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9.4: Derived Adaptations - Early Hominin Dentition

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    The Importance of Teeth

    Teeth are abundant in the fossil record, primarily because they are already highly mineralized as they are forming, far more so than even bone. Because of this, teeth preserve readily. And, because they preserve readily, they are well-studied and better understood than many skeletal elements. Even in the sparse hominin (and primate) fossil record, teeth are, in some cases, all we have.

    Teeth also reveal a lot about the individual from whom they came. We can tell what they evolved to eat, which other species they may be more closely related to, and even, to some extent, the level of sexual dimorphism, or general variability, within a given species. This is powerful information that can be contained in a single tooth. With a little more observation, the wearing patterns on a tooth can tell us about the diet of the individual in the weeks leading up to its death. Furthermore, the way in which a tooth is formed, and the timing of formation, can reveal information about changes in diet (or even mobility) over infancy and childhood, using isotopic analyses. When it comes to our earliest hominin relatives, this information is vital for understanding how they lived.

    The purpose of comparing different hominin species is to better understand the functional morphology as it applies to dentition. In this, we mean that the morphology of the teeth or masticatory system (which includes jaws) can reveal something about the way in which they were used and, therefore, the kinds of foods these hominins ate. When comparing the features of hominin groups, it is worth considering modern analogues (i.e., animals with which to compare) to make more appropriate assumptions about diet. In this way, hominin dentition is often compared with that of chimpanzees, gorillas (our two closest relatives), and/or modern humans.

    The most divergent group, however, is humans. Humans around the world have incredibly varied diets. Among hunter-gatherers, it can vary from a honey- and plant-rich diet, as seen in the Hadza in Tanzania, to a diet almost entirely reliant on animal fat and protein, as seen in Inuits in polar regions of the world. We are therefore considered generalists, more general than the largely frugivorous (fruit-eating) chimpanzee or the folivorous (foliage-eating) gorilla.

    One way in which all humans are similar is our reliance on the processing of our food. We cut up and tear meat with tools using our hands, instead of using our front teeth (incisors and canines). We smash and grind up hard seeds, instead of crushing them with our hind teeth (molars). This means that, unlike our ape relatives, we can rely more on developing tools to navigate our complex and varied diets. Our brain, therefore, is our primary masticatory organ. Evolutionarily, partially in response to our increased reliance on our hands and brain, our teeth have reduced in size and our faces are flatter, or more orthognathic. Similarly, a reduction in teeth and a more generalist dental morphology could also indicate an increase in softer and more variable foods, such as the inclusion of more meat. These trends begin early on in our evolution. The link has been made between some of the earliest evidence for stone tool manufacture, the earliest members of our genus, and the features that we associate with these specimens.

    General Dental Trends in Early Hominins

    Several trends are visible in the dentition of early hominins. However, worth noting is that all tend to have the same dental formula. The dental formula is a method to characterize how many of the different kinds of teeth are present in the mouth. Going from the most anterior (front) of the mouth, this includes the square, flat incisors; the pointy canines; the small, flatter premolars; and the larger hind molars. In many primates, from Old World monkeys to great apes, the typical dental formula is 2:1:2:3. This means that if we divide the mouth into quadrants, each should have two incisors, one canine, two premolars and three molars. In total that is eight teeth a quadrant, for a total of 32 teeth. In humans, this number can be variable. Unlike in other apes, it is not uncommon for people to have only two molars in one or more of their quadrants. One explanation for this is that, because of our processed foods, there are fewer dietary constraints—that is, less pressure to have many teeth for additional processing. Furthermore, with smaller mouths and faces, fewer teeth may be advantageous. All early hominins have the primitive condition shared with other great apes.

    The morphology of the individual teeth is where we see the most change. Among primates, large incisors are associated with food procurement or preparation (such as biting small fruits), while small incisors indicate a diet which may contain small seeds or leaves (where the preparation is primarily in the back of the mouth). Most hominins have relatively large, flat, vertically aligned incisors that occlude(touch) relatively well, forming a “bite.” This differs from, for instance, the orangutan, whose teeth stick out (i.e., are procumbent).

