The evolution of primate diet and dentition

Evolutionary process has lead to many changes among all types of animals, including changes to body size, mass, cranial capacity, and even changes to oxygen intake levels and reproductive behaviors. Evolution has also lead to changes in the diets of many mammals. These types of changes are evident in the number and types of teeth, as well as in the sexual dimorphism in teeth between males and females.

These changes in teeth have been well documented in the teeth of primates over the course of evolution. Diet changes are correlated with both the dentition and the type of digestive tracts seen in primates. Additionally, terrestrial monkeys as well as apes show sexual dimorphism in teeth between males and females. This paper will discuss changes in the dentition of primates throughout history, and will examine theories proposed to explain these changes.

The origins of primates have been examined extensively for the last several decades. Theories understood previously to be factual have been questioned in light of much new evidence, and this new evidence has been examined from a variety of different viewpoints (Ciochon, et al., 1). Since primates in the modern sense of the word emerged approximately 50 million years ago in Africa, we will begin our discussion at that point (Fleagel, et al., 3).

These primates, generally grouped into the suborder of Plesiadapiformes, are known only to modern scientists by tooth remains. According to fragments found in Africa, these early primates were mostly fruit and insect eating animals (Milton, 89). Their dental records show three incisors, one canine, four premolars, and three molars on both the top and bottom jaw, with blunt cusps (Bramblet, para. 2). In the late Paleocene primates, the structure of the molars and incisors began to alter, showing longer incisors and variable molars, suggesting the beginnings of dietary differences, including consumption of increased plant materials (Bramblet, para. 3).

These primates, according to most scientists, lived on plant foods such as fruits, leaves, gums, and plant stalks. Insects and eggs are believed to have made up the remainder of their dietary intake. It is reasonable, then, to assume that much of the protein for early primates was obtained from vegetable sources (Eaton, para. 3). Complex carbohydrate and simple carbohydrate intake would have been minimal, resulting in less contribution to dietary energy (Eaton, para. 3).

The next developmental stage of primates, those of the Eocene euprimates, achieved a slight grade of development, seen in the fossil dentition records (Bramblet, para. 9). These primate teeth had evolved with the shearing of the crests of the molar teeth. Such changes indicate a change in diet, to harder, more difficult foods, such as nuts and seeds (Eaton, 5).

The development of the anthropoids, both the Platyrrhines and the Catarrhines, show different evolutionary responses in terms of teeth formation. The Platyrrhines, believed to evolve from euprimates, appear to have retained the same dietary intake, as shown in their dentition structure. Their top and bottom jaws consisted of two incisors, one canine, three premolars, and three molars. The Old World anthropoids, on the other hand, lost their second premolar, and instead retained the third and fourth premolars, equating to a diet more in line with the first primates (Bramblet, para. 11).

It was during the Miocene and Pliocene eras that the anthropoids began to separate into the hominids and cercopithecoids. With this evolution came an apparent drastic change in diet, as seen by the dentition records. The consumption of harder and more abrasive foods occurred, including nuts, seeds, and other forms of vegetable fat, which increased the access to food energy. More importantly, animal foods, or meat, became an important part of the primate diet. This is evident in decreased molar size and alterations in incisor shape (Easton, para. 5). Furthermore, thicker molar enamel became apparent, as did increased tooth wear and more dull molar cusps (Bramblet, para. 13).

These changes indicate a strong change in diet. Molars with sharper cusps are highly effective for herbivores, in that they are useful for chopping leafy materials into pieces easier to digest. However, the dull molar cusps indicate a change to the crushing of hard foods. Increased tooth wear also shows a diet of more abrasive materials. The thicker enamel and unique molar shapes of the terrestrial cercopithecoids would have been ideal for tearing highly sinewy or tough foods. All these changes point to a diet higher in meat (Bramblet, para. 14).

In the late Miocene and early Pliocene, early humans evolved from their cercopithecoid ancestors. The change was marked, again, through dentition records. Larger mesial teeth, thicker enamel, and a closer proximity of teeth to one another marked an even further shift toward meat eating (Bramblet, para. 16).

There are many theories to explain these changes in diet, as recorded through dentition records. According to Milton (1993), diet was almost solely responsible for the changes in evolution. According to her theory, modern monkeys make a concentrated effort to find specific foods, even when different foods are within easier reach. In watching a group of howler monkeys, Milton noted that the monkeys appeared to select specific foods based not on their ease of accessibility, but on their beneficial compounds. Many plants lack vitamins and proteins monkeys require, and still others lack enough carbohydrates for energy (Milton, 90).

