Teeth

Because tooth enamel is made of sturdy material (hydroxyapatite), teeth make good candidates for fossilization. As a consequence, teeth are the most common part of hominin skeletons that are discovered and collected.

Teeth are distinguishable by species because their shape and size are linked to body size, diet (for shearing, cutting, crushing, grinding) and social behavior (long, sharp canines for mate competition). Differences in the number, size, shape, cusp patterning, and placement of the teeth provide a means of identifying ancestral and related species, and tracking dietary evolution.

If you smile at a mirror it is easy to see that there are different types of teeth in the human mouth. The cutting incisors in the front are like flat spades, the canines resemble small fangs or extra incisors, the premolars (what some dentists calls "bicuspids"), and the molars are bumpy, multipurpose grinding implements. This condition of having a diverse dental toolkit is called heterodonty. The extreme opposite condition, called homodonty, is seen in the jaws of a crocodile where all the teeth are sharp cones used for gripping and tearing.

All primates are heterodontic, but they can be distinguished from one another by the particular dental patterns they display, and the specific shapes and sizes of their teeth.

Humans have generalized dentition indicating omnivory like suids (pigs and their ancestors). In fact it sometimes takes a well-trained paleontologist to distinguish between a fossil suid and a fossil hominin molar if it has been broken or worn down through use in life.

During hominin evolution the canine teeth became increasingly smaller. Overall, humans have small teeth for the size of their head and body and have very little dental sexual dimorphism compared to apes and monkeys. Early hominins get smaller and smaller canines before evidence for meat-eating shows up (around 2 Mya) and this trend indicates that social changes were probably the prime mover for canine reduction, not dietary ones. As canine size lost traction in the mate competition game, either (1) mate competition decreased like in gibbons which form monogamous pair bonds, or (2) other forms of mate competition took hold that had little to do with tooth weaponry.

The biomechanics of teeth and the surrounding masticatory (chewing) apparatus are usually correlated with the types of food being processed. Paranthropus had large, flat, thick-enameled molars and an accompanying suite of strong masticatory adaptations in the bones of the skull (see Chapter 3). The teeth were clearly not built for shearing grass and seem to be adapted to handle tough plant foods such as hard seeds and nuts.

Individuals experience microscopic trauma to their teeth from food and these scars caused by food can be used to infer diet of fossil ho-minins. The pits and scratches on teeth caused by eating different types of foods can be linked to certain types of food based on size, intensity, frequency, and location (e.g., incisors versus molars). The first studies of microwear in the 1970s were performed with a scanning electron microscope (SEM), but thirty years later scanning laser microscopes are the preferred tools. Paranthropus teeth show microwear from hard, but not abrasive food, that was determined to be nuts or seeds with hard cases, supporting the nickname "Nutcracker Man" that Louis Leakey gave to the OH 5 Paranthropus boisei skull. --

Wisdom Teeth

The third molars are called "wisdom teeth" because they erupt at the onset of adulthood between eighteen and twenty-two years of age. Many people do not have large enough jaws to accommodate their third molars and they can become impacted, or stuck inside the jaw when it is time for them to erupt. People with access to dental care will often get their third molars removed to avoid pain, infection, and crowding of the teeth, which leads to malocclusion, more pain, and difficulty in chewing.

It has long been assumed that such problems are the result of finely processed soft foods overtaking the diets of many agriculture-based societies beginning about 10,000 years ago. Eating soft foods for an extended period during childhood does not stimulate the jaw to grow as large as in those who eat a tough or coarse diet, which causes bone to grow and to buttress itself against the forces.

However, one fossil skeleton from a site in France contradicts the agriculture theory for the wisdom teeth problem. The 15 Kya "Magdalenian Girl" from the Cap Blanc rock shelter had impacted wisdom teeth, but they went unnoticed (which is why she was called a "girl") until powerful x-ray imaging allowed scientists to see them inside their crypts in her jaw. Now the specimen has been aged to between twenty-five and thirty-five years at the time of her death and she should now be called the "Magdalenian Woman." Since she predates the earliest evidence for agriculture, soft processed foods were not necessarily to blame for her problem. However, it is possible that pre-agriculture humans were processing wild foods similar to the way later humans would soon start to process domesticated foods.

