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Author
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Topic: Common descent
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New York Wiseguy
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posted 07. April 2002 23:18
The concept as you summarize it from Gould is not exactly new. I quote from some recent writing by Hubert P. Yockey: "Bohr (1933) argued that life is consistent with but undecidable by human reasoning from physics and chemistry. In other words, it may be that we cannot now -- or perhaps ever -- devise the methodological or technical innovations that will allow us to retrace the pathways from nonliving to living systems. Accordingly, on this view, the existence of life must be considered as an elementary fact (or axiom) that cannot be explained, but must be taken as a starting point for biology." Yockey goes on in the same article to refer to horizontal gene transfer as having effectively erased the record, so that "the earliest branches of the tree are probably not knowable." Doesn't that parallel Syvanen's concept?
So it doesn't seem "strange" to me at all.
But now you have me curious enough that I might as well order a copy of Gould. I'll probably want to read it sooner or later anyway. A review in the New York Times Book Review hailed it as a major landmark, but criticized Gould for doing too much propagandizing for his own theories, and unbalancing his citations in favor of his supporters to the neglect of his critics.
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New York Wiseguy
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posted 17. April 2002 04:52
I'm posting again to this topic in the assumption that it is still open. Being new here, I haven't yet learned how one can determine if a topic has been closed. So this is partly an experiment to answer that question.
My understanding has been that Paul Nelson had been planning on posting a description of his application of polyphyly to a complexity theory hypothesis relative to the issue of common descent. (If I said that right.) I've been waiting to see that.
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Paul A. Nelson
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posted 17. April 2002 10:10
Hello again NYWG. You wrote:
quote:
My understanding has been that Paul Nelson had been planning on posting a description of his application of polyphyly to a complexity theory hypothesis relative to the issue of common descent. (If I said that right.)
Not exactly, but the fault is all mine.
Let me see if I can sketch, roughly, what I'm thinking about. (The longer and fully-developed post still isn't done; too much going on right now.) In 1995, the Israeli philosopher of biology Iris Fry pointed out in an insightful and provocative article that many neo-Darwinians hold what she called a "near-miracle" view of the origin of life. (See "Are Different Hypotheses on the Emergence of Life as Different as They Seem?" Biology and Philosophy 10 [1995]: 389-417.) Fry argued that this near-miracle view, in which the probability of the natural origin of life was vanishingly small, was actually a kind of quasi-creationism that made genuine research on abiogenesis impossible in practical terms.
If one wants to learn how life arose naturally, calling the event a near-miracle will not do. As the evolutionary geneticist Walter Fitch explained,
quote:
Before there were genes and a genetic code, the processes must have been physical and the events repeatable, or else we are in the realm of requiring the one lucky event, a special creation, a postulate not significantly different from divine intervention. Of course, it might really have been a lucky event unlikely to occur again anywhere else in the universe at any time, but, if so, it falls outside the realm of science and materialistic explanation.
(W. Fitch and K. Upper, "The evolution of life -- an overview of general problems and a specific study of the origin of the genetic code," in Evolutionary Processes and Metaphors, eds. M.W. Ho and S.W. Fox [New York, Wiley, 1988], p. 38)
All right, then -- let's jump into our handy time machine and set the dial for 1,000 years into the future. Let's suppose that in 3002, we know how life originates; in particular, how it originated on Earth. Stanley Miller thinks this knowledge will be in our hands one day. "After you learn the trick," he said to the Associated Press in 1996, "it'll be easy. The hard part is learning the trick."
So we've got the Trick of Abiogenesis in hand. Now, what will be the character (in terms of the type of physical knowledge) of that Trick?
Let's compare the Trick to making a souffle. Bring together the correct ingredients, heat, time, and presto: a puffy egg entre. With a certain probability of failure, of course, due to contingencies.
But an event very likely to occur, given the right starting and boundary conditions.
A souffle is not a miracle, nor even a near-miracle. Souffles occur in ovens all over the world, spatio-temporally independent from each other, when the conditions are right.
In 3002, we know the Trick of Abiogenesis. We have at the same time, however, been constructing evolutionary phylogenies using the near-miracle assumption (i.e., life began only once, or if it began more than once, that only one origination event survived and left offspring). All the souffles stem by descent from an Ursouffle.
