Chapter 5 – From Whence We Came: Primates

Mammals have arisen from reptilian stock. About 80 million years ago, prior to the mass extinction of reptilian dinosaurs and termination of the Mesozoic era, insectivorous mammals had diverged into many forms. Between 25 and 35 million years ago, about 30 million years after the close of the Mesozoic era, a group of these mammals had evolved certain characteristics that would influence further development toward the group currently referred to as primates. Some of the major changes in mammalian features that led to the primates include an increase in size and complexity of the brain, a broader diet, binocular vision, opposable thumbs and toes, and the development in most cases of thin nails in place of claws. Over time, the brain of these animals continued to increase in size and change in complexity. New types of nervous tissues developed as well. Their tails grew, some forming a fifth limb by being prehensile (New World monkeys). Others retained their tails but were unable to use them in a prehensile fashion (Old World monkeys). Some forms of primates later lost any external evidence of a tail (the Apes) but retained a tail remnant referred to as the coccyx. While primates were destined to disperse to many tropical locations in both the New and Old World, distinction between tailed monkeys and apes (nontailed primates) actually occurred in the Old World, and divergence between apes continued for many millions of years, continuing with the rise of humans. The entire story of the human animal's rise to stardom is a fascinating romp through natural selective changes that led them to become the most dominant animal of the Cenozoic Era and the most influential animal in geological history. Table 5.1 lists the major events and species that occurred during the appearance of chordates and what led to the rise of humans within the past few million years. Our species, often referred to as modern humans (Homo sapiens) has existed on Earth for about 200,000 years. Our dominant status has been accompanied by the retention of old and development of new behavioral expressions, sometimes with a serious divergence of an aberrant nature. It is clear that we are not totally the animals we came from. However, all organisms change over time, a characteristic feature of evolution which is never ending. To thoroughly understand our significance as the primate elite, the entire primate group is examined and discussed in terms of its various anatomical, behavioral, genetic, and phyletic characteristics. The mammalian brain which eventually became associated with primates was undergoing rapid evolutionary changes. In comparing plesiomorphic and apomorphic features of the mammalian brain, there are elements of the primate brains that had been present in reptiles, namely the parts associated with automatic functions. The medulla oblongata functions as a relay station between the spinal cord and other parts of the brain. Control centers for heart rate, blood pressure, and breathing are located there, as well as reflexes that control such functions as vomiting and coughing. Our state of consciousness, nerves that control facial expressions, eye movement, olfaction, and vision are influenced by the medulla as well. Nerves concerned with both fine and gross motor movements that extend from the cerebral cortex to the cerebellum pass through the pons. The cerebellum functions as a central control unit for motor activities, such as learning how to play a musical instrument, as well as helping to perfect such movements. Finally, it assists in making learned movements more automatic. Humans and all other mammals have retained this reptilian brain, as well as adding new sections of nervous tissues to the brain over the subsequent millions of years. The most outstanding new section of the human brain, the section that has increased most in size and complexity, is the forebrain. While there are a number of important functions of this section of the brain, the parts that are uniquely human are primarily parts of the cerebral hemispheres. These structures are subdivided into several separate lobes that are demarcated by gyri and deep fissures. The front of the human brain is associated with the integration of perception, a function that is coordinated with tissues at the back of the brain. The thalamus stands at the intersection between the two, helping humans to understand their sensory perception. As evolution progressed in primates, the brain continued to evolve at a rate so rapid that the process has come to be referred to as “the brain explosion.” Much of the change in brain evolution has occurred in the cortex, resulting in a quite new tissue referred to as the neocortex, the tissue that encapsulates underlying parts of the brain. In humans, it is about 2.5 sq. ft in area and less than an eighth of an inch thick. In adults, it contains about 100 billion neurons that are supported and nurtured by billions of glial cells. Over millions of years, the brain of primates had increased tremendously in size, although brain size and complexity had changed at different rates in different species. While orangutans are somewhat larger than humans in body size, for instance, their brains are 69% the size of the human brain. In addition, the surface of the human brain is more deeply fissured. There are also several types of neurons that appear to be unique to humans and higher primates. Examples are spindle cells that may integrate emotion with motor function and mirror neurons that may encode performance and ideas of action and abstract concepts. It was the continued development of the forebrain that led primates to higher cognitive levels, providing insight into new socially related survival strategies in the form of more complex group living. All primates live social lives in one way or another, and thus many of them band together to cooperate in searching for food, for child care, and for defensive reasons. Their social significance is not as clear in some of the early primate forms. We must not lose sight of the fact that agonistic behavior need not be expressed on a continuous basis. Once dominance is established, dominance and defense may only occasionally require subtle clues in short spurts. Thus, we would expect less agonistic behavior in an organism's day once the initial stress of dominance establishment is complete. While territorial disputes and agonistic behavior are commonly expressed in dominance establishment, they are often emphasized in characterizing primate social behavior. There are other behavioral features that may be just as important or possibly more important to understanding its true nature. Based on studies of 60 different primate species, 10% of their time was found to be spent in performing affiliative interactions (e.g., food sharing, grooming, playing, and huddling) and in establishing alliances. Only 1% of their time was spent in demonstrating aggressive interactions (fighting, visual and vocal threats, and subordinate behaviors). Thus, affiliative behavior has apparently increased along with brain size, being 5% higher in the great apes than in other primates. While agonistic behaviors are important in establishing a dominance hierarchy and group harmony, affiliative interactions form a strong basis for social cohesion in all primates as well. While development of the neocortex and frontal lobes have largely been responsible for the human animal's dominant status within the major ecosystems on the planet, genetic changes and social diversity have sometimes led to the development of abnormal behavioral expressions toward others of our male and female fellow humans, their offspring, and other biotic and abiotic components. To complete the chapter, primates are divided phyletically into prosimians and simians. Evolution of the groups is discussed, emphasis being put on simians. This group is further discussed in terms of its members: New World monkeys, Old World monkeys, and apes. Apes are further discussed in terms of the following: gibbons, orangutans, gorillas, chimpanzees, and bonobos. Most of the final discussion is about the human animal's closest primate relatives. A brief preview of human forms is provided.