.  Finally, I 
>>can't resist going back to the evolving vessel story.  
>
>	I think that there is some confusion here. Arey shows (pg 314)
>pairing of vessels in the embryonic circulation, with what ammounts
>to fusion of the paired tubes,

Right, the heart starts off as TWO tubes which fuse.  


 but the distinctions between the deve-
>lopment within the vetebrate groups is clearely more complicated than
>your account. If changes do occur in otogeny within these major groups,
>we would be concerned that human cardiac ontogeny at least follow
>the mammalian plan, and it does.

Yes, but I find it most interesting that hearts can start off in
one of two different subroutines.  I see no selective reason for
changing subroutines when they both can give rise to two and three
chamber hearts.  



>>Measurements in the chick embryo show that while the heart tube 
>>can easily fill with blood while it is relaxed, it cannot contract 
>>sufficiently to reduce the lumen of the tube to provide for efficient 
>>propulsion.  This mechanical difficulty is resolved by the cardiac 
>>jelly.  It provides the medium by which a force of relatively modest 
>>contraction of the myocardium is transmitted radially toward the 
>>endocardium with resultant closure of the lumen.  Once the 
>>musculature of the ventricle builds up, this jelly is no longer needed.
>>Thus it seems that notions of contracting blood vessels are far too 
>>simplistic.  Even if they contracted, they might not have been able 
>>(as seen by this evidence) to reduce the lumen enough to give a 
>>substantial force of propulsion.
>
>	It is already a matter of record that the course of the
>earliest bird and mammal cardiac development is different.

This evidence was directed at the "vessel becoming a heart" story.




>>Now lets turn to the development of the chambers.  Moore 
>>emphasizes that these events occur concurrently.  After fusing the 
>>two tubes, the tube within a tube elongates and develops alternate 
>>dilations and constrictions resulting in the formation of the various 
>>chambers.  The atrial chamber grows a septum and the completion of 
>>this septum is actually quite involved and depending on the stage, 
>>various perforations are made and discarded.  In fact, because of 
>>fetal circulation, a perforation (the foramen ovale) is maintained 
>>throughout fetal life as the lungs are bypassed.  It isn't until birth 
>>that this hole closes and pulmonary circulation is established.  The 
>>right atrium forms by incorporating part of the sinus venosus.  The 
>>left atrium forms mostly by incorporating proximal parts of the 
>>pulmonary vein.
>
>	The bulbus and adjacent part of the ventricle fuse to become
>the left atrium, the atrium of the original three chambered heart
>becomes the right atrium. Thus the venous circularion enters the
>heart through the old atrium and leaves through the new atrium via
>the aorta and pulmonary vein. The septa appear later in the development
>and the phylogeny. The atrial circulation is established before the
>division of the chambers in effected. In amphibians and reptiles
>the septa are only partially developed, the pulmonary circulation
>is already there, as these animals have fully functional lungs, but
>it isn't until the birds and mammals that the four chanberes are
>completely divided. This is very much the course of the later
>development, the development of the septa and valves occurs after
>the pulmonary circulation is established with the foraman ovale
>retained as a "primative" trait in the fetus until the lungs are
>used.

You seem to think it is phylogenetically significant that the pulmonary
circuit evolves before the development of the septa and valves.  Imagine
what would happen in the heart completed forming before the pulmonary 
circuit developed.  The closed circulatory system would be broken!
The fetus would bleed to death.  As for the foraman ovale being a
"primitive" trait, are you saying reptiles or amphibians have this
perforation?  



>>The interventricular septum concurrently grows to 
>>divide the ventricles.  Now at no time does the septum grow so that 
>>the two atria pour into the left ventricle.
>
>	Wait a minute, the right atrium develops out of the bulbus
>and part of the single ventricle, before a ventricular septum has
>developed far along. So when the atrial circulation is established,
>with sinus venosis being incorporated into the right atrium, with
>aorta and pulmonery VEIN comming out of the VENTRICLE and BULBUS
>to become the left atrium, the ventricle is still not fully divided.

I know.  But in reptiles, the two atria empty into the left side.
See my previous post.  



>>  The system develops so 
>>that with the growing interventricular septum, the right atria pours 
>>into the right ventricle and vica versa. 
>
>	But this is only the case long after the atrial scheme is
>established with an incompletely divided ventricle.


The atria develop so that they are laterally opposed.  If development
reflected phylogeny, they should be close together on the left side
and the right one should shift position concurrent with completion of
the interventricular septum.


>>I find this significant since 
>>this does not reflect the supposed reptile to mammal change (as 
>>outlined in my previous posts).  Furthermore, the manner in which 
>>the left atria forms is most instructive.  It's formation is tied to 
>>the prior existence of the pulmonary vein.  Yet a pulmonary vein 
>>clearly implies an oxygenating organ, and pulmonary artery.  Now a 
>>pulmonary artery obviously implies that the heart must be sending 
>>blood in two directions: to the body AND the oxygenation organ.  
>>Thus, the development cannot be reduced to pieces and parts.  The 
>>whole thing has to be there.
>
>	Wrong! This is not how I understand it at all. The pulmonary
>circulation exists before the completion of the ventricular septum.
>See Arey, 1942 pg 327.


