I expect philosophers of science would tell us that that's just how
science works. Theory interprets data and data supports theory. The
big bang predicts galactic red-shifts, galactic red-shifts confirm the
big bang. Gravitational theory predicts planetary motion, planetary
motion confirms gravitational theory. I expect epistomologists would
tell us that almost all knowledge, not just scientific, is "circular"
that way. But that doesn't mean all theories are equally supported.
More to the point: you don't need to assume branching to give credence
to the nested homologies. They're there in the sequences. They're so
clear that even a computer can find them. :-)
You can see the homologies for yourself by consulting the (increasingly
vast) on-line web-accessible gene databases. A database with a non-
molecular-biologist friendly interface is at
http://biology.semo.edu/web/ag/game.html.
Common ancestry and descent by modification was proposed before the
genetic data was available, so going in that direction: nested genetic
homologies provide supporting evidence for a pre-existing theory. Going
the other direction, nested homologies are produced by computer
simulations of evolution and produced by micro-evolution in laboratory
and field studies; in that sense, common ancestry and descent by
modification is a simple extrapolation suggested by nested sequence
homologies above the species level. Combined with nested homologies at
other levels (genome, development, anatomy), and the perception is that
this theory-data package is a lot less "circular" than most of the other
things one believes.
>In addition, I have heard the avid creationist make the argument that God
>can use a #10 bolt wherever one is needed in all related species. God as
>designer uses the parts he as already designed just as the Dodge engineer
>uses the same #10 bolt to hold the intripid or the neon engine to the
>motor mount.( I have recently had troubles with my neon). Although by
>definition this is out of the realm of science,It still is an interesting
>question worthy of comment by some of you prolific responders out there.
>JONT>
and Art Chadwick adds:
>I would like to suggest that logically this line of reasoning is consistent
>equally with an evolutionary hypothesis and with creation by a God who was
>wise enough to realize (as sometimes we ourselves seem not to be) that the
>more similar two organisms are, the more similar their (whatever) need to
>be.
You can take N-type calcium channel genes from primates or rodents or
birds or insects, stick 'em in Xenopus oocytes (frog egg cells), and
they express channels and function like gangbusters in that alien
environment. Channels from different species function nearly
identically -- despite impressive sequence divergences in all but a few
critical areas. And those divergent regions, which have a lot of
latitude in what is functional, show that suggestive pattern of nested
homologies.
The widespread perception is that the nested homologies go *way beyond*
what is necessary for "common function." That perception isn't created
by any one knock-down drag-out example (although pseudogenes come as
close as anything). The perception is created by reading _Nature_ and
_Science_ (and, if you're really a glutton for punishment, _Cell_ and
_Journal of Molecular Biology_) for several years and seeing one gene
after another after another sequenced, its homologies compared to other
species, and its sequence/structure/function dissected apart. So if one
wants to understand and/or challenge the perception that nested
homologies go way beyond the functional needs, there is, for now, no
shortcut; one needs to dive into the research journals.
Art raises an excellent point with:
>
>The question I would like to see addressed is the following: (specifically
>to the issue raised by Lorne) The very complex (some are 24 pass proteins
>with gates and in the case of the sodium channel, a ball and chain to plug
>the channel temporarily while the gate is reestablished) ion channel
>proteins of insects and humans are essentially identical in structure
>(ignoring for the moment the differences in amino acid composition which
>fit the molecules for their specific environments) and in function to the
>point that most of what we knew about the system in humans we have learned
>from the study of the system in insects (up until the last few years when
>recombinant technology came in). Focusing attention on the minor
>differences that distinguish organisms from one another only serves to beg
>the question about the origin of the complexity itself. For this we are
>left with a giant question mark.
Agreed. Nested homologies have the potential to tell us a lot about
descent by modification, but I expect they won't help us much in
understanding the origins of complexity (specifically, the complexities
common to all organisms).
>If evolution is the correct explanation for origins, then the common
>ancestor of insects and man must have had these same ion channel proteins
>in essentially their current configuration.
I agree. And there's where my information runs out. I know there are
ion channels in plants and slimes and single-celled critters, but I
don't know how much is known beyond that. I don't think there's any
systematic info yet on which channels are found in which phyla, class,
etc. of critters without nervous systems. I do know that just one or a
few mutations can radically change most channels' ion selectivities, so
gene duplication and a few mutations can produce new ion channel types.
But, as you point out, it sure seems like there had to have been several
complex ion channels all worked out and in place pre-Precambrian,
*before* all this multi-cellular diversification happened.
>The same logic applies to
>virtually all of the molecular complexity recognized in modern organisms,
>including the developmental genes and the general scheme of development
>shared by insects and man. Since morphological change is now clearly
>recognized as a manifestation of the variable expression of the underlying
>developmental genes and the molecular complexity shared by all organisms,
>it is not difficult to see that essentially all evolution of shared
>molecular complexity is required to have occurred in the Precambrian before
>there was any record of anything more complex than a carbonized film of
>single cells.
Also true. I find it suggestive that while the Precambrian fossils are
dated about 0.7 billion years old, first life fossils are dated more
than 3 billion years before that. It looks like God wasn't in a rush to
create the molecular complexity required for multi-cellular
morphological diversification.
> I do not see how this differs in scientific value from
>saying that a Creator did it. At least in the latter case we have an
>intellectually plausible scenario.
When I consider how the Creator's natural laws make it possible for
particles to be organized into atoms, atoms to be organized into
molecules, molecules to be organized into complex biological molecules,
complex biological molecules to be organized into cells which eat,
excrete and reproduce, and cells to be organized into creatures which
can sense, process information and communicate; when I consider how the
Creator's natural laws make possible the evolutionary development, from
simple hydrogen and helium, of stars, heavy elements, planets, oceans,
atmospheres, and even simple biological molecules on spaceborn hunks of
rock and ice; when I consider how the Creator's natural laws make
possible the development of a zygote into a baby; when I consider how
the Creator's natural laws make possible an incredibly complex genomic
phase space, only a tiny fraction of which is being explored by existing
species; when I consider how the Creator's natural laws make possible
biological adaptation and complex inter-dependent ecologies; after all
that, when I consider the idea that the Creator's natural laws might
also make possible via non-linear feedback and selection mechanisms the
slow development of biochemical complexity, I hesitate to dismiss that
idea as intellectually implausible.
Loren Haarsma