Striding bipedalism is a key derived characteristic of the earliest hominids, and it became relevant after the divergence of chimpanzee and human lineages. (Bramble and Lieberman 2004) Bipedal gaits such as running and walking did not play a significant role in human evolution as researchers initially thought. There was a misconception about bipedalism, in which it enabled the early Homo Erectus to run faster in open habitats than its predecessors. (Bramble and Lieberman 2004) Contrarily, humans lack the speed of its quadrupedal counterpart, in which elite sprinters can only sustain a maximum speed of 10.2 m/s for less than 15 seconds. (Bramble and Lieberman 2004) In comparison, mammalian cursorial specialists such as horses can maintain a maximum galloping speed of 15-20 m/s for several minutes. (Bramble and Lieberman 2004) Researchers such as Bramble and Lieberman suggested a new hypothesis, in which endurance running might have contributed to human evolution. The null, in this case, would be that there is no significant difference between endurance running for both bipedal and quadrupedal individuals. Bramble and Lieberman took an in-depth approach to examine the physical demands posed by ER, which includes: skeletal strength, stabilization, thermoregulation, and energetics, to create a valid argument for their hypothesis.
While running, humans expose their skeletal system to higher stresses than walking. (Bramble and Lieberman 2004) When the foot collides with the ground, a shock wave is sent from the heel through the spine, then to the head. Compared to walking, running produces a peak vertical ground reaction force at heel strike that is approximately twice as high. (Bramble and Lieberman 2004) To compensate this force, humans can reduce these stresses to some degree through limb compliance and mid-foot striking, but have to disperse the impact forces within their bones and joints. (Bramble and Lieberman 2004) To lower joint stress is to enlarge joint surfaces, and spreading the force over larger areas. Compared to both Pan and Australopithecus, the genus Homo has a greater articular surface area relative to body mass in most joints of the lower body. (Bramble and Lieberman 2004)
Bipedal gaits are unsteady, but there are mechanisms responsible for both walking and running that help ensure stabilization and balance. The trunk and neck of runners are more inclined during running than walking, and as a result, it helps sprinters to lunge forward, especially at heel strike. Independent rotations within the trunk is a key factor in dynamic stabilization during running. While walking, one leg is on the ground, enabling the abductors and medial rotators of the stance hip to counteract the induced rotation of the trunk produced by the forward acceleration of the swing leg. (Bramble and Lieberman 2004)
Maintaining a stable body temperature is ideal for long-distance walking in hot environments. (Bramble and Lieberman 2004) Humans possess many derived features with regard to heat dissipation, which includes eccrine sweat glands for evapotranspiration, and reduced body hair. (Bramble and Lieberman 2004) Also while running, human distance runners breathe through their mouths, to release excess heat. These features helped the earliest hominids to dissipate heat and are essential for ER in hot environments.
Walking uses an ‘inverted pendulum’ motion, in which the center of mass peaks for a relatively long time during the stance phase. (Bramble and Lieberman 2004) The metabolic cost of transport or COT for walking is the same for all mammals, including human bipeds, where it is a U-shaped curve. But what differentiates humans from quadrupeds is the shape of the COT curve during running. When humans transition from walking to running, the COT curve is flat at ER speeds. At higher speeds, running becomes less costly than walking for humans because humans can utilize a mass-spring mechanism far different from other mammals. The tendons and ligaments in the leg stores strain energy during the support phase and release it as recoil in the propulsive phase, to improve ER for humans. Proposed by Ker, the elastic properties of the arch of the human foot play a significant role in storing energy. To support his claim, Ker and his research team devised an experiment on an amputated foot to find significance in energy stored inside the foot.