    While the teeth are often sensitive, evolutionarily speaking, with diet, the canines may be misleading in that regard. We tend to associate pointy, large canines with the ripping required for meat, and the reduction (or, in some animals, the absence) of canines as indicative of more herbivorous diets. In humans, our canines are often a similar size to our incisors and are therefore considered incisiform (Figure 9.9). However, our closest relatives all have very long, pointy canines, particularly on their upper dentition. This is true even for the gorilla, which lives almost exclusively on plants, as you have seen in previous chapters. The canines, in these instances, possibly indicate more about social structure and sexual dimorphism than diet.

    image12-3.pngFigure \(\PageIndex{1}\): In humans, our canines are often a similar size to our incisors.

    Early on in human evolution, we see a reduction in canine size. Sahelanthropus tchadensis and Orrorin tugenensis both have smaller canines than those in extant great apes, yet the canines are still larger and pointier than those in humans or more recent hominins. This implies strongly that, over evolutionary time, the need for display and dominance among males has reduced, as has our sexual dimorphism. In Ardipithecus ramidus, there is no obvious difference between male and female canine size, yet they are still slightly larger and pointier than in humans. This implies a less sexually dimorphic social structure in the earlier hominins relative to modern-day chimpanzees and gorillas.

    Along with a reduction in canine size is the reduction or elimination of a canine diastema: a gap between the teeth on the mandible that allows room for elongated teeth on the maxilla to “fit” in the mouth. Absence of a diastema is an excellent indication of a reduction in canine size. In animals with large canines (such as baboons), there is also often a , where the first premolar (also known as P3 for evolutionary reasons) is triangular in shape, “sharpened” by the extended canine from the upper dentition. Evidence for this is also seen in some of the early hominins such as Ardipithecus, for whom even though the canines are much smaller and almost the same height as the incisors, they are larger than those in more recent hominins.

    The hind dentition, such as the bicuspid (two cusped) premolars or the much larger molars, are also highly indicative of a generalist diet in hominins. Among the earliest hominins, the molars are larger than we see in our genus, increasing in size to the back of the mouth and angled in such a way from the much smaller anterior dentition as to give these hominins a parabolic (V-shaped) dental arch. This is opposed to our living relatives as well as some of the earliest hominins, such as Sahelanthropus, whose molars and premolars are relatively parallel between the left and right sides of the mouth, creating a U-shape.

    Among more recent early hominins, the molars are relatively large, larger than those in the earliest hominins and far larger than those in our own genus, Homo. Large, short molars with thick enamel allow these early cousins of ours to grind away at fibrous, coarse foods, such as sedges, which require plenty of chewing. This is further evidenced in the low cusps, or ridges, on the teeth, which are ideal for chewing. In our genus, the hind dentition is far smaller than in these early hominins. Our teeth also have medium-size cusps, which allow for both efficient grinding and tearing/shearing meats.

    Understanding the dental morphology has allowed researchers to extrapolate very specific behaviors of early hominins. It is worth noting that while teeth preserve well and are abundant, a slew of other morphological traits additionally provide evidence for many of these hypotheses. Yet there are some traits that are ambiguous. For instance, while there are definitely high levels of sexual dimorphism in Au. afarensis, which we will discuss in the next section, the canine teeth are reduced in size, implying that while canines may be useful indicators for sexual dimorphism, it is also worth noting other lines of evidence.

    Dental Trends in Early Hominins

    Trends among early hominins include a reduction in procumbency, reduced hind dentition (molars and premolars), a reduction in canine size (more incisiform with a lack of canine diastema and honing P3), flatter molar cusps, and thicker dental enamel. All early hominins have the primitive dental formula of 2:1:2:3. These trends are all consistent with a generalist diet, incorporating more fibrous foods.

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