Only they don't -- or we have no reason to think they do. Let me quote another heretic, Christian Schwabe of the University of South Carolina (whom I'd love to see contributing here):
quote:
It is clear that if not one but two or more cells had started life, every single conclusion regarding relatedness of molecules or species is doubtful simply because one cannot know whether branches of a tree or similar branches of different trees are being compared. If we admit the possibility that life started from more "individuals" than the solitary Urzelle, we are forced to abandon our interpretation of similarities as automatically implying relatedness.
(C. Schwabe and G. Warr, "A Polyphyletic View of Evolution: The Genetic Potential Hypothesis," Perspectives in Biology and Medicine 27 [1984]: 465-485; p. 468)
This is why I suggest that abiogenesis research and phylogenetic reconstruction are on a collision course with each other. This is also why hypotheses of aboriginal polyphyly, such as Malcolm Gordon's, have profoundly destructive effects on the neo-Darwinian conception of homology.
It turns out that the theory of common descent -- Darwin's Tree of Life (his greatest achievement, to my mind) -- and theories of abiogenesis are about as intimately linked as any two theories can be.
[ 17 April 2002, 10:28: Message edited by: Paul A. Nelson ]
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James A. Barham
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posted 17. April 2002 17:20
Paul:
I have been enjoying your posts, and also I appreciate the guides to the literature (I fully agree about the excellence of the Fry article, in particular---have you seen her book, as well?).
Here is the question---or couple of related questions---that occurred to me as I was reading your discussion of the relationship between the Tree of Life and abiogenesis.
First, isn't it the case that a true phylogenetic taxonomy is already rendered nearly impossible by several other factors, quite apart from how life originated? I am thinking, for instance, of (a) evolutionary convergences (homoplasies); (b) the fact that there is no one-to-one correspondence between genes and phenotypic traits (with one-to-many, many-to-one, and many-to-many relationships being standard)' (c) the problem of endosymbiosis; and (d) perhaps most gravely, the problem of lateral gene transfer among bacteria.
So, in other words, there are lots of problems for taxonomy even if monophyly is basically true. But let's say monophyly is false. Are the consequences for the Tree of Life really as dire as all that? After all, even if life had more than one origin, no one supposes that this will affect taxonomy at anything other than the prokaryotic level. It will still be the case that the prokaryote/eukaryote transition occurred either once, or multiple times, as the case may be. But that transition could very well have occurred only once, even if prokaryotic life had multiple origins, and conversely, even if life had a single origin, the prokaryote/eukaryote transition could still have occurred multiple times in different prokaryotic lineages.
In short, it seems to me that the two issues---Tree of Life and origin of life---have little to do with each other directly. Both problems are extremely difficult and complex, but solving one will by no means ipso facto resolve the other one.
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Paul A. Nelson
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posted 17. April 2002 20:53
James Barham wrote:
After all, even if life had more than one origin, no one supposes that this will affect taxonomy at anything other than the prokaryotic level.
Well, there's the rub.
Decouple homology from material descent (as would happen with multiple origins of life), and the consequences for biological -- i.e., historical or phylogenetic -- inference begin to percolate upwards through the Tree of Life.
Think about it this way. The base of the monophyletic Tree rests on very widely shared biochemical universals. But if those universals could arise deterministically, independent of each other, then they are no better markers of material descent (template causation) than the geometries or chemical homologies of crystals currently are. A quartz crystal in Sweden shares no common ancestry with another quartz crystal in Australia. Quartz happens.
Once the tree begins to come apart at its base, one has to locate stopping points higher up to forestall further fracturing. But the possibility of polyphyletic origins affects those points as well.
[footnote: I have seen Fry's book. I liked the Biology and Philosophy article better, although the book has much to commend it.]
[ 17 April 2002, 21:49: Message edited by: Paul A. Nelson ]
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Evan
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posted 17. April 2002 21:20
I’m glad to see this thread revive.
