Question

1. Why is it important for a hominid to have a more centralized to anterior foramen magnum position?

2. What does the shape of the pelvis indicate about its function in hominids?

3. What are some benefits of standing upright and moving as a biped rather than as a quadruped?

Answer 1

1. In orthograde (upright), bipedal humans have a cranium that rests atop the vertebral column: the anterior position of the foramen magnum helps to balance the mass of the head above the vertebrae, and its inferior orientation reflects the anatomical relationship between the cranium and vertebral column. In pronograde (with a body more parallel to the ground), quadrupeal apes have a cranium that projects anterior of the vertebral column, and thus the foramen magnum is positioned and oriented posteriorly.

The foramen magnum functions as a passage of the central nervous system through the skull connecting the brain with the spinal cord.

2.The shape of the lower half of the pelvis (along with soft tissues, including the pelvic floor muscles) dictates the shape of the birth canal and shows important differences between apes and modern humans related to neonatal head size.

The fossil record of the human pelvis reveals the selective priorities acting on hominin anatomy at different points in our evolutionary history, during which mechanical requirements for locomotion, childbirth and thermoregulation often conflicted. In our earliest upright ancestors, fundamental alterations of the pelvis compared with non-human primates facilitated bipedal walking. Further changes early in hominin evolution produced a platypelloid birth canal in a pelvis that was wide overall, with flaring ilia. This pelvic form was maintained over 3–4 Myr with only moderate changes in response to greater habitat diversity, changes in locomotor behaviour and increases in brain size. It was not until Homo sapiens evolved in Africa and the Middle East 200 000 years ago that the narrow anatomically modern pelvis with a more circular birth canal emerged. This major change appears to reflect selective pressures for further increases in neonatal brain size and for a narrow body shape associated with heat dissipation in warm environments. The advent of the modern birth canal, the shape and alignment of which require fetal rotation during birth, allowed the earliest members of our species to deal obstetrically with increases in encephalization while maintaining a narrow body to meet thermoregulatory demands and enhance locomotor performance.

3. Standing up on the hindlimbs allows a quadruped to strike and manipulate its opponent with the forelimbs over the same range of motion used during locomotion. Humans are capable of striking with substantially higher force and energy from bipedal than quadrupedal posture.

The bipedal posture does provide a performance advantage for striking with the forelimbs. The mating systems of great apes are characterized by intense male-male competition in which conflict is resolved through force or the threat of force. Great apes often fight from bipedal posture, striking with both the fore- and hindlimbs. These observations, plus the findings of this study, suggest that sexual selection contributed to the evolution of habitual bipedalism in hominins.

Aside from its energetic efficiency, bipedalism also has the advantages of raising the head, and therefore allowing a wider range of vision in a grassland environment, and of freeing the hands for carrying items or for tool use. Despite these advantages, bipedalism also has considerable disadvantages.