Craniofacial Form and Evolution

It's time to give my preliminary review of the evolution of craniofacial form I've observed over the past six weeks.

Again, I've been to Lexington, KY, Springfield, IL and now Tucson, AZ, collecting shape data from prehistoric crania and mandibles from a number of Native American populations. My research is designed to test the hypothesis that the shift from hunting and gathering to agricultural subsistence resulted in decreased robusticity in numerous aspects of the crania and mandible. This is known as the functional masticatory complex hypothesis. The FMCH posits that the causal mechanism behind this trend of decreasing craniofacial robusticity, or craniofacial gracilization, is the transition to a diet consisting of a higher proportion of soft, processed foods. Simply put, according the the FMCH, an individual needs to generate less bite force to chew corn meal as opposed to nuts, seeds or dried meat. As a result, an agricultural diet places lower demands on the bones and muscles of the face than does a hunting and gathering diet. Thus, with the shift to an agriculture-based diet, the cranium and mandible became less robust through evolutionary processes or through the regular, developmental responses of bone, or through both. "Robusticity" is a term that paleoanthropologists have struggled to define. Loosely, it includes aspects of bone thickness, overall size, as well as the presence of certain "robust" features, such as pronounced bony browridges.

On to the review. After each paragraph, I provide labeled drawings which identify the anatomical points referenced in the text.

In Lexington, I studied a prehistoric population of Archaic hunter-gatherers. During this period, we see definite development and growth of features throughout the facial region. This growth extends from the zygomatic arches on the sides of the face, inferiorly down the mandibular ramus to the gonion of the mandible, and forward from there to the zygomaxillary suture, the alveolar areas of the maxilla and mandible, the mental eminence which marks the most anterior aspect of the chin, and gnathion, the most posterior aspect of the chin. But this development should not be viewed on a feature-by-feature basis. Rather, it is spread across the entirety of the face and jaw.

Now to Springfield, where I collected data on a prehistoric population which, over millenia, transitioned from hunting and gathering to a subsistence economy heavily reliant on maize agriculture. Here, after a period of relative stability or even growth, we do observe the total loss of developed features in certain regions of the face and mandible. This loss seems to take place in an abrupt, step-wise fashion. If evolutionary processes are at work, I don't yet completely understand them. In any event, the first disappearances are in smaller spots--at the alveolar region at or near upper first adult molar, and a sort of "trimming" in the upper cheek region, at and around the zygomaxillary suture. This is followed by similar but more broadly occurring phenomena of loss along the lateral corpus and lower ramus of the mandible. Mostly unaffected, however are (1) the alveolar region from the midline of the face to the canines, (2) the midline region of the jaw from the alveolar margin (infrandentale), through the mental eminence, down to the most posterior aspect of the chin (gnathion), and (3) development along the zygomatic arch and mandibular condyles. All of this seems to be a series of transitional forms on the way toward final manifestation.

Finally, in Tucson, I am collecting data on a prehistoric population of well-established agriculturalists. Here, we see only one substantial change: the complete loss of features of the chin (from the mental eminence to gnathion), except for retention at infrandentale.

Below I provide photographs taken during my six weeks on the road. This chronological sequence demonstrates the physical reality of what I've described above. And while we may all have different opinions of the end result, from brilliant to, well, cheesy, I suggest that we have reached for the foreseeable future what we in the business refer to as an evolutionarily stable state.

Lexington, early stage . . .

Springfield, early stage. Note: ALL SKULLS (EXCEPT MINE) ARE PLASTIC CASTS. NO DISRESPECT TO THE DEAD.

Springfield, transitional forms

Tucson, final form!

Postscript: Yes, I took some of these pictures of myself. But there was a journalistic purpose to it, I guess.

I'm back

I had bad Indian food today at the University of Arizona food court. It made me think of my friend Nikhil, who hasn't called me back, which makes him a bad Indian. Where are you NM?

If summer 1969 was the "Summer of Love," . . .

. . . summer 2009 may go down as the Summer of "Hey, what are your plans this afternoon?"

Two, not one but two couples I know well eloped in the month of August. Each couple has already been together longer than most marriages last, which I think bodes well.

Congratulations all. And well done.

Confounded!

Damn cranial deformation. I can't use geometric morphometrics to analyze whether cranial robusticity diminished in populations that switched from hunting and gathering to agriculture if the agricultural populations engaged in the cultural practice of artificial, intentional cranial deformation. Look at these folks. They're all adults...

The crania are all from a Central Illinois prehistoric agricultural population. In the last picture, the lighter colored cranium on the right is not intentionally deformed, and provides a good comparison. Clearly, I can't use the posterior cranium data from the deformed individuals. The problem is, I might not be able to use mid-cranial data either. I'm not sure exactly how far forward on the cranium intentional posterior deformation has an effect. I suspect far enough that it could raise and widen the posterior aspect of the frontal bone, where I take several landmarks. Where else? Temporals? Probably. Maxillary? At least where they articulate with the temporals.

Cranial deformation could even affect the shape of the mandible: it could, for example, change the shape of the condyles, change overall shape by altering the angle of muscle attachments, or push the ramus (posterior portion of the mandible) farther forward.

Cranial deformation isn't present on all individuals in this Central Illinois population. But even that presents something of a problem. With the naked eye, it's not easy to tell the difference between an individual with a naturally broad, round cranium side-to-side (brachicephaly) and an individual with cranial deformation.

Like I said, not everyone in this population has artificial deformation, so this is not a total disaster. But it is very annoying.

Background reading:

http://www.clas.ufl.edu/users/nparr/index_files/Page398.htm