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

Comparative Anatomy of Primate Locomotion Lomotor pattern is reflected in many aspects of the anatomy, including the relative length of the arms and legs, as seen in the intermembral index. In contrast with hominoids, monkeys carry their bodies parallel to the ground. The spine forms an arch supported by the limbs; the trunk is long and narrow. The hominoid back does not play an important role in locomotion. The body is vertical or semivertical to the ground. The back muscles are small and the spine is relatively inflexible. The positioning of the shoulder girdle also differs.

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.

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. Other features of the skull reflect the relative importance of the sense organs. 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.

Comparative Anatomy of the Skull Photographs by Dodie Stoneburner

Comparative Anatomy of the Skull (cont’d) Photographs by Dodie Stoneburner

The Evolution of the Primate 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. The neocortex covers the entire cerebrum in the Anthropoidea. Many convolutions increase its surface area. 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.

Erect Bipedalism and the Human Brain In the evolution of the human pelvis, a repositioning of the sacrum has created a complete bony ring through which the birth canal passes. The need to pass the large human brain through this opening has resulted in the human newborn having a brain less than 30 percent of its adult size. The brains of other animals are almost completely developed at birth. Thus the human infant is very dependent on others for an extended period.

Primate Dentition Mammals are characterized by heterodonty. There are four different kinds of teeth: incisors, canines, premolars and 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. Figure 11.17 from Primate Adaptations and Evolution by John Fleagle, copyright 1988, Elsevier Science (USA), reproduced by permission of the publisher. This material may not be reproduced in any form or by any means without the prior written permission of the publisher.

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 dental arcade is U-shaped, with parallel tooth rows. From Dentition of Living Primates by D. R. Swindler, p. 173. Copyright 1976, by permission of the publisher Academic Press, London.

Modern Hominid 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. Hominid teeth have relatively thick enamel. From Dentition of Living Primates by D. R. Swindler, p. 180. Copyright 1976, by permission of the publisher Academic Press, London.

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. This shelf rarely occurs in hominids. 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.

Molecular Biology and Cytogenetics 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 percent of their DNA.