The common ancestor of humans and chimpanzees wasn't a chimpanzee. It was something that no longer exists.It was an ape, though. Likewise apes did evolve from monkeys, just not any of the monkeys around today.
It's more accurate to say that humans and chimpanzees share an ape-like ancestor, and that apes and monkeys share a monkey-like ancestor. The phylogeny we use for existing animals can't be projected onto their ancestors. The common ancestors of monkeys and apes weren't more monkey than ape.
Yes, they were. What you are saying only applies to positively defined groups (monophyletic or holophyletic to use the technical term). Monkeys are defined negatively (they are paraphyletic), including all primates that aren't apes, and some are closer relatives of modern apes than to some other modern monkeys. Thus, it's fair to describe their common ancestor as a monkey, whereas it isn't fair to describe the common ancestor of humans and chimps as either.
But paraphyletic and polyphyletic groups aren't allowed in cladistic taxonomy. I guess you're a Linnaean?
It doesn't matter, because monkeys aren't a formal taxonomic category. :) For the record, I agree with the argument by more traditional phylogeneticists that ancestral organisms can't be classified without using paraphyletic groups, but I think their use should be minimized and otherwise support cladistic systems.
[According to DNA analysis bonobo chimpansees are more closely related to humans than to common chimpansees. This means that both kinds have a common ancenstor which would likely have been classified as a chimpansee also. Since humans are also descended from this same common ancestor (through a later branching with proto-bonobo chimpansees), then humans probably are descended from chimpansees. Perhaps they should all be placed together in the sapiens Genus: homo sapiens, bonobo sapiens, pan sapiens. This is likely a claddistic approach.]
Not at all. First, Homo is the genus, sapiens the species. Second, just because they have a common ancestor, doesn't mean they have to be the same genus. Either all three could be placed in Homo, only humans and bonobos could, or all three could be given separate genera. The last seems the most likely in practice.
[oopse, This is terrible. I can't even tell what species and genus I am. OK how about this one: Homo sapiens, Homo bono, Homo troglodytes.]
Actually, there's only one choice in the matter of genus and species differentiation, and it has nothing to do with how you track ancestry. Last I checked, humans and bonobos can't produce a living crossbreed, so they're in different genera. The same is true of the other two pairs of species.