CHAPTER 8: COMPARATIVE STUDIES: ANATOMY AND GENETICS
Primates display a variety of locomotor patterns including branch running and walking, ground running and walking, New World semibrachiation, Old World semibrachiation, vertical clinging
and leaping, true brachiation, quadrumanous locomotion, knuckle walking, and erect bipedalism. The primate skeleton is relatively generalized and has retained many traits of the generalized mammal.
The skeletons of living apes and humans are adapted to suspensory behavior. Some of the skeletal features needed for suspension include a short, broad trunk, fewer lumbar vertebrae in the spine, clavicles that extend backwards, a scapula that lies on the back of the trunk, a shallow socket on the scapula, and the ability to rotate the arm 160 degrees. The human skeleton has become specialized for erect bipedalism. Some modifications that made this possible include the development of the lumbar curve, changes in the shape and orientation of the pelvis, changes in the function of the gluteal muscles, elongation of the leg relative to the arm, and the evolution of a short, stout big toe and an arch of the foot.
One of the most significant characteristics of the hominins is the development of the hand as a fine instrument of manipulation. The primate hand has five fingers and fingernails and is covered with fine epidermal ridges. The thumbs of Old World monkeys, apes, and humans are opposable.
The skull is a very complex part of the skeleton. In primates, the occipital condyles are located underneath the skull. In humans, they are positioned almost directly in the center of the skull, achieving a good balance of the skull atop the spine. In general, the primate facial skeleton is reduced in relationship to the rest of the skull. The nasal apparatus is reduced in size and the eyes are encased in bony sockets located on the front of the skull.
Within the primate order, we see the progressive enlargement of the brain volume and development of the cerebral cortex. The human neocortex makes possible higher mental activities. The increase in the size of the human brain is reflected in the increased volume of the cranium, or brain case.
The primates have retained a fairly unspecialized tooth structure. The New World monkeys have three premolars per quadrant of the mouth while the Old World anthropoids have only two. General characteristics of hominin dentition are a reduction in tooth size, lack of a sectorial premolar, a parabolic dental arcade, lack of a diastema and projecting canine, vertical implantation of the incisors, early eruption of the canine, a differential wear pattern of the molars, and thick tooth enamel. The human jaw is shorter, relative to the skull, than is the ape jaw. The muscles that operate the jaw are smaller. The modern human skull lacks a sagittal crest and is characterized by small or absent brow ridges and a slender, nonflaring zygomatic arch.
Comparative analysis of human and nonhuman chromosomes have provided information for determining evolutionary relationships. Human and chimpanzee chromosomes can be easily paired up with the exception of human chromosome 2, which appears to have evolved from the fusion of two smaller chromosomes. By a comparison of protein and DNA molecules from various living species, the evolutionary closeness of these species can be estimated. From this data, phylogenetic trees can be drawn to illustrate the most probable evolutionary relationships.
