From: salem@pangea.Stanford.EDU (Bruce Salem)
Subject: Re: Developing ideas on hearts.
Organization: DSG, Stanford University
Date: 16 Apr 92 04:08:32 GMT

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 first thing to note is the cardiovascular system is the first to 
>form.  Blood actually begins to circulate in the embryo by three 
>weeks.  Moore writes, "this precocious development is necessary 
>because the rapidly growing embryo needs an efficient method of 
>acquiring nutrients and disposing of waste products".  Now recall 
>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. [...]"

>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, 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.

>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.

>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.

>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.

>  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.

>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.

>  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.

>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?

>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.

	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.

>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. 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.

	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.

>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?

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

	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.


>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.

>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. 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.

	The idea of chance contingencies, like the formation of Pangea
that dried up all the amphibian habitats and made ones nice for reptile
to thrive, are not a direct part of the evolutionary process, but they
are criticel to history.

Bruce Salem