Ideally, yes. Unfortunately, it seldom rises to that level. I've debated this issue with far too many creationists who didn't have the slightest idea what the facts were, but by gosh, they *knew* I was wrong anyway, and probably a pawn of the devil as well.
The facts themselves are not in dipute.
Surely you jest. Creationists very frequently dispute the facts when they raise uncomfortable questions about their position. For a recent example, see post #301:
"Totally false. There is no such evidence at all. Before one can make such a claim one has to show that even one single mutation has created greater complexity in any organism. There is no such proof."And on the flipside, creationists often make wild claims as if they were fact, like the ICR pamphlet that for several years claimed, "The billions of fossils found are all of highly complex forms of life." This is not only wrong, it is blatantly contrary to the known facts.
The assumption of the evolutionary biologist, that homology implies common ancestory, most certainly is.
By itself, certainly, homology can only *imply* common ancestry, and reasonable people can disagree about some particular homologies. Conversely, it would take an *unreasonable* person to disagree about more clear-cut homologies.
*However*, when coupled with evidence from DNA and other molecular analysis in their various forms, fossil features, present and past geographic distribution, stratigraphic distribution, intermediate forms, ontogeny and developmental biology, genetic change rates, and so on -- when all these divergent lines of evidence converge to the same answers, one has to be dogmatically stubborn to avoid admitting the obvious.
In your treatise, you have made salient points regarding the anatomy and physiology of certain shark species, points that are not in dispute. You can thank the hard work of many scientists over many years for that summary. However, it is a great leap to suggest that isolated similarities between a vast array of disparate animals can be hand selected as if from a buffet, in hopes of reconstructing an orderly transition from one organ system to another, even less one species to another.
All well and good, but that's not what I was doing.
What I was doing -- and I believe I was quite clear on this -- was demonstrating that gore3000's "can't get there from here" claim falls flat since there *are* plausible gradualistic "baby steps" from his "before" picture to his "after" picture, and not only are they simply arguably plausible, the intermediate steps demonstrably ACTUALLY WORK because they *do* work in various species.
The fact that the mammalian range of placental types (and the lack thereof) shows the sort of gradation one would expect from an evolutionary sequence was just icing on the cake and offerred as food for further thought. In order to "prove" the sequence further types of evidence would be necessary, including DNA analysis, fossil intermediaries, etc. etc. But since you bring it up, I'll mention that all the evidence to date strongly supports the obvious -- that mammals evolved their present-day placentas from ancestors which did not have them.
So, herein lies the issue: homology need not imply common origin. If it did, one might assume that botanical phloem, complete with specialized cells with seive function and companion cells were more primative versions of fenestrated vascular endothelium supported by pericytes. An absurd notion to be sure,
Homology "need not" imply common origin, sure. But it's a confirming piece of evidence when combined with dozens of other independent lines of evidence.
And so far I'm not aware of a single homology which clearly *violates* a presumed common ancestry. (Note the use of the word "clearly" in that last sentence -- things which merely appear similar but have fundamental differences, like your example above, don't count.)
but no less so than the cladistic hierarchy of binary fission to hammerhead yolk-sac placenta to platapus pouch to human placenta to astronaut. Regards.
I quite clearly indicated that hammerhead placentas were derived independently from mammalian placentas (and marsupial placentas idependently from mammalian placentas as well), so take your straw man elsewhere.
It's also quite clear that the fundamentally *different* nature of the hammerhead/marsupial/mammalian placentas preclude them from being mistaken for being homologous, so again your cautionary tale misses its mark. If you meant to throw doubt on *valid* examples of homology, and the evidence they provide for common descent, you have failed and appear to be acting disingenuously.