I'd love to see a species-by-species breakdown of which primates have one and which don't. I'll bet that the distribution tracks lineages, just as you note that it does in the four anthropoids.
Guess what? You're right (doesn't it feel good to be justified?). Try Scott, GB 1980, "The primate caecum and appendix vermiformis: a comparative study.", J. of Anatomy 131:549-63
quote:
The examination of the caecum of two groups of cynomolgus and rhesus monkeys, two orang-utans and a chimpanzee, as well as an extensive review of the available literature, confirmed that the length of the caecum, relative to that of the colon, decreased as the position of the species in the primate scale rose. Although absent in prosimians and New World monkeys, there was evidence that the appendix vermiformis began to develop in certain Old World monkeys and became fully developed in the anthropoid apes, showing that, far from being a vestigial organ, it has actually developed progressively in primates.
I think it's very interesting that this author downplays the "vestigial" nature of the appendix in certain primates. However, I think he's using a misleading definition of vestigial. As you suggested, it depends on the lineage - and the diet to which they are adapted. For instance, colubine monkeys (old world), are primarily folivores and have developed a foregut digestive system - completely doing away with an appendix, which the lineage never developed (although it does have a caecum). Another major lineage of folivorous monkeys - the Cebidae (new world) - are hindgut fermenters and have only a caecum. Great apes appear to be the only lineage of primates that have developed an appendix. (see, for example, BANR 2003, "Nutrient Requirements of Non-Human Primates", NAP, ppg 22-26). In humans, out of all the hominins, the appendix is highly reduced, probably related to the change in diet when our ancestors left the forest for savannah in the Pliocene. Although still serving a dietary function in the great apes, the only apparent function in humans is as a small part of our immune system - IOW a coopted function, which is my definition of vestigial.