From: bk186@cleveland.Freenet.Edu (Julie Thomas)
Subject: More on three and four chamber hearts
Reply-To: bk186@cleveland.Freenet.Edu (Julie Thomas)
Organization: Case Western Reserve University, Cleveland, OH (USA)
Date: Thu, 9 Apr 92 05:04:22 GMT


I have often found that the more I learn about biology, the more 
easier it becomes to become skeptical of evolution.  Let's consider 
our three  to four chamber heart transition in more detail.  At first, 
I must confess that I thought this was really not a big deal.  I could 
envision some regulatory mutation working to to simply extend the 
interventricular septum so it completely seperated the right and 
left ventricles (although typical neo-Darwinian gradulism seemed 
out of the question).  My problem was that I could see no selctive 
advantage in this.  The standard line about increased efficiency 
(meaning evolution works to increase efficiency?) didn't hold up to 
scrutiny (as far as yours truly was concerned).  But now I must 
change my tune and become even more skeptical as evolving a four 
chamber heart was MUCH more than just completely the 
interventricular septum.  

See my cartoons below for a schematic of the two types of heart.




Reptilian heart

                   1     
                    !      
                    !        2 3
                    !***! !*/ /*
                  *             *
                 *      !        *
                 *   R  !  L     *
                  *     !       *
                    *   !     *
                      ******

Mammalian heart

                    1
                     !   2     3
                     !  /     /
                     !*/**** /*
                  *       !     *
                 *        !      *
                 *   R    !  L   *
                  *       !     *
                    *     !   *
                      ******

1 = Pulmonary trunk (blood leaves for the lungs)
2 = Right atrium (blood enters from the lungs)
3 = Left atrium (blood enters from the body)
R = Right ventricle
L = Left ventricle
! = Interventricular septum


If you look at these figures, it becomes quite obvious that the ol' 
"complete the septum" story just won't work.  For if the reptilian 
septum was completed, this would kill the reptile since the blood 
from BOTH the right and left atrium could not enter the right 
ventricle, thus it could not be oxygenated by the lungs.  

In other words, blood from the right atrium must first travel 
through a portion of the left ventricle.  In mammals, not only is the 
interventricular septum complete, but the right atrium opens into 
the right ventricle.  Thus, the change that would have to occur is far 
more than a growing septum.  The plumbing of the heart would also 
have to change simultaneously.  Any tales about how a shifting 
atrium occurred?  


But let's also look at interesting developmental consideration.  In 
humans, when the interventricular septum is completed in about 3 
months gestational age, the atrial septum still possesses a 
discontinuity where there is cross-talk between the two atria. 
 Thus, development does not show a "2 complete atrial chambers /1 
ventricular chamber --> 2 complete atrial chambers/2 compete 
ventricular chambers" transition which is what was supposed to 
have happened in the reptile heart to mammal heart transformation. 


~It always seems that if we find a similar pattern in embryological 
development, we are supposed to have evidence for evolution.  But if the
pattern differs, well........     Evolution can be such a paradigm!  :)