My point was directed at the notion that the in series heart evolved
into a parallel series heart.  I'm not wrong., as this point had nothing to
do with the competion of the septum.


>>  If this evolved, surely some sort of 
>>saltation is called for as gradualistic microevolutionary changes 
>>must travel through many levels which are not only lacking 
>>advantages, but would actually be disadvantageous.
>
>	Wromg, again. Both the development and the hearts of reptile
>and amphibian show that a three chambered intermediary could exist,
>with two atria, pulmonary circulation, and a partially divided
>ventricle.

No, I'm talking about the evolution of the pulmonary circuit having
to be concurrent with the division of the heart.  Surely, between these
states, I see problems.  As one might say, what good is a 1/2 pulmonary
artery?



>>Does all this really help the notion that "hearts evolved"?  I really 
>>don't think so.  First, let me sound like a creationist.
>
>	Do you believe what you are sounding like?


Am I a creationist?  No.  Do I believe what i wrote below?  Yes.
If you think my belief is wrong, let's hear the evidence.


>>In this case, 
>>ALL mutations which affect the heart in an observable manner are 
>>deleterious (although I'm quite sure there are many neutral ones 
>>which are not picked up due to a lack of mutant phenotype).  When 
>>development is changed, we get a heart DEFECT.  Thus, the 
>>development of the heart must be fine-tuned so that significant 
>>changes result in deleterious effects.  This is certainly not a 
>>friendly environment for notions of reshaping the system due to 
>>mutations.  
>
>	This is just the argument for a designer. Nature can't
>do it by herself, so she needs help from the big strong He-man
>creator.

No, it's an observation of fact.  I'm not making an argument.  I'm
questioning your story.  



>	Whether it is fair to cast you AS a creationist, the
>argument you give falls apart if the changes don't occur all
>at once and the advantages they have aren't compounded all
>at once. Suppose that having mixing of venous and arterial
>circulation in a ventricle that is not fully divided is not
>a fatal, or even significant problem for the amphibians and
>reptiles we know, but with the same pulmonery circulation,
>some ancestral mammal like reptiles had some individuals who
>developed variation in the degree that the ventricular septum
>closed the ventricles, then grsadualism and seclection could
>indeed operate if those hearts that closed were marginally
>more efficient than those that did not. A variation that involves
>a simple amplification of an existing structure would be
>strongly selected for in animals that must maintain high
>energy output, as preditory mammal-like reptiles.

You forgot to introduce the story with a "once upon a time" and end
it with a "they lived selectively happy ever after".  I can SUPPOSE
many things, but is the supposition true?  How about another story?
How about one where the pulmonary circuit itself evolves?  How about
one where the atrial septum grows?  As for your interventricual septum,
you forgot to mention that this story would cut of the continuity
between the right atrium and right ventricle in the reptile.



>>Secondly, the development of the heart underscores the final forms 
>>of different hearts, namely, there are significant differences.  If 
>>similarity counts as evidence for evolution, differences ought to 
>>count against.  None of this is certain, of course, however it ought to 
>>raise some red flags when we hear stories about hearts "a-changin". 
>
>	I am not so struck with the differences, especially as they
>are at the level of the class taxon, and these classes had different
>modes of life.

If I were to probe this, such clean cut distinctions would quickly
blur away.


 What is interesting is that we are not seeing the
>main line ancestors, so we see the ends of a tree based on a single
>step in alternative development. Further, there are many common
>steps, even though you emphesize the differences, There are the
>paired circulation, that fuse, then the thickening and crimping
>of the thickened part into a three chambered heart.

Sure there are similarities.  So what?  Evolution is about more than
similarities, it's about transformations.  Here is where differences
are often ignored, although they shouldn't be.



>	Please remember that the trick question is to be suckered
>into saying that the development follows closely the phylogeny, it
>probably reflects it only in general terms.

No, the "trick" is to stack the cards so that only evidence for
evolution can be gleaned.  But that's an aspect of a paradigm.
We just need to be aware of it and not allow our critical minds
to be mesmerized by "half-stories".



>>Thirdly, the similarities need not imply relationship.
>
>	Meaning that if you group lower taxa into higher ones, the
>grouping has no genetic significance at all? You must be saying
>that it may not imply common descent. What is your alternative
>view? And what does similarity mean? If it is God's plan, can
>we question what it means?

I have no alternative.  I'm simply doubting your story.   


