From: bk186@cleveland.Freenet.Edu (Julie Thomas)
Subject: 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 00:36:37 GMT


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.

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.  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.  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.  

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.


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 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.  The system develops so 
that with the growing interventricular septum, the right atria pours 
into the right ventricle and vica versa.  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.  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.

Does all this really help the notion that "hearts evolved"?  I really 
don't think so.  First, let me sound like a creationist.  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.  

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". 

Thirdly, the similarities need not imply relationship.  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.  

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.  

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.