I have read both the discussions here and the more lengthy discussion at ARN on this issue. I understand (and in fact mentioned this in a previous post) that the “bottom” of the tree of life could well be “bushy”, with multiple origins and diverse ways in which genetic material was exchanged. Therefore, common descent in the sense of a strict convergence upon one single common ancestor may well be a simplistic and outdated notion.
But I think this aspect of the notion of common descent is obscuring the notion as it is more commonly understood, and as I originally intended to discuss: the idea that starting with creatures that are alive today, a chain of parent-child relationships has existed stretching backwards in time for at least 500 million years.
I don’t see how any of the discussion concerning abiogenesis and the origins of early life bear on his question.
Let me be more specific.
Start with me, and then visit my parents - still living. Then borrow Paul’s time machine and visit my grandparents, and then their parents, and so on, one generation at a time for the past 5 million years, until the time of some ancestral hominid. Would there ever be a break in this family chain - a time where one of my ancestors came into existence in a way other than being born of their parents?
1) If one thinks common descent in this sense might not be true, what evidence might lead one to think this?
2) If one thinks common descent in this sense might not be true, what are possible hypotheses about what does happen - what would one see instead of a birth when my ancestor came into existence?
==========================
Now, the moderator has made it clear that discussions here at ISCID need to be more than a rehash of evolution / ID arguments that can take place elsewhere. Therefore, at the risk of being redundant, I offer the following summary of what has led me to this question:
Long ago, a poster named Tom Stalnaker asked the question: given that CSI can not be created by natural forces alone (as per Dembski), what would we see at the moments that CSI enters the world by design. That led me to offer the hypothesis that CSI is created when genetic events are manipulated at the quantum level – altering probabilities without otherwise “miraculously” intervening in causality (again being motivated by suggestions of Dembski’s.)
Therefore, my hypothesis includes common descent (in the sense I am discussing here) as a critical component - that a sequence of intelligently designed but slight alterations in genetic material over a number of generations can lead to substantial changes in organisms, and therefore account for the succession of creatures we see in the fossil record and the diversity of life currently existing.
This hypothesis is more specific than most ID hypotheses, can account for various misgivings people have about the adequacy of natural evolutionary processes to account for macroevolutionary changes, and yet accepts the obvious fact of parent-child descent.
So at the level of, for example human/prehuman ancestry for the last five million years, why would anyone doubt common descent? What evidence would lead one to this conclusion? To repeat, it seems like all the discussion that has taken place concerning phylogeny here and at ARN do not bear on this question.
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James A. Barham
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posted 17. April 2002 21:49
Like Evan, I was having a hard time understanding Paul's point about the alleged importance of monophyly for the Tree of Life notion. However, after thinking it over, I believe I may have grasped the issue that we are all dancing around. Correct me if I am still wrong, Paul!
Paul wrote:
"Decouple homology from material descent (as would happen with multiple origins of life), and the consequences for biological -- i.e., historical or phylogenetic -- inference begin to percolate upwards through the Tree of Life."
Finally, I realized that Paul seems to be assuming that if monophyly goes by the boards, then this in itself would undermine our evidence for transformism as such---hence the idea the the "consequences for biological . . . inference being to percolate upward" etc.
But monophyly itself is just an inference that has been drawn from the presumptive evidence for transformism. It is hardly the case that monophyly counts as evidence for transformism, since the evidence for transformism is much stronger than the evidence for monophyly. I just don't see that multiple origins of life has the devastating consequences for the transformism hypothesis that Paul believes it does.
What is this "presumptive evidence" for transformism, by the way, according to me? Well, mainly the fossil record, of course. Paul's point about crystals is well taken, and I for one believe that life has much more lawlike properties (consider the amazing convergences) than most folks do. But if crystals grew in complexity over time (not jus in size), and if we had a fossil record of decreasing complexity with depth, then yes we would have pretty good prima facie evidence of direct descent, even if the fact of multiple origins might mix things up, and make it impossible to distinguish the individual lineages with certainty.
