Thursday, July 22, 2010

The Role of Chin

Image caption: Homo Erectus, Homo Floresiensis and Homo Sapiens skulls on display. Note the chin structures of the two most visible and complete skulls of the three.

The well-known ideas circling out there about the human chin are pretty much in general agreement about its potential adaptive nature, they are just not in unison about what that adaptive nature is. To get a glimpse of what these ideas entail, we'll take a preliminary look at a recently published journal by Zaneta M. Thayer and Seth D. Dobson, titled Sexual dimorphism in chin shape: Implications for adaptive hypotheses.

Here is the abstract:

The chin, or mentum osseum, is one of the most distinctive anatomical traits of modern humans. A variety of hypotheses for the adaptive value of the chin have been proposed, ranging from mechanical stress resistance to sexual selection via mate choice. While the sexual selection hypothesis predicts dimorphism in chin shape, most biomechanical hypotheses preclude it. Therefore determining the presence or absence of significant sexual dimorphism in chin shape provides a useful method for differentiating between various adaptive hypotheses; however, this has yet to be done due to a lack of quantitative data on chin shape. The goals of this study are therefore: (1) to introduce a new method for quantifying chin shape and (2) to determine the presence or absence of sexual dimorphism in chin shape in a diverse sample of modern humans. Samples were drawn from recent human skeletal collections representing nine geographic regions. Outlines of mentum osseum contours were quantified using elliptical Fourier function analysis (EFFA). Fourier coefficients were analyzed using principal components analysis (PCA). Sexual dimorphism in chin shape was assessed using PC loadings in the pooled geographic sample, and statistically significant differences were found. These findings provide the first quantitative, morphologically based evidence in support of adaptive hypotheses that predict dimorphism in chin shape, including the sexual selection hypothesis. - Abstract ends.

The effects of chewing and/or talking on chin development sound reasonable enough, but uncertainty about either theory lingers on. If we take the effects of chewing or mastication on the evolutionary development of the prominent chin for example, and if one assumes that the earliest anatomically modern humans had this trait [there is little reason to assume otherwise, given their taxonomic status as "anatomical modern humans"], then one might ask why the prominent chin could not have developed even before the emergence of anatomical modern humans. After all, like their predecessors, the earliest anatomically modern humans were primarily hunters/hunter-gatherers; wide-scale agricultural economy caught on relatively late within the context of modern human bio-history. Both types would have been subjected to more less similar masticatory stresses. To this end, the effects of cooking on mastication and the possible role of the chin in that scenario had been offered in some quarters as a possible matter of relevance to the development of human chin shape.

Though the issue of whether Homo Erectus produced "controlled" fire has been a matter of debate in some circles, there is an understanding that they did however produce and use fire. Having just discovered fire, what are odds that the discoverer will learn that it burns in very short time, and thereof, its usefulness as a "heater" against freezing temperature and as a tool that makes the prey more manageable to eat? Remember, the Homo Erectus were ruled out as creatures capable of assuming a sedentary lifestyle until recent findings suggested just that very thing, long before anatomically modern humans even existed. Certainly the phylogenetically younger Neanderthals used controlled fire, yet images at our disposal of their cranial remains generally do not feature prominent chin. The mastication theory weakens with those sort of things kept in mind while the speech-theory picks up the momentum. Speech is widely understood as uniquely human.

Floating in the internet, an extract from the study reads as follows...

The most common biomechanical explanation for the chin is that it acts as a buttress against masticatory stress. However, recent evidence suggests that this hypothesis is unlikely...

More recently Ichim et al. (2007) have suggested that speech production is associated with mechanical stresses acting on the mandibular symphysis due to tongue and orofacial muscle activity. Thus, they argue that the chin is an adaptive response to resist stresses caused by oblique contractions of the genioglossus muscles during speech. Computer simulations provide results that are consistent with the orofacial stress hypothesis (Ichim et al., 2007), but this hypothesis has yet to be fully tested.

