DNAunion

DNAunion: Okay, I've given just a bit of thought to a new way of looking at the TCA thing. First, let's remember Behe's actual impossibility claim:

DNAunion: I don't see any flaws at all in that line of logic, when properly understood, that is: it seems sound. If a system is IC, then it cannot have originated by a direct evolutionary route (consisting of a continuous, gradual series of simpler functional precursors that preformed the same function by the same mechanism). Assuming that holds, then using a sort of symbolic logic:

If IC, then !DE

Where IC stands for "The system is an IC biochemical system", and DE stands for "The system could have originated by a Direct Evolutionary means".

Then it necessarily follows that:

If DE, then !IC

Thus, it is legitimate to look at whether or not something could originate in a direct evolutionary manner as a means of trying to determine if a system is IC. If the system COULD arise directly, then it is known not to be IC.

Of course, this check does fail, though, if a system cannot arise directly - in that case, a definitive conclusion cannot be drawn from the above logic alone.

Okay, so does this apply to the TCA cycle? Geez, now that I think about it, NO. A very quick look shows that the TCA might be able to form gradually since many of its intermediates are useful for other purposes, but a partial TCA cycle does not "peform the same function by the same mechanism" as the full TCA cycle. So by Behe's definition of a direct route, it couldn't arise directly (or at least, from this simple examination, it couldn't), which means we can't make a definitive conclusion based on this logic.

So I guess my "little insight" here is not applicable to the TCA cycle itself, but might be able to be used for other systems.

[ December 08, 2002: Message edited by: DNAunion ]</p>

Nic Tamzek

Interesting...

This gets back to what I've occasionally re-realized about IC and IC arguments -- that is, sometimes

1) "evolvability" (direct or in-general, depending) is considered part of the definition, whereas

2) other times it is considered the conclusion whereas IC is defined on other criteria

So sometimes IDists will say "look at all these multiple-parts-required (Etc) systems" and then say "systems with these characteristics can't evolve".

When critics respond with "well, here's how the systems could evolve" then the IDist will say "well, if they could evolve then they're not IC"...which begs the question then of whether or not anything in biology is IC!

This subtle definitional two-step has been crucial to what success the ID argument has had.

As a t.o. poster once wrote, all this proves is that a circular argument is irreducibly complex.

(note, DNAunion v2002 is not in the generalized group discussed above, it appears that he is one of us Darwinians now...)

DNAunion

DNAunion: Version 2002 :-)

DNAunion: And I am arguing that, under the correct circumstances, one can look at the issue from either direction, legitimately.

1) If we examine any biochemical system and decisively conclude that it is IC, then we know that it could not have originated by a "direct" evolutionary path.

Therefore, it follows that:

2) If we find a "direct" evolutionary route to any biochemical system, then we know that it isn't IC.

Do you reject the logic I used (originally) to show this? If so, why?

[ December 08, 2002: Message edited by: DNAunion ]</p>

Nic Tamzek

The problem is that IDists virtually always do this:

1) "This system has multiple parts required, therefore it is IC (and they usually leave out well-matched etc."

2) "Such systems can't evolve" (sometimes but not usually they say something about indirect pathways, followed by the unsupported assertion that indirect pathways are wildly unlikely)

The thing is, it's easy to show that multiple-parts required systems are common. My point is that this says nothing about evolvability.

Even direct evolvability is not ruled out by IC I think, consider the hypothetical case of the addition of a part which increases the sensitivity of the clotting cascade, and later becomes indispensable through coevolution of the parts.

nic

DNAunion

DNAunion: Can we pick something that is less involved than the blood-clotting cascade. What about the nice, neat three-part cilium (microtubule "paddles", dynein "motors", and nexin "linkers"). Can you express your argument in relation to it?

[ December 08, 2002: Message edited by: DNAunion ]</p>

LiveFreeOrDie

DNAunion gets butt kicked.

<a href="http://www.christianforums.com/threads/29446.html" target="_blank">http://www.christianforums.com/threads/29446.html</A>

lpetrich

As to the origin of the Krebs Cycle, Gunter Wachtershauser has proposed that it dates back to the earliest life, which he proposes had been dependent on iron/sulfur chemistry on mineral surfaces -- a sort of Haeckelian Urschleim that had inhabited the clay and mud of hydrothermal vents.

The original version of it would have been run in the reductive direction, with the cycle accepting carbon dioxide and hydrogen to form more complicated molecules. The more-familiar oxidative direction would have come later.

And the Krebs Cycle has a property that neatly fits into this picture: all of its members are acidic, meaning that they can easily be attracted to the metal ions of mineral surfaces.

Here is <a href="http://www.skepticfiles.org/evolut/abiogenf.htm" target="_blank">a short article comparing W's scenario with some others</a>. And <a href="http://www.wilenskyminerals.com/ScientificAmericanArticle.htm" target="_blank">a more detailed article</a>, which includes some experimental demonstration of the prebiotic-synthesis catalytic capabilities of certain minerals.

<a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=122257 77&dopt=Abstract" target="_blank">Bryan Davis's article on molecular evolution before the origin of species</a> also uses Wachtershauser's concept -- it uses biosynthesis steps from the Krebs Cycle as an amino-acid "code age" measurement.

And a very interesting result is that the "oldest" protein by this criterion out of the 10 examined is ferredoxin, which is an iron-sulfur-containing enzyme that handles hydrogen equivalents. And that its ancestral version had a negatively-charged "tail" that can easily stick to a mineral surface.

[ December 12, 2002: Message edited by: lpetrich ]</p>

Mike H

It is rather interesting to note that such a reductive Krebs (TCA) cycle is found in such organisms such as Chlorobium tepidum, which is an anaerobic photoautotroph where carbon dioxide is fixed from the environment. It is also of note that this reductive cycle is (at least in part) powered by sulfide as the reductant, which is made by various geochemical cycles. One might be tempted to suggest that this provides a small piece of the puzzle, as the early Earth atmosphere was known to our best knowledge to be anoxic, and such a system provides a way to fix carbon dioxide for biosynthetic purposes using readily available reductants. (This of course was also put forth by TIGR and researchers at Penn State, who published C. tepidum's genome earlier this year.)

Also of marginal interest is that in a recent solution NMR structure of PsaC (a subunit of cyanobacterial Photosystem I), it was found that the C-terminus adopts a disordered helical structure. The reason this may be of interest is that PsaC is a somewhat removed member of the quite larger ferredoxin family whose C-terminus structure in the bound state to the PS I core is distinctly different. It is certainly quite possible that this feature of being able to "stick" to various surfaces may also apply to fledgling biological structures. [Ref. for structure: Antonkine, M.L. et al. (2002) "Solution Structure of the unbound, photosystem I subunit PsaC, containing [4Fe-4S] clusters F(a) and F(b). Conformational change upon binding to Photosystem I." J. Biol. Inorg. Chem. 7:461-472. ]

Anyway...

[ December 12, 2002: Message edited by: Mike H ]</p>

Clutch

Ouch. Cringe-making.

DNAunion

DNAunion: Hmmm…looks to me like DNAunion's butt didn't get kicked at all.

http://www.christianforums.com/threads/31780-1.html