I guess I mean to say simply that the real reason that those of us who believe in evoluion do so is that the fossil record certainly appears to be telling us that living things have changed over time, and the transformism hypothesis seems overwhelmingly the most likely explanation of the fossil record, even if we do not understand how speciation is possible at the genetic level. (I of course believe there is a powerful internal teleological component that has to be taken into account, and that "chance" explains nothing.) But surely the Omphalos scenario is a stretch, in this day and age, and it seems to me that the idea of myriad independent creation events that just happen to be arranged in time in such a way as to look like evolution is nothing more than an Old Earth Omphalos scenario.
At any rate, to return to the original point at issue, since monophyly is inferred from the appearance of evolution, and not the other way around, I really don't think that multiple origin events are fatal for evolutionism.
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Paul A. Nelson
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posted 17. April 2002 21:57
James Barham wrote:
I really don't think that multiple origin events are fatal for evolutionism.
They're not, of course. Lamarck and Haeckel were polyphyletic evolutionists, as are Malcolm Gordon and Christian Schwabe today.
But there are deep inconsistencies in how evolutionists think about such concepts as homology, and the path into those inconsistencies may come out -- through a forest of careful analysis -- into some surprising places. ;-) More later.
Incidentally, Agassiz and Cuvier saw much of the same fossil record as we do today, yet neither was an evolutionist. "Change over time" does not yield naturalistic evolution, unless a couple of other assumptions are tagged on.
[ 17 April 2002, 22:09: Message edited by: Paul A. Nelson ]
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Evan
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posted 17. April 2002 22:17
Paul Nelson writes, “Once the tree begins to come apart at its base, one has to locate stopping points higher up to forestall further fracturing. But the possibility of polyphyletic origins affects those points as well.”
Why?
Once asexual replication, and especially sexual reproduction became the norm, it seems to me that the types of events that might even hypothetically account for a polyphyletic base would no longer apply. Massive amount of gene transfer, or incorporation of one type of an organism by another, or whatever else might apply to biochemistry and single-celled organisms 3 billion years ago does not apply at all to multi-celled animals such as a pre-hominid, or an early amphibian or a creature at the reptile / bird transition.
The differences between these two are so significant that I don’t see why issues about monophyletic origins of single-celled organisms (where we have some ideas about mechanisms that might have been at work) would apply to issue of, say the transition from pre-hominids to humans.
On this point, then, I am unclear as to how far “up the tree” Paul thinks “further fracturing” might take place. In particular (to paint as wide a contrast as possible for the sake of discussion), is there any possibility in Paul’s view that human beings could not be related by common descent to pre-hominids of five million years ago?
And if this is considered a possibility, is there evidence and/or conceivable mechanisms for such a break in the chain of common descent that is in any way analogous to or based on concerns related to the origins of life?
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edmund
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posted 18. April 2002 00:40
quote:
Decouple homology from material descent (as would happen with multiple origins of life), and the consequences for biological -- i.e., historical or phylogenetic -- inference begin to
percolate upwards through the Tree of Life.
There seem to be two very distinct questions tied up in this statement.
1) How likely is abiogenesis? (I.e., are multiple origins of life likely?)
2) How similar are the products of these abiogeneses likely to be?
The second question is the key. If abiogenesis is not only likely but completely deterministic-- if there is some law which guarantees all new life forms to be identical-- then it becomes impossible to construct phylogenies beyond a certain level. This is no different than homoplasy. In a nutshell, any forces which render phylogenetic characters nonindependent makes them phylogenetically uninformative.
However, I don't know of any reason why multiple abiogeneses would produce indistinguishable products. For example, the exact details of genetic molecules and the genetic code are arbitrary; apart from common descent, is there any force which logically makes them roughly the same for all organisms?
In the absence of such a force, Schwabe's point does not hold, and multiple origins do nothing to undermine phylogenetics. What undermines phylogenetics is any force which mimics common descent: horizontal gene transfer and homoplasy, for example.
Paul, are you suggesting that there might be some other force which causes apparent homologies? I'd also like a clearer picture of exactly how these consequences percolate upward through the phylogenetic tree, and to what level-- are we talking about a separate origin for humans, or a separate origin for tetrapods, or what exactly? I think Evan is making some very good points and I'm interested in hearing what your response is.
