Difference Between Human Species And Mammals
Human beings are mammals. Generally speaking, mammals are four-limbed, active creatures with well-developed brains. Humans share many key physiological attributes with their fellow mammals, including warm blood, a four-chambered heart, and young that are born live and nourished on milk secreted from the mammary glands of the mother.
Human beings are also primates, members of the mammalian order that includes lemurs, tarsiers, monkeys, and apes. Primates are more or less flat-faced creatures with a large cranium (the part of the skull enclosing the brain), small nose, and forward-directed eyes.
One factor that sets humans apart from other primates, and other mammals, is their erect posture. Humans walk upright on two feet. This arrangement involves a variety of unique structural features. For one, the human skull is balanced neatly on the vertebral column, or the backbone. The hole through which the spinal column passes into the skull, called the foramen magnum, is set relatively far forward on the underside of the skull. In apes, by contrast, this opening is set farther back, so that the weight of the skull is concentrated, not on top of the backbone, but in front of it.
The curvature of the spine is also different in humans. The backbone of four-limbed animals, including the apes, forms a simple curving arch. Because of a human’s erect posture, the backbone is thrown into an S-shaped curve. The second curve—the small of the back—brings the body’s center of gravity directly above the hips so that the head and chest are balanced above the hip region. The fused hipbones, attached to the base of the spine, form a girdle that transmits the weight of the body to the legs. The bone of the heel gives a solid support to the foot. From the shoulder girdle, made up chiefly from the shoulder blade, the bones of the arms hang freely. Humans’ long legs allow them to have an increased stride length compared to that of apes. At the same time, humans’ low center of gravity reduces inertia, or drag, when they walk or run.
Another physical attribute that differentiates humans from other animals might seem ordinary, but its consequences have been profound indeed: It is the human hand. While fairly typical of a primate hand, with five straight and relatively short fingers, the human hand differs in many respects. The thumb is relatively long compared with the other fingers, and its range of movement is extensive. The pulp pad of the thumb can also be opposed to the pulps of all the other fingers—an ability that other primates do not have. The muscles of the hands, as well as those of the upper arm, permit not only a power grip for firmly grasping objects, but a precision grip as well, for more delicate work.
These highly developed manual skills—the ability to grasp and hold objects while using a great degree of precision, to make complicated tools, to write, paint, sculpt—all set humans apart. Indeed, the freeing of the hands made possible by an upright posture was an essential development in human evolution (which we will examine in the next chapter). Tool use, however, would be quite impossible without another key attribute—perhaps the most important attribute that separates humans from the other animal inhabitants of the planet: the human brain.