Vitamin C

Helen · Jun 21, 2003

Much has been made of the fact that both humans and apes need to supplement their diets with vitamin C -- and this fact is used as evidence that we are related to apes.

It is much more complicated than that; and there may be an explanation apart from the standard evolutionary one.

First of all, this article
http://www.grisda.org/origins/12096.htm
chronicles something very interesting. Vitamin C is synthsized in different organs in different animals. It can be synthesized in the liver or the kidney. Birds, for instance, synthesize vitamin C primarily in the kidneys. Mammals synthesize it primarily in the liver.

I am curious how evolution explains this. Especially since we are -- evolutionarily speaking -- all 'related' to fish, where, if and when vitamin C is synthesized at all, it is synthesized in the kidneys.

So here is the evolutionary picture as it seems to be indicated:
Vitamin C synthesis does not exist in most fish.
Vitamin C synthesis exists differently in most birds than in most mammals. Therefore there must have been -- again evolutionarily speaking -- an ACQUIRING of the ability to synthesize vitamin C in two different ways as fish mutated into other forms of life.
And then the ability was selectively dropped in various forms of life.

It's not the dropping of an ability that would concern me if I were an evolutionist -- but the acquiring of that ability when it did not first exist. And then to show that it was not even acquired consistently (speaking of where it is synthesized) in 'closely related' animals.

Here is a possible alternative take on the thing.

We know that mutations occur. We know that if the mutation results in the death or inability to procreate in an organism, that this mutation will spell an end to that particular line.

Therefore, it can be said with certainty that if the mutation -- if this is what happened, which I am willing to go with here for the sake of argument -- disabling the synthesizing of vitamin C occurred and the animal and population and species survived, it could only have been because their diets were sufficient not to have minded that mutation.

In short, the heavy vitamin C enriched diet came before the disabling of the genetics related to it.

Among vertebrates, perhaps, those with the widest and least restricted preferences regarding diets were those who simply never noticed that mutation, whereas others which may have suffered that mutation died (meaning, also, we would not know about the presence of that mutation!). Heavy consumption of fruits and leafy foods would have had to have preceded the loss of vitamin C synthesizing ability, not followed it.

If this is the case, there is no need to claim relationship at all among any two groups of organisms on the basis of vitamin C synthesis -- it would be better to look at preferred diets as the cause and not the result of changes in this area.

The Galatian · Jun 22, 2003

Vitamin C is synthsized in different organs in different animals. It can be synthesized in the liver or the kidney. Birds, for instance, synthesize vitamin C primarily in the kidneys. Mammals synthesize it primarily in the liver.
Click to expand...

And yet, it's the same gene involved. How can this be? Hmm... Did you know that the same genes that produce immune cells in humans and birds, use different organs? In humans, it's primarily undifferentiated cells in the bone marrow. In birds, it's in the bursa of Fabricious, an outpouching of the cloaca.

It seems that the particular organ in which this gene is switched on is not as important as the common origin of the gene.

I am curious how evolution explains this.
Click to expand...

I can think of two possible ways: (I don't happen to know but I'll find out)

A. During development, the cells responsible for these functions migrate to different locations in the early embryo.

Following gastrulation, the paraxial mesoderm becomes visibly segmented into a reiterated series of tissue blocks called somites. Somitogenesis involves cell-cell interactions that convert a field of identical cells into a set of individual compartments, each of which is patterned along the anterior to posterior axis. spt acts near the top of the pathway to get cells to the right place at the right time; we would like to identify other segmentation genes. - Sharon Amacher, geneticist.

Mesoderm gives rise to the cells that mediate the immune system;

http://www.dkfz-heidelberg.de/zbi/pub/ResRep00/pdf/A0500.pdf

however, the final destination of these cells is coordinated by higher level genes, and they can end up in tissues derived from ectoderm (skin), mesoderm (bones) or endoderm (Bursa of Fabricius).

As you also know, the genes for vitamin C are found in all cells, and all of them can potentially produce it (except for organisms like humans, who have the gene, but in a damaged and non-functional state)

Especially since we are -- evolutionarily speaking -- all 'related' to fish, where, if and when vitamin C is synthesized at all, it is synthesized in the kidneys.

So here is the evolutionary picture as it seems to be indicated:
Vitamin C synthesis does not exist in most fish.
Vitamin C synthesis exists differently in most birds than in most mammals. Therefore there must have been -- again evolutionarily speaking -- an ACQUIRING of the ability to synthesize vitamin C in two different ways as fish mutated into other forms of life.
Click to expand...

Humans, non-human primates, guinea pigs, the red-vented bulbul (an Asian bird), the Indian fruit-eating bat, rainbow trout and Coho salmon are the only animal species lacking the complete enzymatic machinery to synthesize Vitamin C.

http://www.positivehealth.com/permit/Articles/Nutrition/vitc3.htm

Perhaps in these animals, as in the immune system, the cells responsible for this function ended up in a different organ during development.

And then the ability was selectively dropped in various forms of life.
Click to expand...

Since vitamin C is required in all metazoans, the only way this could happen would be if the population had a ready source of daily vitamin c in their diet.

It's not the dropping of an ability that would concern me if I were an evolutionist -- but the acquiring of that ability when it did not first exist. And then to show that it was not even acquired consistently (speaking of where it is synthesized) in 'closely related' animals.
Click to expand...

It seems to be almost universal in animals.

Here is a possible alternative take on the thing.

We know that mutations occur. We know that if the mutation results in the death or inability to procreate in an organism, that this mutation will spell an end to that particular line.

