Biliskner wrote:patiodog wrote:Well, f-man, about all I can do for a sheep is knock 'em down and trim their hooves -- and even that ends up being pretty comical.
Was really struck in puppy embryology last year just how profound the homologies are. Development really does recapitulate phylogeny. (How else to explain three different sets of kidneys at three different times in development? -- one pair homologous to those of a fish, the next to those of amphibs and reptiles, and the final pair being the mammalian kidneys we know and love.)
i really (no really) do want to put this into my own words but i figure if you're really questioning the concept as a scientist would, then you can also click & read:
http://www.answersingenesis.org/tj/v15/i1/homology.asp
Quote:'In fish and amphibia the kidney is derived directly from an embryonic organ known as the mesonephros, while in reptiles and mammals the mesonephros degenerates towards the end of embryonic life and plays no role in the formation of the adult kidney, which is formed instead from a discrete spherical mass of mesodermal tissue, the metanephros, which develops quite independently from the mesonephros.'33
I apologize that I was a little fuzzy on my phylogeny -- I only really care about mammalian development and anatomy. Nonetheless, this is a statement presented as refutation that simply restates the argument for embryology as a mirror of evolution. The fact is, both the pronephros and the mesonephros are the
functional kidneys of the developing embryo at different times. The mesonephros is incapable of concentrating urine, and so is unsuitable for an animal (like a reptile or a mammal) that spends most of its life out of water -- but it is perfectly functional during embryonic development. (Bits of it live on, by the way, in the adrenal glands and in the gonads.)
I did read the page, and raised my eyebrows a few times -- such as when the author states, "The eye is hypothesized to have evolved independently as many as 60 different times." So I follow the reference. "Climbing Mt. Improbable." Not a peer-reviewed article, but a popular book. So, what, I'm supposed to go out and find this book to find out who has done investigations and come up with 60 different instances in which "the eye"? (As if there was only one "eye" -- hell, even some unicellular critters, like Euglena, have photosensitive capabilities.)
Aright, another thing, for the hell of it...
Quote:For example, in sharks the alimentary canal is formed from the roof of the embryonic gut cavity; in frogs it is formed from the gut roof and floor; and in birds and reptiles it is formed from the lower layer of the embryonic disc or blastoderm.
I'm not sure what is meant here by the phrase "embryonic gut." The alimentary canal
is the gut. All I can figure is that the author has substituted the unfortunate term "embryonic gut" for "blastocoel," which is the fluid-filled space inside the embryo before it has started to fold on itself. The egg structures of different animals varies according to the nutritional needs of the critter -- different amounts of yolk lead to different arrangements of cells that will give rise to the adult animal during cleavage.
Amphibs, for instance, need little yolk because so much of their development occurs after hatching (metamorphosis and whatnot). Most of the fertilized egg (or zygote) ends up becoming adult cells. The embryo develops in a sphere around what little yolk there is.
Birds, at the other extreme, have high metabolic demands and a relatively long hatching period, and so have an enormous amount of yolk (that egg in your refrigerator is a single cell). Cleavage and early development take place on the surface of this enormous yolk, so superficially the process looks very different. However, when you compare what's really going on as though each were a flat map of the world, both processes are remarkably similar. The germ layers may be distributed differently in relation to the yolk and thus to the "embyronic gut," but the germ layers give rise to the same structures.
And then there's the attempt to attack biochemical evidence...
Quote:The homology argument from biochemistry parallels the argument in anatomy. Evolutionists suggest that just as the study of comparative anatomy has found evidence of anatomical homologies, likewise research on
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the biochemistry of different organisms has revealed biochemical homologies. In fact, the biochemical similarity of living organisms is one of the most remarkable features of life
. Cytochrome enzymes are found in almost every living organism: plant, animal and protist. The enzymes of the citric acid cycle are also almost universally distributed. Chlorophyll a is found in all green plants and almost all photosynthetic protists. DNA and RNA are found in every living organism and, so far as we can determine, contain the same hereditary coding mechanism. The fact that underneath the incredible diversity of living things lies a great uniformity of biochemical function is difficult to interpret in any other way but an evolutionary one. Presumably these molecules were put to their current use very early in the history of life and almost all modern forms have inherited the ability to manufacture and use them.'36
The fact that animals are 'so similar in their chemical make-up' has long been used to support Darwinism.37 But extensive biochemical research has revealed that the simplest reason for biochemical homology is that all life requires similar inorganic elements, compounds and biomolecules; consequently, all life is required to use similar metabolic pathways to process these compounds. Most organisms that use oxygen and rely on the metabolism of carbohydrates, fats and proteins must use a citric acid cycle which is remarkably similar in all organisms.
