We share many physical characteristics with other primates, because humans and other primates developed from a common ancestor. This ancestor, now extinct, lived an arboreal, or tree-dwelling, existence. The evolution of some primate traits into human traits came about much later.
Primate Traits One important characteristic of all primates is a complex and highly developed brain. Compared to other animals, primates have brains that are larger in relation to their overall body size.
Primates also have sophisticated eyes that distinguish minute details – an adaptation to the ancient dim forests. The keen vision of primates is due to the position of the eyes at the front of the face. This position produces stereoscopic vision, or the ability to perceive objects in three dimensions. Special eye cells called cones also contribute to primates’ keen vision. These cells distinguish color and enable the eye to see sharp images.
A third primate characteristic is a hand with five digits. These digits include an opposable thumb-that is, a thumb that can be positioned opposite the fingers to grasp branches and objects.
Long arms with flexible shoulders and wrist joints are another feature of primates. Two bones in the forearm enable primates to rotate their hands a full semicircle; shoulder joints enable them to move their arms in many directions. Together, these structures and the grasping hand permit primates to swing from branch to branch. Some primates are able to maintain an upright sitting or standing posture during certain activities such as feeding.
Primates also share the same four types of teeth – incisors and canines for tearing, and broad premolars and molars for grinding and chewing. Together these teeth enable primates to eat both plants and other animals.
The earliest human possessed so many ape-like characteristics that scientists sometimes have difficulty telling whether fossil bones are those of a primitive ape or a human. As evolution continued, however, humans developed the distinctive traits that characterize them as species.
The most important human feature is a brain larger that that of any other primate
Chimpanzees, for example, have a brain capacity of about 500 cm3 (30 cu.in.). Humans, however, have an average brain capacity of about 1,400 cm3 (85.5 cu.in.). The expansion of the human brain results in the vertical forehead typical of humans.
The ability to stand and walk upright under all conditions is another distinctly human trait made possible by several specially adapted structures. The pelvis, the girdle of bone that includes the hip bone, is wide and slightly curved. This permits it to support the upper part of the body. The broad rear of the pelvis provides a large area for anchoring the walking muscles. The S-shaped spine rising from the pelvis provides support and balance. The head sits erect at the top of the spine. Even the human foot is designed for standing and walking upright. Basically flat, is contains an arch for support. The large toe is not opposable, but lies parallel to the other toes. In this way the large toe is adapted for walking instead of grasping. More than any other characteristic, upright posture with the erect head creates the distinctly “human” appearance. This posture, with the eyes at a high level, enables humans to see distant objects.
Human teeth and jaws are also distinctive in size and shape. The canine teeth of monkeys, apes, and other primates are long and sharp. These canines are useful for tearing food. Human beings have smaller. More even teeth than other primates. Human canines are only slightly longer than incisors and are used to hold food as well as to tear it. The premolars and molars, the back teeth are specialized for chewing and grinding, are broader than they are in other primates. The human jaw is shaped like an arch, while the jaw of other primates has a rectangular shape.