Personally, I doubt very seriously that phylogenetic reconstruction will ever approach the origin of life. James Barham is right-- there's too much noise in the way. In fact, I'm skeptical that we'll ever be able to approach the origin of life scientifically. There are too many ways in which it could have happened and I don't think we'll ever have the necessary information. It would not surprise me if that question remains forever beyond the reach of scientific study.
--edmund
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Paul A. Nelson
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posted 18. April 2002 09:21
Edmund asked:
Paul, are you suggesting that there might be some other force which causes apparent homologies?
I'm suggesting that we just don't know.
We have no canonical theory about how genetic information, for instance, arises ab initio, i.e., at the origin of life itself. In 1968, on the same page where he proposed the "frozen accident" theory for the origin of the genetic code, Crick said:
quote:
The theory seems plausible but as a theory it suffers from a major defect: it is too accommodating. In a loose sort of way it can explain anything. A second disadvantage is that the early steps needed to get the system going seem to require rather a lot of chance effect.
(F. Crick, "The origin of the genetic code," Journal of Molecular Biology 38 [1968]: 367-379; p. 378)
Hence my time machine, where, a thousand years from now, we do have a canonical or experimentally well-attested theory about abiogenesis. But what would that theory tell us, if we had it in hand?
Perhaps that living systems, if they are going to arise or exist at all under terrestrial conditions, will exhibit a very narrow range of properties falling into the same sphere as known Earth life -- even down to the particular molecular details. Norm Pace balks at "particular molecular details," but he sees deterministic origins for pretty much everything below that. See his challenging article, "The universal nature of biochemistry" (PNAS 98 [2001]: 805-808.)
And why not? We rest our judgment that this couldn't happen on our total ignorance of how it did happen.
Several months ago, Jack Szostak gave a fascinating talk at the University of Chicago where he described the remarkable molecular convergences that his lab, and a couple of others, had achieved with their ribozyme engineering experiments. Quite independently, the labs had "evolved" the same structures under the same selective regime. [I've got the notes at my office; sorry for the lack of detail right now, I'm writing this from home.] In related experimental work, Szostak and his colleague Kourosh Salehi-Ashtiani argued for "multiple origins for the hammerhead ribozyme" (Nature 414 [2001]: 82-84). In the last sentence of their paper, they conclude:
quote:
Our results show that, despite the dominance of contingency (historical accident) in some recent discussions of evolutionary mechanisms, purely chemical constraints (that is, the ability of certain sequences to carry out particular functions) can lead to repeated evolution of the same macromolecular structures. (p. 84)
Here's a mind-opening thought experiment. Pick any molecular system, and ask yourself how you know that that system could not have evolved more than once. If the answer is, "Well, it's just too improbable," ask yourself how you assigned the relevant probabilities.
I'll say more about how far up the fracturing may go later.
P.S. I should say that my thinking in this thread is being done within the confines of methodological naturalism, strictly for the sake of argument.
[ 18 April 2002, 12:04: Message edited by: Paul A. Nelson ]
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New York Wiseguy
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posted 20. April 2002 12:27
Paul:
This was composed after seeing your first post in the "renewal" of this topic on 17 April, before seeing the additional discussion which ensued. I feel, nevertheless, that I made some points very similar to those made by Evan and James Barham, albeit in a different manner, so I hope the following will be worthy of attention.
In commenting on your most interesting post, our relationship will be more congenial if I proceed from the bottom up. I. e., I find the essence of your bottom line quite persuasive, although I might want to quibble with some details of the process by which you arrive there, and also with some details of the conclusions which you reach from what I consider to be the "essence".
What I consider the "essence" is the (for me) greatly increased plausibility that the earliest stages of biological evolution, plus abiogenesis, were polyphyletic processes. I see less plausibility, however, that the later stages, including speciation, involved polyphyly to any significant extent. Some of my quarrel with the sources you cite, and I suppose also with your total argument, is what appears to be an unwarranted extension of polyphyly from its more plausible domain of early life into the realm of more recent evolutionary events.
As a matter of fact, the way I'm now looking at it, the increased plausibilty of polyphyly in abiogenesis and the early stages of evolution serves to increase the plausibility of a completely naturalistic origin of life, and an increased likelihood that the earth is not the only place in the universe where life has originated and evolved to its present level of development.
