Re: Sequence Homology

David Campbell (
Fri, 19 Dec 1997 19:25:17 -0400

>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
Don't omit creation by a wise God through evolutionary processes.
The premise that similar organisms need to be similar in other ways
is not necessarily true. Organisms living in a similar environment but not
closely related often have some similarities (overall shape of whales and
fish, for example) but also significant differences. Conversely, genes
have been transferred across wide taxonomic gaps and remained functional,
even though they differ in many details from the naturally present gene.

>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.
>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. 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. 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.
It's not necessary for the full potential of the ancestral genes to
have been realized in the ancestor. Although some of the same genes are
involved in eyes of protostomes and deuterostomes, the eyes are very
different. This would suggest that the ancestral eye was rather poorly
developed and evolved in quite different directions after its origin. In
fact, the ancestral form may not have had an eye at all but merely some
light-sensitive compound that was used for other purposes. This compound
could later be co-opted for use in making an eye by various descendants.
A real example of this is in the latest American Scientist (Jan-Feb
1998), in an article by A. Dean. The ancestral enzyme was converted for
use on a second substrate in eubacteria and in eukaryotes independently.
By studying the structures of the enzymes in various organisms, he was able
to change the enzyme's substrate by mutating just 6 amino acids; he cited
another study in which a single mutation could work the change between two
It is possible to examine a few of the biochemical issues in the
fossil record. The oldest eukaryotic cells known are over 1.5 billion
years younger than the first prokaryotic cells, and well-developed
multicellular organisms are much later still, so there are long periods of
time involved. Another potential source of information comes from looking
for distinctive organic molecules in the fossil record. If a molecule has
durable, distinctive decay products (lipids are generally the best bet),
finding traces of it in the fossil record gives a younger limit to the time
of its appearance. Something with distinctive biological use is especially
informative, e.g., chloresterol is made only by animals (as far as I know),
so finding fossil chloresterol or chloresterol decay products indicates "An
animal was here". A few biological processes have distinctive effects on
carbon isotope ratios and can be looked for in any carbon sample.

David Campbell