This is a very eukaryocentric view of life. Most prokaryotes (and many unicellular eukaryotes) do
not use the citric acid cycle. In
alleukaryotes that do use the TCA cycle (same thing, shorter to type), this takes place in a little membrane composed compartment inside the cell that bears remarkable similarity to the few bacteria that also use the TCA cycle, and it is generally accepted among people who study this sort of thing that this little compartment is (gasp) is descended from the same ancestor as the TCA-using prokaryotes.
Quote:Furthermore, the metabolism of most proteins into energy produces ammonia, which is processed for removal in similar ways in a wide variety of organisms. What evolutionists must explain is why billions of years of evolution have not produced major differences in the biochemistry of life.
Because the consequences of changing these systems is very severe.
Why is this a problem? Life is constrained within very narrow functional limits. Look at how little variation there is in the vertebrate body plan for macroscopic evidence of this. As in embryology, so-called "intermediate forms" have to remain living.
Quote:Despite the many significant differences between the two basic cell forms (eukaryotes and prokaryotes), they are both
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remarkably similar on the biochemical level
. Procaryotes and eucaryotes are composed of similar chemical constituents. With a few exceptions, the genetic code is the same in both, as is the way in which the genetic information in DNA is expressed. The principles underlying metabolic processes and most of the more important metabolic pathways are identical. Thus, beneath the profound structural and functional differences between procaryotes and eucaryotes, there is an even more fundamental unity: a molecular unity that is basic to life processes.'40
There's an implicit red herring being used here. The author is acting as though it is variety in function that has been used to bolster the evolutionary argument, and this is simply not the case. It is variety in
form that has been so used. The bits of ribosomes that are less important to functioning do change, and change considerable. Looking at RNA sequences, the eukaryotic ribosome is only about 50% homologous to that of an archeobacteria, even though the functional parts are highly conserved. It is in exhaustive examination of the differences between less important parts of molecules like the ribosome that it has come to light that they are consistent with the story of evolution as it was already told. Changes that would affect function are selected against, while those that do not affect function are not. Moreover, the degree of difference between the unimportant bits of molecules can be used to determine how closely related they are. Our ribosome is virtually (or perhaps entirely) identical to that of a lizard, which is, like ours, roughly 50% different from that of an archeon in a thermal vent. If all life had been created over the course of a week, we should all differ from each other by the same amount.
Quote:Although many biochemical similarities exist in life, millions of biochemical differences exist that are inexplicable via evolution. Many of these differences do not provide a selective advantage as implied by the claim that Darwinistic mechanisms have fine tuned life for the past 3.6 billion years.
This is simply woeful misunderstanding. The conceit (and its been a very useful one) is that differences initially arise as a matter of chance and only disappear if they prove deleterious in relation to the other options out there. It is environment that determines which varieties are deleterious and which are advantageous. Botanists and horticulturists are very aware of this.
Quote:Creationists suggest that such differences exist due to the need for ecological balance and because the Creator chose to employ variety. Also, were one compound in an organism to be altered, scores of other compounds with which it interacts would often also need to be changed so that the entire biological system could function as a harmonious unit.
This isn't really the case, either. Altering an organ might necessitate changes in other organs.* Altering the vast majority of proteins (remember, this is the section on biochemistry) does no such thing. The protein either works or it doesn't. An enzyme either makes a product that the next enzyme can work on or it doesn't. A structural protein either forms a matrix which protects and nourishes the cell that produces it or it doesn't.
* This reminded me of something else on the page that bugged me. There was a demand to find "nascent organs." Problem is, there is no need to do so to explain vertebrate variety. All vertebrates are incredibly similar in their organ layout. Obvious difference like limb morphology (and a limb is not an organ) are not reflected by internal anatomy. Even the elaborate four-chambered stomach used by cattle and other ruminants is demonstrably a variation on the simple stomach that we've got -- and this can be observed in the embryo and even to a certain extent in the calf before it is weaned. It would be very useful for the ruminant to have evolved (or to have had designed) a new organs capable of fostering the microbial fermentation of cellulose** -- theirs is a pathology-prone system -- but, alas, such change was not in the cards.
** Better still, a benevolent designer might simply have given them a gene for cellulase so that they could digest the stuff on their own, but 'twas not to be.
Long story short -- I'm immensely underwhelmed (disappointed, even) the reference which seems to be the cornerstone of your arguments.