It appears to me the emphasis on common descent, in the absolute meaning of the term, is becoming counterproductive, and something like a red herring. Most of the as yet unanswered questions in the explanatory puzzle have no relevance to the question of whether all life arose from a single common ancestor. That explanation which you suggest could come by the year 3002 might conceivably show that there were self-replicating objects, of a design which today we have no conception, which somehow came into being multiply in an environment whose charactistics we likewise today have no conception. And that there were further processes by which these self-replicating objects evolved, very slowly, into DNA and procaryotic cells. And each of those steps could likewise have required environments with today can only be conjectured.
Working my way further back in your chain of reasoning, Stanley Miller's suggestion that there is one "trick" waiting to be discovered seems to me the height of optimism. His experiment I would regard as quite significant, in that prior to it, it was believed that amino acids could be synthesized only by life processes, and he disproved that notion. Whether or not he had the right recipe for the primordial soup is less important than that simple fact. But, in my opinion, Miller provided only a suggestion as to an answer to one very tiny part of the entire explanatory puzzle, perhaps just one question out of thousand which need to be answered. I. e., there is not just one "trick" to be discovered, but instead 999 more tricks. In other words, there's a substantial common area of agreement between myself and James Barham (although he's a bit more smitten by Ms. Fry than I am).
And, speaking of Fry, she seems to me to be engaging in teleological argument, as shown by her terminology. The very term "near-miracle" strikes me as an oxymoron. It's either a miracle or it isn't. Furthermore, what is "vanishingly small" supposed to mean? Something is either zero or non-zero; can't be both. Nevertheless, her arguments are still thought-provoking, but would appear less teleological (and equally persuasive) if she said "of very small probability" in place of "near-miracle" and "extremely tiny" instead of "vanishingly small".
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James A. Barham
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posted 21. April 2002 17:04
New York Wiseguy:
I think that your disagreement with Fry is largely verbal, rather than subtantive. As I understand it, you in fact agree that the probability that even a single protein (much less a cell) could have just fallen together purely by chance is so small as to be impossible for all practical purposes (even if not logically impossible, or with absolutely zero probability). (One of the best discussions of this is Walter M. Elsasser, Reflections on a Theory of Organisms, Johns Hopkins UP, 1998, pp. 49--57.)
I have written that such an event would be "tantamount to an abrogation of the laws of nature," hence indistinguishable from a miracle. Would you agree with that? If so, I think that is all that Fry is trying to say.
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Paul A. Nelson
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posted 21. April 2002 21:43
Hi NYWG,
Let me comment briefly on Iris Fry's article, and a couple of other matters. You write:
quote:
she seems to me to be engaging in teleological argument, as shown by her terminology. The very term "near-miracle" strikes me as an oxymoron. It's either a miracle or it isn't.
Actually, I misquoted Fry; her term is "almost miracle." But she should speak for herself:
quote:
While the continuity thesis assumes the stepwise, probable emergence of biological organization, the "almost miracle camp" regards the emergence of this organization as a real puzzle. We are dealing here, says Jacques Monod, one of the most pronounced representatives of this position, not so much with a "problem" but with "a veritable enigma" (Monod, 1974, p. 135). ... Not willing to consider any other physical option than chance, Monod is, thus, left with a real enigma. Under his assumptions he has to envisage the whole process of evolution as a huge lottery. Luckily for us, he says, "our number came up in the Monte Carlo game" (Monod, 1974, p. 137). However, based on the "virtually zero" a priori probability of the emergence of life, we are dealing, according to Monod, with a unique and unrepeatable event. We cannot exclude the possibility, he says, that "the decisive event occurred only once" (p. 136).
(Iris Fry, "Are the Different Hypotheses on the Emergence of Life as Different as they Seem?" Biology and Philosophy 10 [1995]: 389-417; pp. 396-397)
Fry opposes this viewpoint, saying that it "implies in fact a creationist position" (p. 389). She is no teleologist, and in her book on the origin of life pointedly critiques the intelligent design position.
