2.8 Systematic hierarchy
Activity 7
Timing: 0 hours 10 minutes
This clip builds on the idea that development recapitulates systematic hierarchy, by trying it out with the wrist bones of hominoids.
At the end of the clip, Dr. Patterson refers back to the Andrews and Martin diagram (Figure 7), illustrating the hominoid tree (‘Cladogram for the Hominoidea’. This figure is repeated below the clip.
Figure 8 Venn diagrams for the classification of the Homininae
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Dr. Colin Patterson
We can try out that idea with the wrist bones of hominoids. The first condition, in the egg and the very early embryo, is to have no bones or cartilages in the wrist - the skeleton hasn't yet started to develop.
The next stage is to develop nine carpals. And the final stage, in African apes and us, is to fuse two of them, leaving eight carpals. Given those three conditions, we could convert them into a Venn diagram, with each condition characterising a group. The group with no carpals is the whole of life, including plants and bacteria. The group with nine carpals happens to be mammals. And the group with eight is African apes and us, the subfamily Homininae.
Now the hominoids have another character that behaves like this, with ontogenetic or developmental evidence on transformation. The character is reduction of the tail. All adult apes have just a rudiment of a tail, but during embryonic life they develop a long tail, like all other vertebrates, and then it becomes reduced to a vestige by differential growth. So we can get another Venn diagram from the tail, and I've combined it with the carpal diagram. I'm sure you get the idea. We’re building up a picture of groups and sub-groups, and with just those two characters, the carpals and the tail, we could get a simple tree from the Venn diagram.
I mentioned one other feature shared by us and African apes, the frontal sinus, which is a space in the bones of the face developed during ontogeny, as an outgrowth or expansion of the ethmoid sinus. And again, we could use development from the general, absence of sinuses, to the more particular, presence of an ethmoid sinus, to the still more particular, presence of the frontal sinus. And so we could get another series of groups to add to the Venn diagram.
Now all this probably seems much too simplified and, in real life, characters are often much more difficult to sort out. There's a good example in the hominoid tree, where the chimp is linked to both the human and gorilla lines. This is a case where there are two conflicting sets of characters, the ones labelled 7a and 7b on the tree. Just to take a couple of examples, chimps are linked to us by having the premaxilla and maxilla fused in the adult. But chimps are linked to gorillas by having six vertebrae involved in the sacrum, instead of the five that we have and orangs have. And chimps and gorillas also share a number of features of the arm and hand, associated with their habit of walking on the knuckles.
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Figure 7 Cladogram for the Hominoidea, from Andrews, P. and Martin, L. (1987) Cladistic analysisof extant and fossil hominoids, J. Human Evol., 16, 101–118, Figure 3 (redrawn)