Further archaeological evidence will shed light on this in the future. --

TOOL USE

Humans are not the only animals to use tools. Sea otters, naked mole rats, and owls are just some of the other tool-users on the planet. Dolphin mothers teach their daughters to use sponges to protect their snouts from stinging creatures while foraging on the sea bottom. Within the primates, habitual tool use (like the use of hammers, probes, and scrapers made of rocks or vegetation) is mostly limited to orangutans, chimpanzees, bonobos, gorillas, and capuchin monkeys (New World monkeys). Gorillas and chimpanzees (especially from the Tai Forest in Cote D'Ivoire) crack open oil palm nuts by smashing them between two rocks with a "hammer and anvil" technique. Gorillas have been observed to use sticks and branches as crutches to cross water, and chimpanzees will use leaves as sponges to sop up drinking water.

There is a distinction, however, between using a found object as a tool and modifying an object for use as a tool, something very few animals do, aside from chimpanzees and humans. Chimpanzee females and their daughters will prepare a stick and then dip it into a termite mound to extract the nutritious insects. The brush-like tip of a termite fishing probe is made by pulling a plant stem through clenched molars, straightening it to a point by wetting it with saliva, and then carefully manipulating the fibers by hand or mouth. Sticks and branches are also used to kill much larger prey. Chimpanzees have been observed to use prepared branches as spears to kill bush babies (see the section on "Scavenging and Hunting" in this chapter).

Certainly early hominins were using plant materials as tools, but these artifacts do not preserve like stone tools do. Because chimpanzees do not manufacture stone tools in the wild—that is, they do not strike off bits with the intent to use the newly modified rocks—stone tool artifacts are attributed to the handicraft of our hominin ancestors. Even when capuchins and chimpanzees are taught to make stone tools in captivity, they fail to produce even the simplest Oldowan-like tools.

Cognitive power determines the ability to make stone tools but so does anatomy of the wrist and hand. There are a few hand and wrist bones of australopiths and they show derived features like a flexible wrist, a long thumb, and broad fingertips that are related to humanlike manipulation and the capability to produce stone tools. Surprisingly, evidence dealing with the enlarged gluteus maximus muscle in humans also contributes to our understanding of hominin tool use. Mary Marzke and her colleagues pointed out that an Australopithecus ilium from South Africa shows that the gluteus maximus muscle that attached to it in life was large and similar to that in humans. Relative to that in chimpanzees, the human gluteus maximus arrangement provides a mechanical advantage during tool use. Our gluteus maximus controls the rotation and movements of our trunk which keeps us balanced and stable when we are using tools, throwing objects, clubbing, digging, and basically doing anything that involves vigorous arm movements. Thus, it appears from the hands and the buttocks that, by 3 Mya, australopiths had the body for serious and habitual tool use. But again, bipedalism and tool use must be linked, because the gluteus maximus muscle is also important for running. Since

Figure 5.3 Acheulean hand axes and other tools literally litter the landscape at the early Pleistocene site of Olorgesailie located in the Great Rift Valley of Kenya. Photograph by Holly Dunsworth.

running (as opposed to walking) is a series of one-legged jumps, the gluteal muscles are highly engaged in the activity.

Stone tools are found in association with evidence for hominin meat-eating behavior. By the early Pleistocene there are sites like Olorgesailie, Kenya, that show how important stone tools had become. There are hundreds of hand axes and other stone tools scattered across the ground and associated with H. erectus remains nearby (Figure 5.3). The ubiquity of the hand axes and their consistency in shape in space and time (over 1 million years!) begs the question of their function. Certainly they served, as their name implies, as handheld axes for skinning and butchering animals. Plus, the flakes that came off of them as they were made would have been useful for cutting implements as well. But, they also would have been useful for digging up tubers or perhaps for digging for underground water sources. One hypothesis calls on their aerodynamic nature to postulate that they were launched at prey as "killer frisbees." It is easy to imagine such a scenario from the looks of Olorge-sailie.

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