However, in the howler monkey population, Milton noted that the monkeys appeared to combine food sources in order to obtain all the nutrients they needed. One type of fruit, rich in carbohydrates, may have lacked fiber or protein. The monkeys would then travel long distances to find plants to supplement the missing fibers or proteins. Milton noted that, had the monkeys simply eaten what was in the area, they would have been missing necessary amino acids (Milton, 91).

Milton theorized, then, that the monkeys evolved in teeth and digestion to allow and necessitate the ingestion of a large variety of foods, in order to ensure a balanced diet. Had the monkeys relied on one source of food, they would have had to travel much father to continuously locate the same plant material. Additionally, since some trees produce certain fruits only short periods in the year, the monkeys evolved in order to adapt to a continuously changing agricultural environment (Milton, 92).

Another theory of the dietary evolution of primates involves the increased capability of the primate in later eras to extract animal fats. According to Steffanson (1960), an increase of meat within the diet would have produced more animal proteins. This, in turn, would have allowed for the stature increase in evolution. However, this is probably a byproduct of the situation causing the increase in meat intake. Steffanson theorized that increased brain capacity, which allowed for the primitive use of tools, is responsible for the change in diet. With crude tools, early humans could have gained access to brain tissue or fat that would otherwise be inaccessible. Through this new tool, early primates, through scavenging and hunting, could have adapted over time to digest meant and animal proteins (Eaton, para. 7).

Still another theory of the evolution of diet is the changing climate of both the Old and New Worlds. The early Miocene era brought to Africa and Europe a period of unusual mild or warm climate. This climate, over time, became cooler, with more dry airflow. As the climate changed, the cercopithecoidea began to divert from their frugivore nature. The moist forests of prior times allowed for fruit and plant vegetation to grow wildly. However, as the cooler, dryer air began to drift through, many of this plant life altered. As a result, the primates were forced to evolve to allow for digestion of different food sources (Bramblet, para. 17).

Still another theory of why the diet change occurred in primates is nearly opposite that of Milton's. According the Leonard and Robertson (1992), the early Homo erectus altered their dietary intake in order to more easily obtain the necessary levels of caloric intake for larger body mass. According to their theory, as the body mass increased, so too did the need for calories to burn. Had primates remained frugivores, they would have had to gather food across very wide areas in order to maintain their body mass. With the addition of hunting and scavenging, primates could obtain the caloric intake needed in a much easier fashion (Leonard and Robertson, 191).

Additionally, Leonard and Robertson note that the evolution of the human brain may have had a great deal of influence on diet adaptation. Homo sapiens use far more energy in a resting state, according to the researchers, due to their larger brain. While the brain makes up only two and a half percent of total body mass, it accounts for almost 25% of resting metabolic needs (Leonard and Robertson, 180). Thus, the Homo erectus species would have had to increase their caloric intake immensely, simply to allow for the brain to continue developing.

In addition to general changes in dentition, researchers have also noted sexual dimorphism in tooth size in primate fossils (Plavcan, 27). In males, the canine teeth tend to be longer, and sharper. Additionally, there appears to be greater variation in size and shape of male canines, in comparison to the relatively stable size and shape of the female canine. Often, the size differences between male and female teeth are seen more in the height of the crown of the tooth than in the length or width of the lower portions of the tooth (Plavcan, 29).

Further dimorphism can be seen in the anterior surface of the canine teeth. Researchers note that, in males, there is a groove that runs the length of the tooth, whereas in females, this groove in generally absent. Additionally, even in cases where the female tooth does have this groove, it is much less pronounced than that of the male primate (Plavcan, 29).

The mandibular premolar also appears to show signs of dimorphism. In males, this tooth is longer, and overlaps with the canine teeth, acting as a sharpening tool. While this can also be true for females, the evidence suggests that, in females, the overlap appears to be a response to the male overlap, rather than a functional tool for sharpening the teeth (Plavcan, 30).

It is not just the size and shape of the canine teeth that shows sexual dimorphism in primates. Researchers have also found that the non-canine teeth of males are approximately seven to ten percent larger than those of the female primate. Furthermore, the density of the teeth in males tends to be far lower than that of females, generally due to the smaller jaw size of the female primate (Plavcan, 31).

Again, there are many theories as to the reasoning for the sexual dimorphism in tooth development in primates. One such theory relates to body mass. According to some, body mass is directly related to tooth size. Since the female primate tends to be much smaller in body mass than the male primate, it follows that the female would have smaller teeth in general. This relationship between body mass and dentition is easily explained, in that larger mass requires more energy to move. With larger teeth, male primates would be able to chew meat more effectively, thus equating to higher levels of protein, carbohydrate, and caloric intake (Plavcan, 32).