Large animals, including humans, save energy for running by using the flexible structures in their legs and feet. (Ker et. al 1987) Ker hypothesized that the arch of the foot also contributes to energy conservation and allows running to be more efficient. The null in this experiment indicates that there is no significant difference in energy storage in the arch of the foot. During running, when the foot is in stance phase, it is capable of storing strain energy and releasing it as an elastic recoil. (Ker et. al 1987) In his experiment, Ker wanted to know if the energy stored as strain energy in the arch of the foot would be enough to make running efficiently. To test his hypothesis, Ker sampled an amputated foot and compressed it between a load cell and an actuator. Two steel blocks were placed under the foot with a distance of 13 mm apart from each other, to adjust the horizontal movements involved in flattening the arch of the foot. (Ker et. al 1987). And a steel rod was placed inside the foot to represent the tibia and cut at an appropriate angle for stabilization of the load cell. (Ker et. al 1987) To imitate the forces equivalent to running at 4.5 m/s, a load of 6.4 kN was used and controlled throughout the experiment.
To measure the strain energy, Ker used a line graph that depicted the load on the y-axis and displacement on the x-axis. The graph shows a positive correlation between load and displacement. The graph clearly indicates the foot is capable of storing elastic energy and releasing it as an elastic recoil. Ker used a scatter plot to see the strain energy needed for a particular load. Using the line of best fit, a load of 6.4 kN would require 17 J of strain energy. (Ker et. al 1987) Lastly, Ker also obtained data to show the amount of strain energy the plantar aponeurosis, the long and short plantar ligaments, and the spring ligaments conserved. Although the graphs were clear and precise, the evaluation of the data is confusing. The author does not mention how to calculate strain energy from the graphs shown. But the data does indicate that the total energy accumulated in each stance phase of a 70 kg man running at 4.5 m/s is 100 J. (Ker et. al 1987) Therefore 17 J or more strain energy is present in the arch of the foot, 35 J in the Achilles tendon, and the rest stored in other tendons. (Ker et. al 1987) In contrast to Ker’s studies, Taylor and Roundtree looked into the energy consumption of both biped and quadruped locomotion while running.
Unlike Ker’s experiment, Taylor and Roundtree’s experiment looks at energy cost in a broader spectrum. Taylor and Roundtree conducted a study to compare bipedal and quadrupedal locomotion by measuring oxygen consumption in primates. Through recent measurements of energy cost in quadrupedal locomotion, Taylor and Roundtree predicted bipedalism individuals would exert more energy than quadrupeds. Their null hypothesis predicts there will be no significant difference in energy consumption in energy consumption between bipedal and quadrupedal locomotion. In this experiment, Taylor and Roundtree sampled two chimpanzees and two capuchin monkeys and trained them to run on either two or four legs on a treadmill, the control. Regardless of running on two or four legs, the result of this study shows both primates expend the same amount of energy. Based on this data, the relative energy cost of both bipeds and quadrupeds should not be an argument for the evolution of bipedalism.
From all the articles, endurance running had most likely contributed to human evolution. Researchers initially thought bipedal gaits was a result of human evolution. Studies from Bramble and Lieberman provided examples of thermoregulation, stabilization, skeletal strength, and energetics to prove endurance running was derived from the genus Homo and has contributed to the evolution of the human body. Ker’s experiment provided further evidence, in which the arch of the foot has significance in storing strain energy. From Ker’s analysis, the graphs clearly show the arch of the foot stores 17 J or more, which makes running more efficient for humans. The tests from Taylor and Roundtree proved that energy consumption was the same for bipeds and quadrupeds. Their conclusion reveals the insignificance of energy cost between quadrupeds and bipeds, and it should not result in the evolution of humans. Although the experiments were thoroughly handled, there are some key aspects missing in both studies. In the Taylor and Roundtree experiment, it would be more convincing to see different species to perform the same task than primates. In Ker’s experiment, adding more samples to the study and using a different experimental group would improve the accuracy of the experiment. The amputated foot gave promising results, but it hardly resembled an actual human foot, regarding elasticity. Human evolution is a topic with endless possibilities, but future research on opposable thumbs can be very promising because the earliest hominids and even present day humans rely on grip for everyday tasks, such as grabbing and climbing.