Once upon a time, a Roman author named Quintus Ennius wrote: “how like us is that very ugly beast, the ape!” It was quoted by Cicero, and from him Bacon, Montaigne and various others. But always it was thought that apes (simia, literally “the similar ones”), which in that time include monkeys and what we now call apes indifferently, were distinct from humans in every meaningful way. As Cicero said after citing Ennius, the character is different.
But then along came a Swedish botanist turned generalist, Carolus Linnaeus, in the 18th century, and despite being a creationist, he put apes, monkeys and humans in the same group (Primata, literally, “the first ones”), and worse, apes and humans into a single genus, Homo. He wrote to Johann Georg Gmelin
It is not pleasing that I placed humans among the primates, but man knows himself. Let us get the words out of the way. It will be equal to me by whatever name they are treated. But I ask you and the whole world a generic difference between men and simians in accordance with the principles of Natural History. I certainly know none. If only someone would tell me one! If I called man an ape or vice versa I would bring together all the theologians against me. Perhaps I ought to have, in accordance with the law of the discipline [of Natural History].
Nevertheless the theologians objected to humans and apes being placed into the same class no matter what the reason, and in 1775, Blumenbach revised the classification so that Humans were the sole members of Homo and Chimps the sole members of Pan. No real reason was given, as this was both intuitively (read: “religiously”) obvious, and the period in which Authorities got to make classifications based on what seemed best to them, stated or unstated.
Which brings me to the continuous Creationist canard (no, it’s not a duck): Why are there still monkeys if humans evolved from monkeys?
There are two sides to this question: one is whether any modern view of evolution requires that there only be one instance of a “type” and once it has been evolved out of, it should go extinct. This is a silly belief that itself is based on ideas that predate even Linnaeus – that each “position” on the “scale of nature” once occupied by a lineage, must become empty when that lineage moves upward. No theory of evolution has held this view for at least 200 years, even before Darwin. If we did evolve from monkeys, then monkeys do not all have to go extinct just because another kind of monkey (i.e., us) has evolved.
The second side to the question though is this: were our ancestors monkeys at all? And the answer to this is subtle.
There are basically two ways to classify things in biology. One is by identity – if group X is the same in some important manner to Y, then X + Y form a group based on that identity. The biological term for identity of characters here is homology, a term proposed by Richard Owen in 1843. It means the same organ under all variations of form and function. All organisms that have a heart form a single group – no matter if the hearts are single chambered, double chambered, or four-chambered. But organisms that have some kind of pump that is not “the same” as the heart are not in that way homologous – if, say, the “heart” in that species develops out of the anus or something, and not in the thoracic part of the body.
The other way is to classify by similarity. Something is in the same class as another thing if it resembles the other. Similarity is not identity – the anus-heart would be classified as similar to the thoracic heart in virtue of a similar task or even activity and structure. To say that humans are not like beasts is to classify by what seems important to use as a similarity measure to us. The biological term for a trait that resembles others because of form or structure is homoplasy. Bats’, birds’ and insects’ wings are homoplasious – similar because of what they do, not because they are the same parts used.
Something can be the same even if it is not very similar, and groups made by identity are called taxa (singular taxon), whereas groups made by similarity are types. Classifications of taxa are called, naturally, a taxonomy. A classification based on types is a typology. These are often confused, even by scientists.
So were our ancestors monkeys? Each way of classifying gives a different answer. On the identity criterion, humans fall naturally into several increasingly larger groups: Homo is in Hominini, which includes several now extinct Homo species and chimps; Homininae, which includes hominids as well as gorillas; Hominidae, which also includes orangutans, and Hominoidea, which includes gibbons. Hominoidea is referred to as the African Great Apes, although the gibbon and orangutan live in Asia. It a part of Catarrhini, or the Old World (African and Eurasian) monkeys.
So, if you classify by taxa, any immediate ancestor of our species was a member of Homo, Hominoidea (and hence the apes), and Catarrhini (or the Old World Monkeys). Hence our ancestor was a monkey, because we are monkeys (and apes).
But “monkey” is typically understood to mean a Primate that has a tail, and so it includes also the New World (American) monkeys: the Platyrrhini. But apes do not have tails, so “monkey” defined by similarity as a type is basically Primate minus Hominoidea. This is like saying that a cookie (or biscuit in the sensible English speaking part of the world) is whatever is left after a bite has been taken out of it. It is what taxonomists call a paraphyletic group: a group that is everything left over by some exclusion of a part that would normally be included.
Now, our ancestors were never New World monkeys. The term “monkey” therefore refers to animals that include organisms that don’t share our ancestry further down the tree. It’s a type, not a taxon.
Ordinary language is typical. That is, ordinary terms like “monkey” refer to things that resemble each other is ways that may not even be scientifically natural. It’s best when making a scientific claim to use scientific terms, because they refer to natural things, natural classes. So a scientist would say “humans evolved from hominoids, which evolved from catarrhines, which evolved from an ancestral primate.” An ordinary speaker would throw their hands up in despair and say “Just tell me, did we evolve from monkeys or not?!” They are speaking past each other. “Monkey” has no scientific meaning.
Some scientists, though, think that this is just logic and language chopping. Of course whatever it is that humans have as their distant ancestor would have been called a “monkey” in ordinary (that is, typical) terms. Geogre Simpson once said exactly that:
On this subject, by the way, there has been way too much pussyfooting. Apologists emphasize that man cannot be the descendant of any living ape—a statement that is obvious to the verge of imbecility—and go on to state or imply that man is not really descended from an ape or monkey at all, but from an earlier common ancestor. In fact, that earlier ancestor would certainly be called an ape or monkey in popular speech by anyone who saw it. Since the terms ape and monkey are defined by popular usage, man’s ancestors were apes or monkeys (or successively both). It is pusillanimous if not dishonest for an informed investigator to say otherwise (1964, p. 12).
This passage is much beloved of creationists, for it seems to be an obvious contradiction to the view that evolution does not say that our ancestors were monkeys. Simpson in fact was of the old school (note that this is written in 1964, before the form of classification I call “by taxa” was developed – it’s known professionally as “phylogenetic taxonomy”, or “cladistics”). But even so, read what he says carefully: No living ape is our ancestor (or even very much like our ancestor to an anatomist). So if by “ape” (or “monkey”) you mean a chimp or a macaque or an orangutan, no, we are not evolved from these ugly beasts.
So when someone asks if we evolved from monkeys, tell them “Yes, if by “monkey” you mean a primate; no, if you mean Primate minus Hominoid”. Of course at some very early and distant time our ancestors were monkeys, but not recently.
Now, back to the “why are there still monkeys?” part of the question: on the older view of evolution that was the common idea of evolution for a century prior to Darwin (both the evolution of organisms, or languages, and of social institutions), if a lineage had evolved, it moved “up” the ladder as a whole. On the Darwinian view, only one part of a species evolves into the next (and there’s no “next step” – a species evolves into whatever suits the local conditions of the population it evolves from; it may be bigger brained or smaller brained, or for that matter bigger or smaller). The rest of the species remains. So we end up with an increase in the diversity of life, which is, I think, the single most important point Darwin ever made. Monkeys remain because we are monkeys, and so are chimps, orangs, and all those other primates. All of them remain because they evolved by the multiplication of taxa.