Now since humans and guinea pigs are widely separated species evolutionally speaking, and there are many other species in between which are closer to humans than guinea pigs, it seems to me that this is a proof against evolution. Otherwise all the species in between man and guinea pig would be lacking the gene to make vitamin c.
True enough, IF the two species shared the SAME mutation. NOwhere is this stated in the article - what is claimed is that the exact same mutation occurs in the primates, showing their (our) common descent. The mutation in the guinea pig affects the same gene, but it's not the same mutation as the one found in the primates. I don't have a link, but here are some printed references that explain the two distinct mutations:
Nishikimi et al. J Biol Chem 267: 21967, 1992; Nishikimi et al. J BIol Chem 269:13685, 1994
Also, your comments on blood got me thinking. Did you know that all cheetahs are almost as alike as identical twins? In particular, they can accept skin grafts and blood from each other. People are all one species, but cannot accept each others' blood unless the types match. On the other hand, human and chimp hemoglobin are identical. Not similar, identical.
Personally, I find shared mistakes to be very strong evidence of common descent. I mean, is there any other expanation?
Same goes for repressed genes that occasionally are expressed (and are then called birth defects), like the whales with legs. Again, is there any other explanation than inheritance from a legged ancestor?
As to the guinea pigs, some mammals can and do manufacture vitamin c, that's why I do not consider this having to do with descent. Same with the chimp hemoglobin. All scientists agree that man and monkeys split off somewhere a long time ago so if both man and chimp have it, then the other apes should have it too and this does not seem to be the case from what you say. The cheetah is interesting, had not heard about it and it is certainly curious.
Now as to your point, yes it could show descent, however, there are so many curiosities in living things which cannot be accounted by descent - the platypus, the bat, and many others that it seems to me that what we have is a designer who used some parts of his "building blocks" in different places with as far as evolutionary theory goes no sense at all (although I am sure there was good reason since the species in which this happened work well). I don't expect you to change your mind over the above, however, it is another way to think about these curiosities.