One might reckon that the hominid predecessors of modern humans had not yet mastered the art of speech (language); they could well have grunted to one another in certain tonal variations or applied certain behavioral codes or body language to send their message, like other creatures in the animal kingdom resort to, but that's anyone's guess, short of speculation from cranial structure. Meanwhile, from the study,...

The speech hypothesis gives a different explanation, related to stresses due to activity of the tongue and mouth/face muscles. To produce speech, humans engage in an "un-natural" use of their mouth, and the chin can be viewed as an adaptation to handle the added stress produced by speech.

Some have raised the possibility that the Neanderthals likely spoke with one another as the modern humans do today, yet again if that were to be assessed from chin shape alone, then one would be inclined to argue against such a possibility, on the account that Neanderthal skull remains rarely have prominent chins. Neanderthal skulls are generally characterized by projecting faces with receding chins. There is nothing better than images in effectively illustrating this [click on all images for greater resolution]...

Image caption: Neanderthal compared with two modern human specimens.

Image caption: Neanderthal skull

Image caption: Cro-Magnon "The Old Man" specimen.

Take note of the differences between the chin profiles featured on the Neanderthal specimens and those of the designated Middle to Upper Paleolithic modern human specimens.

Continuing on with the issue of speech influence on chin development, the question of how the distinct phonological traits of languages of different cultures factor in here had cropped up before, but as the authors of the study note, it is the action of tongue and orofacial muscle that has prime bearing, which can pretty much be said of just about any speech regardless of culture. In other words, it is the action of the tongue, mouth and facial muscles accompanying speech in general that is the determining factor of the attainment of a well-defined chin in modern humans, i.e. due to the associated stresses brought to bear on the mandibular symphysis, rather than the phonological forms of language. Orofacial activity is an elementary feature for any speech.

A prominent or strongly defined chin has been part and parcel of cranio-facial record of modern human paleontology right from the earliest available specimens. Does this then call for the assumption that modern humans from the very onset of their being had mastered speech when their most immediate hominid ancestors on the other hand had not? This would present the event as a spontaneous miracle of some sort, rather than an evolutionary outgrowth of a succession of events as human social relations matured. If the single-event scenario seems too radical of an idea, then one is left with the possibility that phylogenetically immediate predecessors of modern humans must have mastered speech before the emergence of the latter [modern humans]. If so, then evidence is wanting, because it certainly is not that of a prominent or well-developed chin.

On the other hand, if we were to entertain the single-event scenario, then it would have to be treated as a spontaneous one—given the featuring of the chin from the earliest anatomically modern human records onwards, which contradicts the development of the chin as an artifact of evolution. That contradiction makes little sense in light of the speech-theory, as the theory proposes that the development of the chin was a response to the stresses that accommodate the action of speech, and henceforth, a relationship between the two. A spontaneous appearance of the chin concurrently with speech activity logically suggests chronologically convergent but independent events. It should also be pointed out that much of the existing languages of humans are determined to be no older than the Upper or late Paleolithic.

In contrast to the two common theories examined above, which both have to do with certain movements of the jaw and effect of respective stresses on the mandibular symphysis, another theory that has come out in an effort to explain away chin development in humans, is that revolving around sexual dimorphism. This is the primary argument presented by Zaneta M. Thayer and Seth D. Dobson. They note:

A less well-established adaptive hypothesis, but one worth considering, is that chin shape variation is a consequence of sexual selection (e.g., Hershkovitz, 1970). Psychological studies of facial attractiveness suggest that a ‘‘broad chin’’ in males is correlated with social dominance, which some females may prefer in a potential mate If the sexual selection holds then we expect not only for humans to have chins, but also men to have broader chins than women. By broader, we don't mean, of course, in absolute size, as men are generally bigger than women...

Our study provides the first quantitative evidence of sexual dimorphism in chin shape among a geographically diverse sample of modern humans. The presence of sexual dimorphism appears to refute mechanical explanations of the chin that preclude sexual dimorphism, such as the masticatory and orofacial stress hypotheses (e.g. Daegling, 1993; Ichim et al., 2007)....

