From: bk186@cleveland.Freenet.Edu (Julie Thomas)
Well, I was about to leave this topic, but it seems another actor has
walked onto the stage, Mr. Lungfish. Let's set the context for his
part in the story. (Using Torey's and Feduccia's "Morphogenesis of
Vertebrates).
The structure of the heart in the Chondrichthyes and actinopteryian
Osteichthyes conforms to the general pattern of four-chambers-in
series. Blood from the body enters the sinus venosus, then the
single atria, then the single ventricle, and leaves via the truncus to
travel to the gills, where it is oxygenated, and then goes to the body
WITHOUT returning to the heart.
As we know, terrestrial vertebrates have pulmonary and systemic
circuits operating in parallel along with divided hearts. How did
this rather major change come about? To put it simply, what good is
1/2 a pulmonary artery, 3/2 of atria, or a 'lung' that is not connected
to the circulatory system?
Well, we don't have any living rhipidistian crossopterygians which
are said to have been the ancestors of the first amphibians. But
wait, we do have that living fossil, that member of the other group
of Crossopterygii, the coelacanth. Since this critter hasn't changed
over the hundreds of millions of years, and it is another
crossoptterygian, you would sort of expect some crucial insight
from this critter's heart. According to T&F (pg 422) their:
"internal anatomy is.....so specialized as to be little indicative of the
basic conditions we should like to know".
Now I don't know about anyone else, but this looks like hand-waving
in response to information that is not supportive or even
contradictory of our story, "The Heart That Evolved". It probably has
only a fish heart! Oh well.
Anyway, have no fear, as we do have the living relatives (so it is
said) of the crossopterygii, the Dipnoi, or lungfishes. Yes, indeed,
the lungfish have a four-chambered heart which operates with a
parallel circulatory system. According to this twist in the plot, the
lungfish reflect the state of the original amphibian ancestor, and the
reptilian ancestor, along with the bird and mammal ancestor. Thus,
the origin of the four-chambered heart has been solved, and they all
lived happily ever after.
But hold on a minute. If you ask me, Mr. Lungfish makes the heart
story even more implausible. Why? Because we are STILL left with
a lungfish which evolves a 2 atria/2 ventricle heart with pulmonary
circuit system from fish with a 1 atrium/1 ventricle heart and NO
pulmonary circuit. The origin of this four chambered heart is still
not resolved, and in fact, the situation is worse. For now it is no
longer possible to appeal to the three-chambered hearts of
amphibians and reptiles to serve as intermediate structures! We are
left with the same problem, but now we're empty handed.
The amphibian/reptilian heart evolved AFTER the the lungfish heart,
so certainly any information from these systems is not applicable.
In short, not only do we have no fossil evidence, but we have now
lost our developmental evidence. Thus, we have NO evidence, but we
have a belief that is supposed to be scientific.
Now, I suppose one could appeal to the amphibian heart/reptile heart
as "possible" states which "might have occurred". But I wouldn't
even accept these speculations. Why? For one thing, it's easier to
lose a structure than gain one. That is, the amphibian/reptilian
heart would be "intermediate" examples where the four chamber
structure was LOST, not acquired. A situation where a heart is
changed so that the septum no longer completely forms is NOT
evidence for the ability to acquire new chambers and new
circulatory systems. It is no longer possible to use the
amphibian/reptilian systems as intermediates to explain how some
fish evolved a four-chamber heart in parallel. Mr. Lungfish, while
thinking himself the hero of the story, really turns out to be a
villain.
But what of our reptile to mammal transition? Let's not forget to
let Mrs. Crocodile play her part. She too has a four chamber heart,
much like that of mammals and birds, making her heart more similar
to a humans than to a lizard. In fact, her interventricular septum is
structured differently than other reptiles, causing T&F to assert:
"this septum is a vertical septum rather than a horizontal one , and appears to be a new evolutionary
development, which is not to be homologized with the
interventricular septa of lizards, snakes, and turtles". (pg 425).
Now I'm confused. How is this septa a new evolutionary development
if it, like that of the bird and mammal, supposedly reflects that
lungfish to mammal/bird line? Anyway, the significant point is that
the septa are not homologous. Now let's go back to that old reptile
to mammal transition problem. If you'll recall, the problem I alluded
to was that in reptiles (but not crocs) BOTH the right and left atria
empty into the left ventricle. If Mr. Lungfish's contribution to
the plot is considered, the problem is not how to separate the atria,
but now it's how to sequester them. Either one seems like trouble to
me. What selective advantage was there in taking a heart where the
the right atrium enters the right ventricle and shifting it so that it
enters the left ventricle where the blood can now mix?
This problem is further emphasized in the amphibians. According to
T&F, (pg 423):
"modern Amphibia, with their incompletely divided hearts, may
illustrate degeneracy rather than primitive intermediacy between
fishes and amniotes".
