Are humans descended from monkeys?

Are humans descended from monkeys? It’s an interesting question. Too many well-meaning scientifically-minded individuals are quick to reply with a defensive “No! We just have a common ancestor!”

But that’s not a very good answer. It ignores the fact that the question is ambiguous, and that the definition of “monkey” is ambiguous. Sometimes the answer could be yes.

Does the question mean: Are humans descended from a modern species of monkey? Then the answer is no.

Does the question mean: Was the most recent human ancestral species, that wasn’t some kind of human, a monkey? Then the answer is “No, not unless you classify apes as monkeys.”

Does the question mean: Was at least one human ancestral species a monkey? Then it depends on the definition of “monkey”. The rest of this post will assume this interpretation.

People might say “no” because they know too little. They know that the answer to “Are humans descended from X?” is false for most values of X, and they just assume it is false when X=monkeys.

Or they might say “no” because they know too much. They’re accustomed to a particular scientific definition of “monkey”, and they assume everyone else is, too. The following quote might come from a member of the latter group:

Once again, humans are not descended from monkeys.

— Richard Dawkins, The Greatest Show on Earth

Here’s a taxonomy diagram that includes all the monkey candidates:

Any group positioned inside another group is descended from the common ancestor of that outer group. So, all we need to do is identify the regions representing monkeys, color them, say, purple, and then check whether humans are inside a ring of purple. If so, we’re descended from monkeys. If not, we’re not.

The groups with the X-ed out pattern have no living members, that are not a member of some child group.

I am not claiming that this diagram accounts for every possible definition of “monkey”. One could define the term in such a way that some geometric region (i.e. a region excluding its child regions) contains both monkeys and non-monkeys. But it’s good enough for my purposes. Assume that each geometric region is either all monkey or all non-monkey.

There are two living groups of monkey-like creatures. The ones native to Africa and Asia are known as Old World monkeys. The ones native to South and Central America are known as New World monkeys. Evidently, the New World monkeys (Platyrrhini) sometimes not considered to be monkeys. Though I don’t know what we’re supposed to call them, if not monkeys.

The Old World monkeys are definitely monkeys, or at least some of them are. There seem to be at least four different definitions just of the term “Old World monkey”:

  1. Cercopithecidae, a narrow group that includes all the living Old World monkeys (excluding apes).
  2. Cercopithecoidea, a broader group that includes Cercopithecidae, and some of their extinct cousins.
  3. Catarrhini, a broader group that includes the apes.
  4. Catarrhini, but with a special rule to exclude the apes, and possibly some other (extinct) groups.

It’s fair to classify at least the Old World monkeys (let’s say
Cercopithecoidea) as monkeys:

If we include the New World monkeys, then at least these regions are monkeys:

If, additionally, monkeys evolved only once, then we have to connect the groups, going back to at least their most recent common ancestor, the Simiiformes:

And in this case humans are descended from monkeys.

Any way you slice it, if the answer to the titular question is no, then (using what I think is the most sensible interpretation of the question) at least one of these two surprising statements must be true:

  1. New World monkeys are not monkeys.
  2. Monkeys evolved at least twice, independently, from non-monkeys.

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