Dentition and diet in early hominids

Early hominid teeth changed substantially over time. A number of fossil apes of the Middle and Late Miocene had a dental pattern featuring low-cusped, grinding molars with relatively thick enamel. In females of some species such as Ouranopithecus, Kenyapithecus wickeri, and Gigantopithecus, the canine teeth were small in size compared to living apes like chimpanzees. Living chimpanzees, bonobos, and gorillas differ from the pattern of these fossil apes, as they all share molar teeth with relatively thin enamel and high crowns, and large canines that project well beyond the incisors and premolars even in females. These substantial differences between living African apes and fossil Miocene apes make it unclear which pattern may be the ancestral condition for early hominids. But this pattern of diversity does suggest that the dental characteristics of hominoids tend to evolve readily in response to dietary changes.

By the time of their earliest known fossil representatives, hominids had established their own, unique dental adaptation. This pattern is present at the earliest clear hominid site, Lukeino (Senut et al. 2001), as well as at a number of Middle Awash localities including Asa Koma, dating to between 5.2 and 5.8 million years (Haile-Selassie 2001, Haile-Selassie et al. 2004) and in isolated mandibles from Lothagam and Tabarin, both dating to between 5 and 6 million years ago. The pattern includes molars that are similar in size and morphology to the teeth of late Miocene apes like Ouranopithecus. There has been some suggestion that these teeth may have varied in enamel thickness (Senut et al. 2001), but systematic comparisons have yet to be performed. The main distinguishing feature of early hominids is a reduction in the size and projection of the canine teeth, in both sexes. Although these canine teeth were reduced in size compared to apes, they still projected beyond the crowns of the neighboring teeth and interlocked with each other (Haile-Selassie et al. 2004). Ape upper canines, like those in living chimpanzees and fossil Ouranopithecus, have a sharpened edge resulting from wear against the lower P3. This pattern of wear is called honing. The earliest hominid canines are not only smaller in size, but tend to lack this kind of honing wear. Some of the canines were worn not on their back edge but instead on their tips, showing that they functioned more like incisors than like ape canines. This pattern of canine size and wear is also found at Toros-Menalla, and is the major piece of evidence that Sahelanthropus may be a hominid. The last fossils with dental characteristics similar to the earliest hominids come from Aramis, also from the Middle Awash region dating to between 4.5 and 4.3 million years ago (White et al. 1994, WoldeGabriel et al. 1994).

After the Aramis hominids there appears to have been a fairly strong change in the hominid dentition. The fossil samples from Kanapoi and Allia Bay, at the southern end of Lake Turkana, are slightly more recent than the Aramis hominids at between 4.1 and 3.8 million years ago. The important changes are in the molar teeth and the size and robusticity of the mandible. Compared to earlier hominids, the molar teeth are larger and have thicker enamel (Ward et al. 2000). The mandible, represented by KNM-KP 29281, is tall--well over twice the height of the molar roots inside the mandible--and like later hominids has a strong reinforcing bar behind its symphysis. However, unlike later hominids, the molar tooth rows are long and parallel, giving the mandibular and maxillary dentitions a very U-shaped occlusal configuration. The canine teeth are similar to those of earlier hominids in size and projection. Like earlier hominids, these canines did not have strong honing wear, but the adaptation to cutting against the lower third premolar was not entirely gone, as evidenced by the single-cusped P3 in the KNM-KP 29281 mandible (Ward et al. 2000).

The teeth from Laetoli, Maka, and Hadar appear to form a single series of continuous morphology spanning from 3.7 million to slightly less than 3 million years ago. The basic elements of the dental morphology of these hominids make up the core adaptation of one of the most successful and long-lasting hominid lineages. Over a dozen well-preserved mandibular pieces are preserved, including complete or near-complete mandibles from each of these three sites (White 1977, White et al. 2000, Kimbel et al. 1982). These mandibles are large and thick. They have a distinct buttress along the posterior side of the mandibular symphysis--at the center of the mandible--which is clearly visible in several of the mandibles that are broken at the midline.