Well, there goes the infamous story line! No more can we use
"efficiency" as our selective guide. For hearts in amphibians and
reptiles evolved to a state of inferior efficiency. But what about all
that talk about efficiency? Didn't people REALLY believe it? Or was
it a smokescreen? Wouldn't some amphibian which developed a less
efficient heart be at a disadvantage than one that did not? Afterall,
we heard time and again about how a more efficient heart would be a
selective advantage. If a completely separated heart was an
advantage, then an incompletely separated heart would be a
disadvantage. Thus, if I was supposed to seriously entertain that
just so story, it seems only fair to expect those who believed it to
also take it seriously and explain how evolution worked to select for
an inefficient system among those with efficient systems.
As it turns out, the heart story has taken a turn over some figurative
aortic arch for the worse.
But there is one more aspect of Mr. Lungfish that I want to look at.
Mr. Lungfish has lungs and an efficient circulatory system. These
come in handy when droughts occur. Wait a minute! Isn't this the
scenario for the fish to amphibian transformation? It sure is, yet
Mr. Lungfish hasn't evolved one bit towards wandering onto the land
in over 300 million years of evolution! In fact, Mr. Lungfish has
hardly changed although he has sat poised for 300 million years at
the state and environment which supposedly gave rise to amphibians.
These facts make you wonder about that other story about fish and
amphibians.
Now in response to some of the complaints I've been getting, let me
outline some of the points which I have gotten out of this inquiry.
1. Many believe "hearts evolved". They consider this a scientific
belief. Yet they admit that our evidence for this belief is meager.
So why are people getting upset about me not accepting a belief
which is supposedly scientific yet which lacks good evidence?
2. I have learned again that similarity need not reflect some
phylogenetic relationship. We saw this with sharks and mammals
when it came to the circulatory pattern associated with the yolk
sac. In my mind, this weakens the appeal to similarities to support
evolution. If the vessels of the reptile were not descended from the
shark even though they are identical, then it becomes entirely
plausible that the heart of the mammal was not descended from the
lungfish even thought they are quite similar.
3. The hearts of amphibians and reptiles start out from different
initial states. Thus, this is not an example of "tweaking at the
peripherals". This weighs heavily in my mind against some
evolutionary transformation. On this point, I have proceeded from
agnosticism to skepticism.
4. We are still left with no account of how the four-chambered
heart arrived on the scene. Mr. Lungfish has actually worked to
remove all the developmental circumstantial evidence by placing
this transformation in the fish. If you don't have a clue as to how it
happened, I see no good reason to think it could have happened.
5. The ad hoc nature of "just so" stories have been exposed. Their
only limit is one's imagination. Here is a case where the definitive
word is supposed to rest on someone's musings. Furthermore, the
just so story about efficiency actually turns in on itself when we
realize that hearts evolved to less efficient states. The take home
message is that just so stories are quite like the creationsist's "God
did it" story. In both cases, the "explanation" seems to be able to
explain anything. The only thing just so stories have on the
creationist's stories is that the former are more imaginative. In all
honesty, they usually read like modern myths (to me), an analogue to
the camp fire stories about the distant past which tell us why
things are like the way they are today.
6. I've also learned that the fallacy of card stacking is common in
evolutionary thought. If differences are found which do not support
the story, or even are contrary to the story, we either complain
about them being found in cousins (although this doesn't stop us from
using cousins to support ancestry), or we give them a label and
assign them to a "special" category (an effective way of dismissing
contrary evidence) like "regressive evolution" or "highly
specialized".
7. I've learned that a creature can be poised to transform,
yet can refuse to transform for hundreds of millions of years.
8. Finally, while I fully concur that the creationists are just plain
wrong when they say there is no such thing as an advantageous
mutation, here is a system where all phenotypically observable
mutations are deleterious. To date, the expirical evidence certainly
supports the following hypothesis: -->Concerning the development of
the circulatory system, there is no such thing as an advantageous
mutation<--. I wonder how many other systems this hypothesis
would apply to (BTW, it's both predictive and falsifiable for those
who worry about such things)?
Now if you take this hypothesis, with it's supporting set of data and
add it to the considerations concerning "teaking" in #3, and consider
the lack of evidence and an explanation for the belief that hearts
did evolve, we seem to be falling into a position where it looks like
hearts did not evolve, at least in a neo-Darwinian sense.
All in all, this has been an experience which has shown me more than
the specifics associated with hearts. I've gained insight into the
aspects of the biological world as a whole and have also seen how
the paradigm of evolution functions as a guide. I have enjoyed this.
Now, I'll try to get back to that other topic and post on it later.
Thanks to all those who have interacted!
