Human Origins: Wrong Answers to the Question

Anyone who's spent enough time discussing human origins is bound to have heard this old chestnut at some point:

If we're descended from apes, how come apes are still around?

A variant uses "monkeys" instead of apes.

In an attempt to defend the scientific explanation, people will often use one of two stock answers:

1. If you're descended from your parents, how come they're still around?
2. We're not descended from apes, but we share a common ancestor with them.

Number 1 has a valid point, but it's a bad analogy. Stem-humans originated around 5–7 million years ago, but there aren't any apes sitting around today that are that old. Our parents are still alive (or were not long ago), but our ape ancestors are long dead.

Number 2 is flat-out wrong. We are descended from apes. You could modify that sentence a bit, though, and get a true statement: "We're not descended from any living apes, but we share a common ancestors with them."

So what's the real answer?

The real answer is that "ape", as commonly used, is an unnatural term, and it obscures communication about the subject. Generally "ape" is used to refer to all hominoids except for humans. What's a hominoid? Let's spell that out in some detail:

Hominoidea
|--Hominidae
|  |--Pongo
|  |  |--P. abelii (Sumatran orangutans)
|  |  `--P. pygmaeus (Bornean orangutans)
|  `--Homininae (African hominids)
|     |--Gorilla
|     |  |--G. beringei (eastern gorillas)
|     |  `--G. gorilla (western gorillas)
|     `--Hominini
|        |--Homo sapiens (humans)
|        `--Pan
|           |--P. paniscus (bonobos)
|           `--P. troglodytes (common chimpanzees)
`--Hylobatidae (gibbons)
  |--Symphalangus syndactylus (siamangs)
  |--Hoolock
  |  |--H. hoolock (western hoolocks)
  |  `--H. leuconedys (eastern hoolocks)
  |--Nomascus
  |  |--N. concolor (western black crested gibbons)
  |  |--N. gabriellae (yellow-cheeked gibbons)
  |  |--N. leucogenys (white-cheeked crested gibbons)
  |  `--N. nasutus (western black crested gibbons)
  `--Hylobates
     |--H. agilis (agile gibbons)
     |--H. klossii (Kloss's gibbons)
     |--H. lar (lar gibbons)
     |--H. moloch (silvery gibbons)
     |--H. muelleri (Müller's gibbons)
     `--H. pileatus (pileated gibbons)

Cladogram of extant hominoids, to species level.

(Yes the scientific name of western gorillas really is Gorilla gorilla. And the western subspecies thereof is Gorilla gorilla gorilla!)

As you can see, hominoids form a big, bushy branch of the primate tree, and most of the extant twigs (i.e., species) are among the gibbons (although most living individual hominoids are human—nearly all of the other species shown here are threatened or endangered).

Among living hominoids, we humans are most closely related to chimpanzees (including bonobos). The second runner-up is gorillas, then orangutans, and then finally that big bushy mass of gibbons. These relationships reflect events in our ancestry: the human-chimp split (about 5–7 million years ago), the split from gorillas (about 8–10 million years ago), the split of African hominids and orangutans (about 12–15 million years ago), and the split of great apes and gibbons (about 16–21 million years ago). Each split stems from a population, a group of common ancestors which we share with some other living hominoid group. Those ancestors were hominoids and were not humans; therefore, they were apes.

But, looking at this tree, you can see how unnatural the designation "ape" is. What sense does it make to lump our closest living relatives (chimpanzees) in a group with much more distant relatives (e.g., gibbons)? It would make as much sense as if we invented a group that included all hominoids except for siamang giboons, or all hominoids except for gorillas, or all hominoids except for any randomly-selected group in that cladogram.

(This also points out some problems with the Planet of the Apes movies—chimpanzees would probably just see humans, gorillas, and orangutans as belonging to some group of non-chimp hominoids. And gorillas and orangutans would respectively exclude themselves from such a group, but include chimps. Come to think of it, only the orangutans would be thinking of a natural group.)

Really, the only way to make the term "ape" natural would be to include ourselves. Then we would not only be descended from apes—we would be apes.

The term "monkey" has a similar problem, since some monkeys are more closely related to us than they are to some other monkeys.

Haplorhini
|--Tarsius (tarsiers)
`--Simii (a.k.a. Anthropoidea)
  |--Catarrhini
  |  |--Hominoidea (apes, including humans)
  |  `--Cercopithecidae (Old World monkeys)
  |     |--Cercopithecinae (macaques, baboons, etc.)
  |     `--Colobinae (colobus monkeys, langurs, etc.)
  `--Platyrrhini (New World monkeys)
     |--Aotidae (owl monkeys)
     |--Atelidae (howler moneys, spider monkeys, etc.)
     |--Cebidae (marmosets, capuchins, etc.)
     `--Pithecidae (titis, uakaris, etc.)

Cladogram of dry-nosed primates (haplorrhines), to an arbitrary level of resolution.

A "monkey" is basically any simian that is not an ape (or human). To make it a natural term, we would have to include apes (including humans).

Should we use those terms differently, and consider ourselves apes and monkeys? It would be more honest, phylogenetically speaking. In the end, though, those are vernacular terms and people are free to use them however they want.