The authors take issue with the "mechanical explanations" to the extent that they leave out the role of sexual dimorphism among the acting evolutionary forces on the development of chin. This notwithstanding, the authors run into the loose ends of their proposition. The reader is notified that:

While the presence of sexual dimorphism is consistent with the hypothesis that sexual selection influences variation in chin shape, the degree of overlap between males and females requires further explanation. It is safe to say that the male chin pales in comparison to the more exaggerated ornaments found in other animals, such as the large and colorful tail of the peacock (Pavo cristatus) (Petrie, 1991). The modest contrasts in male and female chin shape indicated by our data (see Fig. 4) do not seem to fit a runaway process of selection driven by female choice (Fisher, 1958). We suggest there are at least two possible explanations for this pattern.

* One hypothesis for why male chin shape is not more exaggerated is that some females may avoid mating with extremely aggressive males...

*Second, the large amount of overlap in male and female chin shape may be due to regional differences in chin shape dimorphism (see Fig. 6). Regional differences in the level of dimorphism would tend to inflate within- sex variance in the pooled human sample, thereby reducing the probability of finding between-sex differences.

The figure 6 in question, is given as follows:

Indeed, the "large amount of overlap in male and female chin shape" would tend to weaken the argument of sexual dimorphism, as the authors themselves correctly observe. Figure 6 above does a good job of illustrating this, with the gray/filled bars representing the female specimens, while the blank/white bars denote the male counterparts. The only male-female samples in the figure that don't overlap, and hence provide a pronounced indication of sexual dimorphism, are those from Southern Africa. Furthermore, the male-female samples of Southeast Asia and Eastern Africa illustrate complete between-gender overlap. It is these "within" region or population variations that compel the authors to offer the first "explanation", revolving around the comparatively low representation of "exaggerated" differences between the gender-grouped samples. The extension of this trend across samples from distinct regional entities presented cause to offer the second "explanation", which is essentially a complaint about the effects of pooling global samples respectively according to gender.

Now of course, neither explanation really offers relief from the apparent weakness of the authors' sexual selection and dimorphism theory. In the first case, all the populations involved have well-developed chins, regardless of any "between male and female" [presumably bi-modal sex-oriented] variations, i.e. whether there is pronounced between gender variations or not. As it relates to the issue of pooling samples from different regions, of course the act of doing so would inflate the within-gender variation, as overlaps between genders from individual regions coupled with the condition associated with the old genetic saying of "more similarities between populations than within" chime in to blur a clear pattern of between-gender differentiation. Additionally, there is no excuse for blaming the pooling of samples, as there is a logical remedy for that: considering  intra-regional between-gender variation, but the readers already know what that entailed!

The gist: Neither theory of the ones mentioned thus far appear to be sufficient on its own to explain the precise nature of the bio-evolutionary history and adaptive quality of the human chin. The development of the chin could very well have been influenced by a combination of the mechanical stresses on the mandibular symphysis through the frequent movement of the jaw both during mastication and speech, and the change in structure of the human face (mid-face) from a projecting one [as in the hominids and Neanderthals] to a flatter one. The latter seems to have been overlooked by many researchers but one that definitely deserves further examination, as the chin would have to adapt to the change in the structure of the face, not just to stresses caused by the movement of the jaw. A flatter mid-face also means that the chin is inclined to project out more. Although sexual dimorphism could figure into the shape of the chin, it does not look to be a prime factor in the development of the chin, and the findings in the study discussed above serve as a good example of that. Furthermore, the idea that sexual dimorphism could be observed in chin shape should not be surprising, since it would be in company with just about other elements of the human body, like say height, shape of the abdomen, shape of the thigh and the tibia regions of the leg, aspects of the cranium, the genital areas, and a whole host of other areas. By the way, by "chin", reference is obviously being made to the prominent chin profile of anatomically modern humans.

*Updates of content herein will be made as soon as new info come to attention.

— Zaneta M. Thayer and Seth D. Dobson, 2010, Sexual dimorphism in chin shape: Implications for adaptive hypotheses.

1 comment:

SC said...

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Arara Sabalu