About "the trick" of abiogenesis. Of course the "trick" will involve many steps.* Stanley Miller was saying, I think, that as we learn how life arises -- as we come to understand the many elements of the "trick" -- we will cease to lean on chance and the cosmic roulette wheel (as Dawkins does, for instance, in The Blind Watchmaker). A stage magician is a thoroughgoing determinist. He has practiced his sleight-of-hand so that it becomes routine. The effect the audience sees appears magical: the performer knows it is not. There are mechanisms and processes behind it all. Not luck. "Chance" is what afflicts the neophyte magician, when he drops the cards and fumbles the rabbit.
About the relevance of common descent and considering the alternatives. When I lay aside the constraint of methodological naturalism, I don't think that naturalistic polyphyly is plausible -- but that's because I don't think that life arose naturally even once. What cannot happen once, of course, cannot happen more than once. But ID theorists learn early to think along multiple tracks. On the assumption of methodological naturalism, polyphyly is as justified as monophyly, indeed more so, I'd say. Darwin's single Tree of Life is not long for this world, and the sooner evolutionary biologists begin seriously to consider alternatives, the happier they'll be. The "almost miracle" position that currently undergirds common descent is inherently unstable.
I still need to say something about the consequences for higher-level polyphyly, but that will have to wait until later this week.
*Here's an interesting tidbit from Fry's paper (p. 399). Funny, NYWG, that you should mention 1,000 steps in the abiogenesis story. Fry quotes Christian de Duve giving the same figure for the number of steps, and the need for very good odds indeed at each step:
quote:
Viewing each step as highly likely, if not bound, to happen under the conditions that prevailed follows from the fact that the number of individual steps must have been very large. Let the probability of each step be even moderately low -- say 50% -- and the combined probability, which is the product of the individual probabilities, soon reaches levels that border on the miraculous (10^300 for as few as 1000 steps).
[ 21 April 2002, 21:46: Message edited by: Paul A. Nelson ]
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Drosera
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posted 23. April 2002 02:55
Paul Nelson wrote:
quote:
About the relevance of common descent and considering the alternatives. When I lay aside the constraint of methodological naturalism, I don't think that naturalistic polyphyly is plausible -- but that's because I don't think that life arose naturally even once. What cannot happen once, of course, cannot happen more than once. But ID theorists learn early to think along multiple tracks. On the assumption of methodological naturalism, polyphyly is as justified as monophyly, indeed more so, I'd say. Darwin's single Tree of Life is not long for this world, and the sooner evolutionary biologists begin seriously to consider alternatives, the happier they'll be. The "almost miracle" position that currently undergirds common descent is inherently unstable.
I still need to say something about the consequences for higher-level polyphyly, but that will have to wait until later this week.
I think that a thorough discussion of common descent/monophyly of life will require a number of clarifications/distinctions:
- what is the definition of life? (speaking in terms of molecular replicating systems)
- what is an "ancestor", e.g. the last common 'ancestor' could have been:
(a) a single bacterium
(b) a single bacterium 'species'
(c) a community of gene-swapping bacterial 'species' that are all descended from:
(i) a single bacterium
(ii) a community of gene-swapping protocells
(d) we could also have the situation of (b) or (c) above but where some genes got transferred into the last common 'ancestor' from protocell lineages which are now extinct
Plus, before the last common 'ancestor' one can ask many of the same above questions about putative communities of protocells etc.
On any of these scenarios it is clear that much later, the pre-last-common ancestor of known eukaryotes acquired mitochondrion via endosymbiosis, and got a lot of its genes transferred to the nucleus. A similar but probably less drastic event occurred for e.g. plastids. These events however appear to be increasingly well-resolved and so probably the 'organismal tree' will be resolvable in spite of the symbiotic transfer.
...etc... Just defining your terms is a very complex question here. However, it seems to me that none of the above hypotheses really compromises the 'Tree of Life' model in a serious way -- trees have roots and branches have a certain thickness (an analogy to the size of a gene pool), there is no particular reason we have to be rigidly literal (demanding that tree of life = absolute clonal lineal descent) at the expense of missing the major pattern (e.g. gene transfer happens is restrained in many ways, e.g. in sexual eukaryotic gene pools, or by the fact that it is easier for closely-related prokaryotes to successfully trade genes, therefore a largely tree-like pattern will still result).
Drosera
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