Well, I was about to leave this topic, but it seems another actor has walked onto the stag

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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!

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