Bipedalism

Introduction

Human bipedalism can be defined as the constant utilization of alternating hindlimbs as a means for movement between two points, according to Fleagle in his 1998 book Primate Adaptation and Evolution. (Begun, p. 9)

Discussion

Among living primates, only humans are bipedal. It is not certain when this unique feature emerged, but it must have been before the 3.6 million-year-old Laetoli footprints were made. Although the prints were not made by completely modern feet, they are unequivocally the prints of bipeds. They are the impressions of feet that lacked a distinctive human rounded ball, or swelling, at the base of the great toe, that had no well-defined arch, and that retained ever so slightly divergent great toes. Somewhat later in time, the well-known 2.9 million-year-old Australopithecus afarensis Lucy fossil is the earliest human ancestor to display the clear skeletal hallmarks of bipedalism. Earlier fossils are either not yet described or lack the two most diagnostic parts, the pelvis and the distal (i.e., lower) femur. (Begun, p. 31)

The gait is a cycle (stride) that consists of a stance phase, midstance phase, and swing phase. The moment the heel strikes, the hip becomes flexed, and the knee is extended while the leg is laterally rotated. The adductor muscles then shift the body's weight over the supporting limb (midstance phase), and the opposite hip and knee are extended. During the increase in forward momentum, the ankle dorsiflexes, and the hip and knee pass the supporting leg. After toe-off, the weight passes the toe and hyperextends the hip joint. This motion makes human bipedality different from the bipedalism of any nonhuman primate. (Hewes, p. 710)

For example, when a chimpanzee uses bipedalism, the cycle differs in its phases due to a lack of full hip and knee extension. In addition, the knee and ankle joints do not pass the hip joint, and the femur does not have a bicondylar angle. Aiello and Dean also noted the absence of abductor muscles and a reverse of the pelvic tilt during the stance phase as additional differences. Altogether, the differences between human bipedality and nonhuman primate bipedality are fourfold in (1) the degree of spinal curvature, (2) pelvic configuration, (3) foot morphology, and (4) biomechanical modifications in related muscles, tendons, and ligaments. (McHenry, p. 187)

Analytical and Critical Thinking

The environment, serving as a selective force, may explain the diversity of primates within various biomes and consequently the various modes of locomotion exhibited by primate species, as Alison F. Richard suggested in 1985 in Primates in Nature. The role of the environment acting as a selective pressure is widely recognized as a major factor (Kingston, 2007). Based on multiple analyses of marine isotopes, ice cores, sediments, loess sequences, pollen sequences, and stable isotopes, environmental conditions have varied throughout the world, including in Africa. Locations at hominin and hominid sites in Africa indicate a mixture of open woodlands, wooded savannas, grassy plains, and closed vegetation. Fluctuations in aridity and environmental changes during the Miocene to the Pleistocene are caused by various factors. Milankovitch cycling, plate tectonics, glacial and interglacial periods, the Messianian salinity crisis, ...