<Figure 6.x Laetoli--Maka--Hadar dentition>
The canine teeth are reduced in this sample compared to earlier hominids. There are still large single canines--especially at the earlier sites of Laetoli and Maka--but these increasingly exhibit wear on the tip and project less beyond the other teeth than in earlier remains. In this sample there is rarely a gap, or diastema, between the canine and the incisors (White et al. 2000), and the canine often takes on an incisor-like function. Most other anthropoids have large canine teeth, and these teeth are often strongly sexually dimorphic. They are apparently sexually dimorphic in these early hominids as well, with strong differences in canine size between the larger and smaller mandibles. The large canines of most primates are not principally a dietary adaptation, but reflect the social aspects of directly fighting or communicating threats. The reduction of the canine teeth in early hominids likely indicates that these social interactions had changed.

One possibility is that social competition, particularly among males, may have reduced in intensity. Such a reduction in male competition is consistent with models of the evolution of bipedalism that involve greater parental investment and provisioning of offspring. On the other hand, competition may have remained strong but may have taken a form for which large canines were useless. For example, the development of weapons such as clubs or accurately thrown rocks would reduce the advantages of large canines. Likewise, the development of more effective vocal communication might reduce the impact of visual signals like the canine teeth. Amid these possibilities, the reasons for smaller canines in hominids remain uncertain, but are clearly linked to the evolution of other features such as bipedality and social complexity.

The most distinctive dental feature of these early hominids is the large size of their molar teeth. The earliest hominids had larger molars than chimpanzees or most Miocene apes. The molars of the Hadar hominids average nearly twice the occlusal area of the earliest hominid teeth. Unlike living humans or chimpanzees, these molars increase in size from the front of the mouth to the back, so that the entire tooth row is elongated. And their large size combined with the smaller size of the canines lead the tooth rows to have a more parabolic shape, diverging from each other further back in the mouth.

The premolars are large as well. The third mandibular premolars are sexually dimorphic. Males lack any trace of honing morphology in the P3, with the tooth more similar in orientation to the P4 and having two distinct cusps. Female specimens tend to have a single-cusp P3 that has a higher angle of rotation from the tooth row. Especially the fourth premolars are larger and more molar-like in function than in earlier hominids. In this way, the area of the postcanine teeth has been increased both by increasing the size of each tooth and by changing the function and form of the premolars.

With low cusps and thick enamel, the large postcanine teeth clearly are used for grinding. These teeth and the powerful jaws that contain them reflect a dietary concentration on lower-energy plant materials, at least during part of the year. The postcanine teeth of the Hadar hominids are perhaps three times as large relative to their body size than most humans, and over twice as large as in chimpanzees. Chimpanzees and humans both eat rather high-energy foods, such as fruits and meat. The large molars of early hominids indicate that such foods were probably eaten more rarely or were unavailable for large parts of the year.

Finally, the incisors are relatively large, possibly with a role in stripping plant material as in living apes.

Two samples from between 3.4 and 3.5 million years ago deviate from the pattern established by the Laetoli--Maka--Hadar sequence. One, from Bahr el Ghazal in central Chad, is not well dated but is likely around 3.5 million years old. The fossil is a partial mandible, preserving the front of the mandible anterior to the first molars, and including canines and premolars on both sides. Unlike other early hominid premolars, which typically have one or two roots, both the P3 and P4 of this specimen have three roots. This unusual feature, as well as the relatively vertical symphysis and relatively thin premolar enamel make this central African specimen stand out somewhat compared to contemporary fossils (Brunet et al. 1995). The other sample is the dental sample from Lomekwi. The teeth from this site, including those in the KNM-WT 40000 skull, have similar morphology and enamel thickness to teeth from other sites, but the sizes of the teeth are at or below the minimum size observed at Hadar or Laetoli (Leakey et al. 2001). Both of these sites have been suggested to represent separate species from the Laetoli--Maka--Hadar sequence as discussed below.


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