Humans are Apes are Primates. But will they remain so?

Question

Not a biologist here as you will propably be able to tell.

I understand that throughout the tree of life each species at the leaves at the same time also belongs to all the parent categories up the tree. So humans are apes and also are primates. The most frequent use of that fact in everyday discussion is in reply to the question "if humans evolved from apes, why are there still apes?". I also get why that question is silly.

I wonder, however, how biology would treat future evolution.

Assume that 10 million years from now our descendants are still around and have developed some trait that conflicts with the current distinction defining the branching of the taxonomic tree. Let them for instance grow some sort of tail again. Possibly to keep their hands free while still clutching some mobile device (I believe taillessness is one of the characteristics differentiating apes from other primates, if I am wrong about that please just substitute any trait that is).

Also assume that todays' other primates and apes are still around in that future.

Would we still say "homo futurensis are apes", even though they have tails?

Or would future taxonomists rearrange the tree and restate the distinctions? If so would that imply that looking back from that future to today us currently tailless humans would also not be apes after all but on a different branch together with h.futurensis? Is there a time-dimension to the layout of the tree of life? I don't mean corrections as we learn more. But changes due to evolution taking its course. The appearance of a new species forcing a remap.

Or would we just keep the tree and drop the tail-thing from the list of characteristics because there are enough other things that define the commonality? If so we could imagine even further evolution changing those traits too. What then?

Or does the new tail not pose a conflict in the first place because it would be something new. "Tail" being a naive name and the new appendage propably being called something else?

Or is the question plain silly, if so please explain why.

Thank you.

Answer 1

Yes, humans will remain members of apes and primates.

The terms "ape" and "primate" are common names representing certain phylogenetic clades. A clade is a group where all members are eachothers' closest relatives compared to members outside the group. Another way to put it is that a clade exists if there is a common ancestor whose descendants include all and only members of that group.

All of today's apes share a common ancestor to the exclusion of all non-apes. The descendants of today's apes will also share that ancestor*, and so will still be within the ape clade. The terms "ape" and "primate" are best thought of as expressions of relationships rather than clusters of traits.

Could humans evolve something very non-ape-like? For argument let's say humans evolved a tail. I suppose colloquially it could become common for people to use the term "ape" to refer only to the tailless apes, but anyone speaking of evolutionary relationships could still use it to refer to all the descendants of today's apes. Then it would just be a matter of clarifying which meaning is intended. Since "ape" is a common name, it might get used more flexibly, but the scientific term for apes, Hominoidea, certainly wouldn't change.

If humans evolved a tail they wouldn't cease to be apes, because "ape" isn't defined as "primates without tails," it really means "all the descendants of [this one ancestor]," which we generally recognize by their lack of tails.

The term "fish" is a similar example. If one calls a shark, a salmon, and a coelacanth all fish, then really humans should be included as well. Otherwise, "fish" stops being a biological term and just means something like "aquatic vertebrates that have gills**."

I'll finally add that it's fairly common for a lost trait to re-evolve, or for a new trait evolved in an ancestor to be missing from some descendants. Often, some additional observation will show that the re-evolved trait is actually different from the lost trait (think, for example of salmon fins vs. whale fins), but not necessarily. This can make it more difficult to accurately reconstruct relationships, but doesn't exclude those organisms from their clade.

---

*This is ignoring the possibility of hybridization, which is extrememly unlikely for ape/non-ape, but can be common in other groups.

**Mostly