Re: How to Account for "Devolution"?


<dougwedel@xxxxxxxxxxxxx> wrote in message
news:dsp9a5$2qu5$1@xxxxxxxxxxxxxxxxxxxxxx
I was recently informed through another discussion in this venue that

Some genes are lost, for instance most mammals >can synthesise vitamin C,
but primates can't.

Where is there evidence that an ancestor of a human ever had the ability to
synthesize vitamin C?
No absolutely reliable typological schema has yet been constructed which can
do more than theorize that "because mammal Y had traits mammal X had, plus
more, and mammal Z had traits mammal Y had, plus more, then Y -- for a
certainty -- was a direct descendant of X and a direct precursor of Z."

While chartings based upon such theoretical assumptions can be made, and are
useful and interesting, they are nonetheless theoretical and not
hypothetical (that is, they offer a possible explanation which has
empirical, but circumstantial grounding, but cannot be tested (which is the
requirement for being a hypothesis).

Another theoretical leap is that of assuming that because mammals today
which resemble mammal Y
of many eons ago have the capacity to synthesize vitamin C, then Y had the
same capacity.

To take two such leaps and attempt to theorize on basis of them that Y (in
the instant case, H. sapiens) or one of 'his' precursors has *devolved* the
capacity to synthesize vitamin C is a hierarchical house of theory cards.

Sailors who die of scurvy give proof that the ability to synthesize
vitamin
C can be essential to survival. How to account for this kind of apparent
"devolution", especially when as we know the genome carries all kinds of
useless baggage in the form of junk DNA or genes that seem to have no
effect
on the phenotype?

It is noted that you used the word "apparent," to qualify the word
"devolution." That is good, but might leave the unwary with the impression
it is a *guess* when it is only a *guess once removed* from a guess. This
is not meant to discourage you from citing any less strained examples.

If errors are made (as we know they are), from time to time in gene copying,
at (or shortly before or after) fertilization, then it is not far fetched
for us to construct a theoretical scenario for a gene coding for synthesis
of vitamin C to have gotten "corrupted" in one individual.

HOWEVER, to construct a theoretical scenario in which the corrupted gene
would have spread from the first individual thence to ALL of a species,
would require veritable contortions of a statistical probabilities
astronomically far beyond chance.

There would have to have been some immensely strong 'attractor' or
'eliminator' to account for so mathematically remote an outcome. Some
possibilities might be:

1. The 'altered' form of the gene which coded for vitamin C synthesis
could have had to offer some ENORMOUS advantage to offset the one it
replaced (this might be testable or at least supportable if,
for example, we could find a very similar sequence that avails some immunity
advantage, or some such thing);

2. The 'altered' form of the gene could have been "paired" with at least
one (and probably more than one) sequence which offered an ENORMOUS
advantage of some kind (this might be almost impossible to trace, because no
controls can be set up to measure for it in fossils for which we have no DNA
samples);

3. Some ENORMOUSLY harmful externality might have selected out ALL members
of the species who ate a particular food which sexually sterilized all who
ate it if they had the ability to synthesize vitamin C; or,

4. Some other ENORMOUSLY specific 'attractor' or 'eliminator' filter would
have had to come to bear.

It should be mentioned that some symbionts may have been "non-self" at one
time in the history of a species and become assimilated by the phenotype.
This is vague, I know, but no more vague than is the non-answer we have, so
far, to the question of how the first prokaryote came to have, as a part of
itself, a mitochondrion. Extending from the theorization that vitamin C
might have been produced by a symbiont which was assimilated into "self" by
some mammals but got "killed off" by, say, some antibiotic process in
something in the diet of human precursors, but not other animals... this,
again, is just guesses based upon guesses.

It would be interesting to hear from anyone working in any of the following
areas of research on the current state of this question (why humans of today
do not have capacity to synthesize vitamin C):

A. Are there any sequences found in the human genome that are so similar to
one (or ones???) found in mammals which code for those mammals' capacity to
produce their own vitamin C, whereby the one in humans is but a possible
copying inversion away from it?

B. If so, does the possibly analogous (possibly once incorrectly copied)
human coding sequence offer some enormous advantage to humans other than
vitamin C synthesis?

C. Are there any species of animal whose vitamin C quota is filled by
internally ecology (i.e., by symbionts carried by all members of the
species)?

D. Are there any other theories and/or hypotheses now considered likely to
serve to account for why humans do not have capacity to synthesize their own
vitamin C, IN LIEU OF the ENORMOUSLY unlikely scenario that it could
have simply gotten "lost" from the human genome... if it EVER WAS in any
direct precursory species PRESUMED to be in the typological line leading to
current H. sapiens?

g


3.


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