Teeth and Jaw: Structure and Mastication Muscles

Note: These are discussed together, as the same evolutionary pressures apply to both.
Hominid dental system is small relative to apes and has decreased in size over evolutionary time. The masticatory system of the great apes is larger than that of humans [Aiello and Dean 1990]. Garn and Leonard [1989 (see quote in the preceding section)], Milton [1987 (p. 106 quote earlier in this section)], and also Leonard and Robertson [1992] report that the dentition of our ancestors decreased from Australopithecus to Homo erectus, coincident with the development of stone tools and increasing consumption of meat (hence decreasing consumption of coarse vegetable foods).

Technology--stone tools--permitted our prehistoric ancestors to hunt, kill, and carve up for transport large animals. All of this was done via tools, obviating the need for the typical carnivore adaptations--strong jaws, sharp teeth, claws, powerful muscles for mastication, etc. Another technology--cooking--may have had a significant impact on human dentition as well.

Potential effect of primitive food processing technology. Brace et al. [1991] present a fascinating hypothesis. Analyzing the evidence from Neanderthal sites in an area referred to as Mousterian, circa 200,000 years ago, they note (p. 46):

[W]e repeat the observation that "The important thing to look to is not so much the food itself but what was done to it before it was eaten" (Brace, 1977:199). If that can be accepted, it should follow that the introduction of nondental food processing techniques should lead to changes in the forces of selection that had previously maintained the dentition.

As their analysis continues, they note the following:

The cooking of food (meat) in earth ovens can be dated back over 200,000 years ago.

The climate of the Mousterian area was cold, and prey animals would freeze solid in winter unless eaten immediately.

Cooking meat in an earth oven thaws out the frozen meat, and in hot weather may offset (within limits) the effects of meat spoilage.

Brace et al. [1991, p. 47] note:

Meat cooked in such a fashion can become quite tender indeed, and in such condition it requires less chewing to render it swallowable than would be the case if it remained uncooked. In turn, this should represent the relaxation of selection for maintaining teeth at the size level that can be seen throughout the Middle Pleistocene. The appearance of the earth oven in the archaeological record, then, should mark the time at which the dental reduction manifest in the Late Pleistocene had its beginning.

They then tie the further reduction in human dentition to the use of pottery, which allows preparation of soups (on which one can survive even if toothless), and allows fermentation as well.

Universal cultural/technological innovations can reduce/change selection pressures. Hence, Brace et al. [1991] correlate the (evolutionary) reduction in the size of human dentition to universal cultural innovations--cooking and food processing--which promoted survival and relaxed selection pressures that favor large, robust dentition. (This is an excellent example of the evolutionary behavior/culture feedback loop discussed in a previous section.) The end result of such selection pressure is our modern dentition, which is smaller and weaker than our prehistoric ancestors.
Milton's comment in Aiello and Wheeler [1995, p. 215] nicely summarizes the situation:

The reduced dentition of early humans indicates that technology had begun to intervene in human dietary behavior, in effect placing a buffer or barrier between human dental morphology and the human gut (and thus selection pressures) and foods consumed.

Appearance of modern human form corresponds with reduced dentition. Brace et al. [1991, p. 48] note that:

The emergence of "modern" human form was uniformly associated with dental reduction. The differences in tooth size that can be seen between the various living human populations, then, were the consequences of different amounts of reduction from the Middle Pleistocene condition. These in turn can then be associated with the different lengths of time that the forces of selection maintaining tooth size have been modified by "modern" Homo sapiens.

The above material supports the view that technology, in the basic form of stone tools and cooking, may have had a significant impact on the form and structure of human dentition--teeth--and jaw muscles. (Cooked food is softer, so it relaxes selection pressures that favor stronger jaw muscles.) Regarding this point, the comparative proofs of diet that fail to consider the impacts of technology and human feeding behavior are therefore incomplete and/or inaccurate.

Other selection pressures on head and oral features:
brain size, posture, and language

Selection pressures are multiple and competing rather than solitary. The above material suggests that changes in diet and food processing techniques relaxed selection pressures for "robust" dentition. However, such factors reflect but one set of selection pressures operating on the human mouth. The mouth is used for more than just eating, after all. Other factors that may impact the evolutionary morphology of the head, mouth, and oral systems include: brain size, consistently upright bipedal posture, and--extremely important--language.
Increasing brain size reduces space available for oral features. The increase in brain size, e.g., encephalization, increases the space required for the cranium (brain vault). This, coupled with the slight posture realignments required for bipedalism, may cause subtle but significant changes in the architecture of the head as a whole. The possibility of impacts from these is mentioned by Radinsky as cited in Hiiemae [1984]. Aiello and Dean [1990] are an indirect reference on this issue--see discussion regarding hominoid mandible and cranium--and consider that a shift in the center of gravity of the head (caused by increasing brain size and bipedal posture) could increase selection pressures for a smaller (lower-weight) mouth and oral systems.

The influence of language on the oral system. Another important selection pressure acting on the human mouth and oral systems was the development of language. As Milton [1987, p. 106] notes:

An innovation such as language could help to coordinate foraging activities and thereby greatly enhance foraging efficiency (see, e.g., Lancaster 1968, 1975).