>>  For example, 
>>the heart tube might not reflect some ancient form.  In fact, it 
>>probably exists only because the embryo needs to recruit a pump as 
>>soon as possible to continue development.  And a tube can form more 
>>quickly than a complicated four chambered heart.  
>
>	So then, it is easy for you to imagine some developmental
>process that begins as a tube, thickens, develops muscle that
>can beat on its own, and becomes a pump?


Yes, I could IMAGINE this.  I could also imagine that aliens landed in
the past and taught men to build pyramids (or whatever).  I just don't
see how one is wrong for not considering such imaginations as truth.
BTW, you don't describe what does occur in mammal heart development.
(Recall the two heart tubes a fusin').

 Dare I say that such
>progressive complexity of structure is also possible under an
>evolutionary process?

Development is under tight control.  It is programmed.  It is
teleological as it moves toward a set state.  Are you suggesting
evolution is also like this?  Are you implying evolution is
"programmed"?




>	Now, why stop there? Why not imagine the rest of the
>steps: Now, crimp the tube into an S shape, incorporate a
>piece of germ layer that is becomming lung tissue, the sinus
>venosus, absorbe the pulmonary vien into the bulbus and incorporate
>both and the pre-existing aorta into a new chamber. Begin the
>growth of septa and valves for the atria, and divide the ventricle.


Why stop there?  Perhaps little intelligent microrganisms that went
extinct invaded the system and changed things.  There's no end to the
possible stories that we can tell.



>
>>In the end, since the evidence is only circumstantial, it's basically a 
>>judgment call.  For my part, speaking from my heart, in an attempt 
>>to circulate some ideas, I am not convinced by the evolutionary story 
>>even when it is pumped up with similarities.  :)   I am unable to 
>>simply hand-wave away the differences, the similarities are not 
>>compelling, and the empirical data gives us reason to suspect that 
>>developing hearts don't like to change patterns, and no reason to 
>>suspect they do or ever have.  
>
>	I think the problem you are having is like the problem of
>the people who challenge us to build all of biochemistry in one
>step from the constituant atoms. You have convinced yourself that
>all the complexity of the four chanbered heart has to be developed
>at once, without a clue to its phylogeny. I think you have made
>a mistake.

No, I have not convinced myself of any of this.  I don't know.  But
you seem to have convinced yourself that "hearts evolved", and I
don't see any persuasive reason for accepting the belief that convinces
you.  I simply see empirical evidence that doesn't fit into your
story.



>>Add to this all the problems with the "just so" explanations.  Would 
>>the three to four heart chamber transition really be such an 
>>advantage?  I already brought up runnin lizards and lazy crocs which 
>>seem to throw a monkey wrench in this little story.  But there is 
>>another aspect.  Mixing blood simply means the concentration of 
>>dissolved oxygen in the blood is going to be less.  Now increasing the 
>>efficiency of the heart usually means getting more oxygenated blood 
>>to the body.  But instead of going through all this organ 
>>restructuring, an organism could enjoy the same benefit with a few 
>>simple point mutations in the hemoglobin so that it would be 
>>saturated at lower concentrations of dissolved oxygen.  In other 
>>words, the oxygen dissociation curve could be shifted to the left by 
>>such a mutation making it possible for the hemoglobin to load and 
>>unload oxygen at lower concentrations.  Thus, if efficiency 
>>was the selective pressure, one would expect this route to have been 
>>monopolized instead of organ transformation since it is more likely 
>>~to arise because it involves simpler changes giving the same benefit
>>s.  These considerations cause me to suspect that evolutionary 
>>reductionism is missing the mark.  Notions of reptiles evolving 
>>mammalian hearts treats organisms as pieces and parts, while if 
>>anything is to be learned from development, it is that organisms 
>>develop and exist as whole beings.  Perhaps the mammalian heart 
>>exists because it exists in a mammal.  
>
>	I think that this "just so" story is wide of the mark because
>what we have is the result of some history of contingencies, leaving
>lots of others not taken. The fact is that a four chanbered heart
>developed from a three chanbered heart via completion of a ventricular
>septum, and not by a biochemical change in the blood.

No, that's not a fact.  It's an assumption/interpretation you make under
the paradigm you hold to.  Since I don't share the paradigm, I find this
consideration illuminating.




 I don't know
>enough about the physiology of modern reptiles to tell you why, and
>indeed making a guess is just as inferential as guessing about the
>phylogeny of a four chanbered heart. Now, maybe it is peak efficieny,
>cardiac output, and not contamination by veneous blood that was the
>selecting factor.

That's the great thing about just so stories.  Since they exist in
the imagination, they can be amended with the imagination in any
way conceivable to get around difficulties.  The rate limiting step
is the richness of one's imagination. And if there are enough
imaginations out there musing, these stories can be reshaped into all
sorts of configurations.  I have never questioned evolution's ability
to catalyze the imagination.  I just question it's validity.


Well, this story is getting old and I'm starting to get bored.  I
think I'll let you have the last word so I can move onto another topic
which has caught my eye to see what stories surface there.  It's been
fun!