I've heard that before that we are one of the only kinds of animal who cannot make our own vitamin C. I've never looked up the details of it though. I guess it makes sense that animals like cats and thing would have to be able to make vitamin C from glucose, since they don't eat all that many vegetables, and in fact doing so would make them sick. But, I wonder why primates can't do it. It seems like a strange thing to lose during evolution.
I guess since they theorize we lived on fruit for a long time, there was no need to make our own vitamin C.
Actually, one of the reasons we even know vitamin C exists is from tests on guinea pigs, who are also unable to make it themselves.