Are humans, apes, monkeys, primates, or hominoids?

Huxley apesI suspect the correct literary answer is that we are Yahoos, but here I want to do what I would ordinarily never dare do: disagree with John Hawks. John takes Jerry Coyne to task for calling humans “apes”:

Humans are hominoids. Hominoidea is a taxonomic group. Phylogenetic systematics holds that taxonomic groups should be monophyletic — meaning that they include all the descendants of one ancestor, and don’t leave any descendants out. Humans are closely related to chimpanzees and bonobos, more distantly to gorillas, then orangutans, then gibbons. All these living creatures are crown hominoids. The Hominoidea includes all these, together with extinct animals like AustralopithecusProconsulDryopithecus, and many others. Chimpanzees are apes. Gorillas are apes, as are bonobos, orangutans, and gibbons. We routinely differentiate the “great apes” from the “lesser apes”, where the latter are gibbons and siamangs. Humans are not apes. Humans are hominoids, and all hominoids are anthropoids. So are Old World monkeys like baboons and New World monkeys like marmosets. All of us anthropoids. But humans aren’t monkeys.

John is not alone in this argument. I have had it also with my coauthor and cladoclast Malte Ebach. Like Malte, John argues:

“Ape” is an English word. It is not a taxonomic term. English words do not need to be monophyletic. French, German, Russian, and other languages do not have to accord with English ways of splitting up animals. Taxonomy is international — everywhere, we recognize that humans are hominoids.

But there is a flaw in this argument, which John himself alluded to before these passages, quoting Coyne:

I believe it was William Jennings Bryan who denied during the Scopes trial that man was a mammal. That one statement laid him low, exposing his Bible-ridden ignorance for what it is. Of course we are mammals, and of course Richard [Dawkins] is an ape.

The word “mammal” is itself indicative of a process of language change. Prior to Linnaeus defining the term, it was not a part of any European language. In 1758 he defined Mammalia as a technical name and by the early years of the 19th century, the vernacular term “mammal” was used by everyone from the meanest uneducated worker through to the most educated palaeontologist. That’s what happens with languages. Experts introduce and revise terms that the folk pick up. Hilary Putnam called this “division of linguistic labor” “according to which such terms have their references fixed by the “experts” in the particular field of science to which the terms belong” [Wikipedia].

If terms like “dinosaur”, “mammal”, and pre-existing terms like “ape” (now not including Barbary Apes, because experts realised they weren’t apes) and “bird” (now not including bats, because experts, Linnaeus being among them, realised they weren’t birds) turn out to need revision, they get revised, and the usage filters down to vernacular use, sometimes. We even now know that whales aren’t “fish” (although, as I will argue, they actually are, and so are we).

Names are not, in themselves, natural facts. If what they denote are natural objects, then nomenclature is critical in classification; otherwise it really is a conventional matter. So the issue is not are the names right, the issue is whether or not they unambiguously denote facts about the world. And here another core problem arises. Vernacular terms like “ape”, “bird” and “tree” get a major revision by technical science. Some terms, like “mountain” or “stone” can be dissolved either into many technical terms, or spread ambiguously across terms of art in ways that make them scientifically meaningless. When the claim is made in the popular press that “birds are dinosaurs” or “humans are apes”, there is a vernacular sense in which this is simply false. Dino feather Every child knows that dinosaurs are flightless things with teeth and no fur that lived more than 65 million years ago (unless they are well educated into the arcane debates in paleontology; never underestimate a motivated ten year old), so how can birds be dinosaurs?

The answer is, of course, that modern phylogenies (classifications based on shared traits that are thought to be the result of evolutionary history) place the group Aves, the taxonomic name for birds, squarely inside theropods (a type of dinosaur). By the rules of technical biological nomenclature, then, birds (Aves) “are” (fall inside) dinosaurs (Dinosauria).

But this is not how vernacular classifications work. Folk taxonomy is hardly rigorous, and since the words often preceded the science, a degree of revision based on science is inevitable. The technical name “dinosaur” itself entered the English language after they were named in 1842 by Richard Owen. So the claim that birds are dinosaurs is a case of a folk taxonomic term (one that agrees more or less closely with scientific usage) being subsumed under a technical term.

The claim that humans are apes is less clear. In folk taxonomy, “ape” is a term that has no comparable scientific meaning. It basically means any primate that lacks a tail and is not human. “Human”, however, denotes a single and scientifically accepted species (or group of species), so here the claim is that the technical taxon falls within a prior folk taxonomic category interpreted scientifically. This is not new, of course, since Linnaeus famously placed humans (Homo) within the same genus as other apes, a classification that was later changed to reflect folk taxonomic preferences (by Blumenbach, and later Oken).

Now the claim is that humans (Homo sapiens) are apes (Homininae), which is a group defined as the African Great Apes. In short, it is a claim that humans are a species of African Great Ape (and therefore a member of Hominoidea, which includes gibbons and orangutans, also included among these apes). The issue is whether or not the taxon name denotes a natural group. And what counts as “natural” in taxonomy is that the group is monophyletic, or is all of the taxa that can be included without any not being included, in that group. Think of it as a Venn diagram:

Dinosaurs and birds

In the older system, Dinosaur meant the blue part. Now it means the whole outer circle. Some continue to speak about “non-avian dinosaurs” but that’s just a holdover from the past, and the Chinese feathered dinosaurs have undercut even the intuitions that made that worth doing. Here is the human/ape case:

Humans and apes

“Ape” has (once again) been redefined by the experts, and to make a rhetorically memorable point, some taxonomists say “humans are apes”, which is the vernacular way to say “members of Homo are members of Hominoidea” without turning off the aforementioned ten year olds. Any professional that continued to talk about “non-human apes” and meant “non-Homo Hominoidea” should be asked to justify why that is a group of interest, especially as new fossils continue to blur the intuitive lines that motivated the distinction.

It is not possible to stem the tide of linguistic change, as the Académie Française has found out repeatedly. If experts can redefine terms influentially, then there is nothing wrong with that so long as it doesn’t confuse the experts. Using paraphyletic terms (that is, group names that denote what is left of the group once a subset has been removed) is a Very Bad Idea that hangs on in science, but it need not hang on in folk usage. And there’s nothing wrong with saying “humans are apes”, because, on the best construal of what those terms denote, they are.

Neil Shubin’s excellent book Your Inner Fish makes a similar point. Where once a “fish” was anything that lived in water (including swans, geese, alligators and crocodiles, whales, and water snakes), it came to mean a vertebrate that had gills and fins and scales. Shubin shows how the Gnathostomes (jawed fishes) includes land vertebrates, including mammals and ultimately us, as well. Language can change…

See also my post “Is Brian Blessed a monkey or an ape?” where, I now recall, I have disagreed with John Hawks once before on this very issue.

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