Question from student answered by email, Aug. 23, 2001.

Dr. Eernisse,

I have [another class] during your office hours so unfortunately I can't make it. Hopefully you can help me via email!
I was reading chapter ten, which is very intense, I must say! Very interesting though. I got confused on page 203, right hand column where it says, "To the cladist, however, the statement that humans evolved from apes says essentially that humans evolved from something that they are not..." I can't sort it out enough to answer review question # 7.

Thanks for your time!

[a very sharp student]

[Dear sharp student],

I think I can help you on that one. To a cladist, humans ARE apes, just like they are primates, and mammals, and amniotes, and tetrapods, and "bony fishes" (Osteichthyes), and vertebrates, and chordates, and metazoans, and eukaryotes, and so on back to members in the clade of all life on Earth.

The point is one of the relationship between a whole and its parts. One cannot say that humans evolved FROM apes because they are part of the ape clade. It is like characterizing two groups, humans and mammals. That makes no sense because humans ARE mammals. For reasons that are obvious, though, many people have a harder time thinking of themselves as apes, and often mistakenly characterize evolution as that theory that says that humans evolved from chimps. To a cladist, humans and chimps share an ape common ancestor and, according to the currently accepted phylogenetic hypothesis (which the evolutionary taxonomists also accept), that ancestor is a more recent ancestor than the more ancient ancestor that humans and all species of chimps share with gorillas. So humans and chimps are equally closely related to gorillas because our ancestry passes through that common ancestral population to get to gorillas.

To an evolutionary taxonomist, humans evolved FROM apes. They would prefer apes to be DEFINED (arbitrarily) as the common ancestor of all apes, and all its descendants, EXCEPT humans. They would remove humans from the ape grouping because they wish to emphasize the ecological or other differences (e.g., brain size) that humans have in comparison to all other members of the ape clade. By doing so, they have created a classical example of a paraphyletic group. Specifically, their "apes" or "Pongidae" is paraphyletic because it only includes "non-ape humanoids" as illustrated by Fig. 10-7, which unfortunately does not give the corresponding cladistic brackets (but see Table 10.1 on p. 198, or this cladogram).

To cladists, Hominidae replaces "Pongidae" plus the old more restricted Hominidae, so all great apes are in one family. The reason the name, Hominidae, is preferred over Pongidae for the combined group name is more esoteric, having to do with provisions of the formal zoological taxonomic code. The technical reason is that Hominidae was proposed earlier than the name Pongidae, so it has taxonomic priority, and Pongidae is rejected as a "junior synonym" (this is not important to study).

The parallel example given in class was "Reptilia" and "Aves" (birds) being accorded the same class rank in a conventional classification, recognizing the different "adaptive zone" that birds have moved into, relative to their reptilian ancestors. In contrast, the cladist would consider birds to be nested WITHIN reptiles or, more specifically, they are therapods, dinosaurs, archosaurs, diapsids, AND reptiles (or sauropsids) as illustrated if you look ahead to Fig. 29-3 on p. 585, and Fig. 28-2, p. 562. Unfortunately, our text is a mixture of cladistic and evolutionary taxonomic approaches. For example, see the classification on p. 577, which has "Suborder Therapoda" in the far right column and says that birds "are descended from this lineage" but then turn to p. 581 and see that birds are treated as "Class Aves." This is classic evolutionary taxonomy!

Thanks for asking,

Prof. Eernisse

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