Inasmuch as language greatly increased foraging efficiency, it increased survival and was an important selection pressure. Hiiemae [1984] points out that fully developed language requires a very flexible oral system--jaws, tongue, esophagus, etc. Thus it is no surprise that the human oral system emphasizes flexibility and is quite different from that of carnivores like cats or crocodiles, who have no need of language, and who lack tools and must use sharp teeth and claws instead. The human oral system reflects the evolutionary selection pressures on humans (and not the pressures acting on cats or crocodiles). Further, thanks to technology (tools and cooking) the human dentition was buffered to a certain extent from the diet's underlying content. That is, simple technology enables us to transform foods, as necessary, to better fit our dentition--and per Garn and Leonard [1989] and McArdle [1996], humans have the dentition of an omnivore that eats soft foods.
Selection pressures on humans due to language are unique. Hiiemae [1984] analyzes the process of chewing and swallowing in mammals, and finds that the process is identical--except in humans. There is a significant difference in the sequence of events in humans, and Hiiemae [1984] describes the difference as (p. 227) "a reflection of a fundamental change." Hiiemae [1984, p. 278] notes:

Man is the only mammal in which "communication" has become a dominant oropharyngeal activity. [Oropharyngeal refers to the area between the soft palate and the epiglottis.] Is it not possible that the single most important change in the evolution of the jaw apparatus in hominids has been the development of speech? To go further: many mammals can call (coyotes can "sing") but "speech" involves the exactly patterned modulation of the basic note emitted from the larynx. That patterning is produced by a change in the shape of the air space in the oral cavity and by use of a series of "stops" which involve the tongue, teeth, and lips.
[These considerations address] a much more fundamental and wide-ranging question--the relationship between the evolution of speech and the changes in both the anatomy and functional relationships of the structures developed to process food.

Design tradeoffs in evolution of the human oral system due to speech/language. The above suggests that the development of that unique human feature--language--may have required changes in morphology. Shea [1992] reports that the evolution of speech was associated with changes in the skull base and pharynx, both of which indirectly impact the jaw and dental systems. The uniqueness of the human oral system is explained by Lieberman [1992, pp. 134-135]:

The human supralaryngeal airway differs from that of any other adult mammal...
Air, liquids and solid food all use a common pathway through the pharynx. Humans are more liable than any other land animals to choke when they eat, because food can fall into the larynx and obstruct the pathway into the lungs...

Human adults are less efficient at chewing because the roof of the mouth and the lower jaw have been reduced compared with non-human primates and archaic hominids. This reduced palate and mandible crowd our teeth and may lead to infection because of impaction--a potentially fatal condition before modern medicine.

These deficiencies in the mouth of an adult human are offset by the increased phonetic range of the supralaryngeal airway.

Cziko [1995, p. 182] reiterates the same information as given above from Lieberman [1992], and adds the cogent comments:

But if the design of the human throat and mouth is far from optimal for eating and breathing, it is superbly suited for producing speech sounds... We thus see an interesting trade-off in the evolution of the throat and mouth, with safety and efficiency in eating and breathing sacrificed to a significant extent for the sake of speaking.

Linkage of language and brain development in evolution of the human oral system. Deacon [1992] notes the seamless integration of language into human nature, and suggests this indicates that language originated long ago. Cziko [1995, pp. 183, 185] mentions the likely language/brain evolution linkage:

The study of our vocal tract also provides hints concerning the evolution of our brain. Obviously, the throat and mouth would not have evolved the way they did to facilitate language production and comprehension while compromising eating and respiration if the brain had not been capable of producing and comprehending language.
The importance of language and the advantages it provides us in communicating, coordinating our activities, and thinking [we "think" in our native language] suggests that, in addition to being a product of our evolution, it also played a large part in shaping our evolution, particularly that of the brain.

The above quote suggests that the evolution of the mouth/oral systems were closely related to the evolution of the brain, via the feature of language.

Human oral system forged as an evolutionary compromise. Thus we observe that dietary changes and food processing are not the only selection pressures acting on the head, mouth, and oral systems. Our jaw, mouth, and throat are the result of a compromise between multiple selection pressures acting on those structures:

The structural impact of food pre-processing (stone tools, cooking),

The oral system flexibility required to support language, and

The subtle changes required to support increased encephalization and upright bipedal posture.

Thus we see that the "big picture" here is not so trivial or simplistic as the claims of the comparative proofs that "humans cannot eat meat because they lack claws, powerful jaws, and the sharp canine teeth of lions." It is a pity that people cling to such simplistic claims--real life is more complicated, but also far more interesting!

To summarize:

Humans do not have claws, razor-sharp teeth, or the other adaptations found in carnivores because from the very inception of the genus Homo we have used technology--at first, and at the least, in the form of stone tools--to serve the same functions as claws, sharp teeth, etc. This buffered humans from the selection pressures associated with the development of the features associated with carnivores.

Additional selection pressures on the human head came from encephalization (increasing brain size), bipedal posture, and the development of language (which required a flexible oral system).

The typical claims made in comparative proofs of diet that humans are not adapted to meat because of lack of claws, fangs, etc., are fallacious and based on the assumption that only one adaptation is possible for the function. In reality, from earliest beginnings, humans have employed adaptive behavior and technology to overcome the limits of morphology.