Therefore, it can be said with certainty that if the mutation -- if this is what happened, which I am willing to go with here for the sake of argument -- disabling the synthesizing of vitamin C occurred and the animal and population and species survived, it could only have been because their diets were sufficient not to have minded that mutation.
Click to expand...

Quite right.

In short, the heavy vitamin C enriched diet came before the disabling of the genetics related to it.
Click to expand...

You've essentially stated what scientists think happened. Great minds...

Among vertebrates, perhaps, those with the widest and least restricted preferences regarding diets were those who simply never noticed that mutation, whereas others which may have suffered that mutation died (meaning, also, we would not know about the presence of that mutation!). Heavy consumption of fruits and leafy foods would have had to have preceded the loss of vitamin C synthesizing ability, not followed it.
Click to expand...

Yep. Selective pressure would not preserve the gene, if there was a ready supply of vitamin c.

If this is the case, there is no need to claim relationship at all among any two groups of organisms on the basis of vitamin C synthesis -- it would be better to look at preferred diets as the cause and not the result of changes in this area.
Click to expand...

It appears, from the evidence that the gene for vitamin c is universal in animals. There are a few species which get sufficient vitamin c in their diet, to remove the selective pressure to keep the gene functional. We have the same gene as the other animals, it's just damaged and nonfunctional. But there was no need for it in primates, whose diet was a sufficient source.

UTEOTW · Jun 22, 2003

First, it is the pathway that is important here, and from fish up through birds and mammals, this pathway is shared. Why is it surprising that over hundreds of millions of years of evolution that some species should follow the path in different areas of the body? It is the same pathway.

According to the reference posted by The Galatian, only two species of fish are known to not have a complete set of the genes necessary for the pathway. Since you have shown that the actual synthesis of vitamin C is often unnecessary in fish, it is not surprising that some might have lost some of the machinery.

Helen, you even give a fairly nice evolutionary path that allows for species to lose this ability and not be severely affected. If the animal's diet already includes sufficient vitamin C then it should not be adversely affected by damage to the ability to synthesis it.

The point with the human angle is not only that neither humans nor primates make vitamin C but also that they share the same disabling mutation.

All you have shown above is that when animals have sufficient vitamin C in their diets that the ability manufacture the chemical can be either not expressed or, in a few specific examples, the ability can be completely lost. This fits evolutionary theory. The rest was just dancing around the edges. Why humans and primates would share the same mutation was not addressed and that is the heart of the problem. Same mutation, same nucleotide of the same gene.

A small section of the GLO pseudogene sequence was recently compared from human, chimpanzee, macaque and orangutan; all four pseudogenes were found to share a common crippling single nucleotide deletion that would cause the remainder of the protein to be translated in the wrong triplet reading frame (Ohta and Nishikimi BBA 1472:408, 1999).
Click to expand...

Link - Section 4.1

[ June 22, 2003, 02:36 AM: Message edited by: UTEOTW ]

UTEOTW · Jun 22, 2003

Here's another shared mutation.

There are many examples of shared pseudogenes of this type; I will describe only one. The steroid 21-hydroxylase gene encodes an enzyme involved in metabolism of steroid hormones. In human DNA, the 21-hydroxylase gene sequence, as well as an adjacent gene encoding "complement C4," has been duplicated; i.e., nearly identical copies of DNA segments lie adjacent to each other, each copy containing a complement C4 gene and a steroid 21-hydroxylase sequence. However, only the "B" copy of the 21-hydroxylase gene is functional; the "A" copy in all humans is a pseudogene, i.e., it contains multiple mutations including an 8 bp deletion that would prevent its function. The corresponding "A" copy sequence of chimpanzee has been examined; it contains the same crippling 8 bp deletion seen in the human pseudogene (Kawaguchi, Am J Hum Genet 50:766-80, 1992).
Click to expand...

Humans and chimps sharing both a duplication and a deletion of the same 8 base pairs in the copy. Could be a coincidence, I suppose.

Link - Section 4.2

mdkluge · Jun 22, 2003

UTEDTW wrote:

A small section of the GLO pseudogene sequence was recently compared from human, chimpanzee, macaque and orangutan; all four pseudogenes were found to share a common crippling single nucleotide deletion that would cause the remainder of the protein to be translated in the wrong triplet reading frame (Ohta and Nishikimi BBA 1472:408, 1999).
Click to expand...

Perhaps a clarification is in order. All of the above primates (all of which cannot synthesize their own vitamin C) share the SAME mutation which prevents them from doing so.

A few other animals are unable to synthesize their own vitamin C. Two questions: (1) Is their inability to synthesize their own vitamin C due to a mutation in the same gene as in the primates? (2) If the answer to (1) is "yes", is it the same mutation in the same gene?

UTEOTW · Jun 22, 2003

Originally posted by M. D. Kluge:
Perhaps a clarification is in order. All of the above primates (all of which cannot synthesize their own vitamin C) share the SAME mutation which prevents them from doing so.

A few other animals are unable to synthesize their own vitamin C. Two questions: (1) Is their inability to synthesize their own vitamin C due to a mutation in the same gene as in the primates? (2) If the answer to (1) is "yes", is it the same mutation in the same gene?
Click to expand...

Good question.

In both guinea pigs and apes, the mutation is in the same gene. However :

LGGLO pseudogenes have indeed been found in both guinea pigs and humans (Nishikimi et al. J Biol Chem 267: 21967, 1992; Nishikimi et al. J BIol Chem 269:13685, 1994). Not only did both groups retain the LGGLO pseudogene as expected, but in each group the gene had been rendered nonfunctional by different mutations.
Click to expand...

Here is a page that gives some of the work comparing the genetic sequences for coding the enzyme in question in various animals being discussed.

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