Comparative Studies: Anatomy and Genetics Chapter Eight Comparative Studies: Anatomy and Genetics

Locomotor Patterns Among Primates Primates exhibit a wide range of locomotor patterns. These include: vertical clinging and leaping arboreal and terrestrial quadrupedalism semi-brachiation true brachiation quadrumanous knuckle walking erect bipedalism

Adaptations for Suspensory Behavior Apes and humans share a number of anatomical features for suspensory behavior. The spine contains fewer vertebrae than in the monkey spine and is relatively inflexible. The scapula lies on the back and the head of the humerus points inward into a shallow socket permitting a high degree of rotation of the shoulder. Hand can be rotated palms up and palms down. Unlike humans, apes have arms longer than legs with elongated, curved fingers and toes. Both apes and humans lack a tail.

Adaptations for Erect Bipedalism In humans, the position of the skull on top of the spine and the development of the lumbar curve have resulted in a trunk balanced over the pelvis. The illium of the pelvis is short and broad. The leg is long and powerful and the thighs angle in towards the knee. The human foot is inflexible with an arch and a non-opposable big toe.

Comparative Anatomy of the Hand Primates are characterized by pentadacytlism and palms and fingertips that are covered with epidermal ridges. Most primates have grasping hands and feet. In Old World monkeys, apes and humans, the thumb can be directly opposed to the other fingers permitting a precision grip.

Comparative Anatomy of the Skull The skull articulates with the spine by the occipital condyles, which are located on the sides of the foramen magnum. The relative position of the foramen magnum is a reflection of locomotor pattern. The eyes of most primates are located on the front of the head, allowing for binocular vision. The lower part of the eye is supported on the side by the zygomatic arch. The eye is further supported by the postorbital bar. In anthropoids, a bony postorbital septum is found behind the eye, which connects the postorbital bar to the brain case, creating a complete eye socket.

The Evolution of the Brain The evolution of the primate brain is characterized by a general increase in brain size relative to body size. This increase is described by the encephalization quotient. In the course of primate evolution, the different areas associated with specific functions have become more clearly defined. The cortical areas associated with hand coordination are three times larger in human brains compared to ape brains, and the expansion of areas associated with language is larger.

Primate Dentition Mammals are characterized by heterodonty. There are four different kinds of teeth Incisors Canines Premolars Molars The types and numbers of teeth are designated in the dental formula. Different diets have different food processing needs and, therefore, different dental adaptations. .

Ape Dentition The incisors of the great apes are broad and spatula-like. The upper incisors are implanted at an angle. The canine is large and projecting. When the mouth closes, the canines interlock each fitting into the diastema of the opposite jaw. The front lower premolar is sectorial and hones against the upper canine. The dental arcade is U-shaped, with parallel tooth rows.

Modern Human Dentition The size of teeth has decreased. The canines are small; they do not project or interlock. The canines do not show sexual dimorphism. The teeth are arranged in a curved, or parabolic dental arcade and there is no diastema. The front lower premolars are not sectorial. Hominin teeth have relatively thick enamel.

The Jaw The human jaw is smaller and shorter relative to the skull than the ape jaw. In apes, the mandible is reinforced internally by a simian shelf. In humans, the evolution of a small jaw has resulted in a chin. The muscles that operate the jaw are also smaller in humans. In gorillas, the temporalis muscle is large and a sagittal crest has developed across the top of the skull, providing the necessary surface area for muscle attachment. Large jaw muscles are also associated with a robust and flared zygomatic arch.

Cytogenetics and Molecular Biology When we examine human and chimpanzee karyotypes, we can easily pair up each human chromosome with a chimpanzee counterpart. The only exception is human chromosome 2 which appears to have evolved from the fusion of two smaller chromosomes. Comparative studies are also made of proteins and DNA. The data derived from these studies can be used to develop phylogenetic trees which show evolutionary relationships. Studies have shown that humans and chimpanzees are very closely related on the genetic level, sharing at least 98.5% of their DNA.