From: colby@bu-bio.bu.edu (Chris Colby)
Newsgroups: talk.origins
Subject: anybody remember the heart discussion?
Date: 21 Aug 92 22:11:54 GMT
Organization: animal -- coelomate -- deuterostome

Awhile back there was a discussion of how the four-chambered heart
(4C --seen in mammals and birds) evolved from the three chambered heart
(3C). As it turned out, it didn't. The 4C heart was the ancestral
state (lungfish had it) and the 3C heart was derived independently
twice in amphibians and reptiles. The 4C heart of birds was apparently
a reversal (it does not develop like the mammal and lungfish heart.)
A question at the time was, why would 3C hearts evolve twice? I have
found a possible reason from reading "History of Life" by Richard
Cowen (a pretty cool book that covers the "basics" -- actually there
is a lot of stuff in it -- of how life developed on the planet.)

When lobe-finned fishes swim, they move their front/right fin
and back/left fin at the same time -- then their front/left
and back/right fin next. Amphibians (then reptiles) inherited 
this pattern and modified it for walking. Since amphibians and
reptiles (at least diapsids) have a sprawling gait, their
body undulates when they walk and they compress their ribcage.
Because of this, they cannot sustain prolonged motor activity because 
they have to stop to breath (next time you're at the zoo, check the 
Herpetology section for confirmation). In this case, a 3C heart is
an advantage over a 4C heart because any blood being pumped to the
lungs during locomotion is useless (perhaps dangerous). It is
advantageous to have all the blood going to the body.

Animals with erect stances (mammals and dinosaurs -- including
birds) don't have to worry about their locomotion shutting down
their breathing and therefore a 4C heart allows for prolonged
walking/running. (Erect stances also opens the door for homeo-
thermy to evolve in animals.)

This is all according to Cowen, but it sounds like a reasonable
(and more importantly, testable) hypothesis to me.

Below is a cladogram giving the "heartedness" of the "bony fish" 
(including lobe-finned, tetrapodal, amniotic, synapsid reptilian, 
placental mammalian fish such as ourselves 8-)

                   1C        4C          3C        3C      4C      4C
agnathans sharks ray-fins lobe-fins amphibians reptiles dinos   mammals
(lamprey  &rays (perch  (lungfish (salamanders (snakes (&birds) (us)
&hagfish)    1  & etc.) &coelocanths) &frogs)  &lizards)    1     1
    1        1      1         1           1        1      1     1
      1        1      1         1          1          1 1      1
        1        1      1         1         1          1     1
          1        1      1         1        1  (diapsid)1 1(synapsid)
            1        1       1        1       1           1
              1        1       1        1      1        1(amniotes)
                1        1       1        1     1    1
                  1         1       1        1    1(tetrapods)
                     1         1       1       1
                        1         1       1  1(lobe-finned fish)
                           1          1    1(osteichthyes-bony fish)
                              1          1 
                                 1    1(gnathostomes-jawed fish)
                                   1
                                   1(craniates)

Chris Colby 	---	 email: colby@bu-bio.bu.edu	---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
 	--Kurt Vonnegut from "Galapagos"
Newsgroups: talk.origins,soc.culture.african.american,alt.rap