Skeletal changes due to erect posture and its implications. | Anthropology Optional for UPSC PDF Download

Introduction

• Throughout the history of primate evolution, humans evolved from non-human primates. This evolution occurred gradually, transitioning from terrestrial quadrupedalism (pronograde) to a semi-erect posture (clinograde), and ultimately to a fully erect posture (orthograde) with bipedal locomotion. This progression of locomotion is the basis for understanding the evolution from simians to humans.
• Humans are distinguished by their ability to stand upright and walk on two legs. We maintain an erect posture, with a vertical back, straight legs, and our arms and hands hanging freely at our sides.
• In contrast, our primate relatives have a different posture when standing. Their backs are sloped, their legs are bent, and their arms and hands hang down in front of their body. The development of an erect posture in humans has led to significant changes in our skeletal system. These anatomical modifications are more prominent in some parts of the skeleton than others, such as the skull, vertebral column, pelvic girdle, femur, and foot.

Changes in Pelvic Girdle due to assumption of Erect Posture

• During the evolution of an erect posture and bipedal gait, the pelvis experienced significant changes, more so than the vertebral column. As a result, the pelvis plays a crucial role in maintaining an upright position and transmitting the body's weight onto the hind legs.
• When an animal stands upright, with its spine nearly perpendicular to the ground, the pelvic girdle must support and distribute the entire body's weight. Consequently, the pelvis becomes flatter, wider, and lower, with the anterior or superior end spreading out into a funnel shape.
• The human pelvis has evolved in a mosaic pattern, with the ischium, ilium, and pubis bones developing at different rates. The ilium changed the most, becoming shorter, broader, and bent back on the ischium, resembling a fan shape. In contrast, the ischium experienced the least change and remains similar to that of living pongidae. The shortness of the human ilium positions the lower pelvis relative to trunk height differently than in other primates.
• Additionally, the broad sacral part of the human ilium aids in weight transmission. The ilio-sacral articular surface is positioned further above the acetabulum in other primates, meaning there is no bony structure opposite the acetabulum. In humans, the lower part of the sacrum lies opposite the acetabulum.
• The human pelvis also has a larger and thicker acetabular region than other primates, which helps support the body's weight on the femur. The acetabulum is situated in the middle of the ilium and ischium, unlike in pongids where it is located below.
• Lastly, the sacrum is shorter and broader in humans, and the pelvic inlet forms a wide, bottomless basin that supports the pelvic viscera.

Changes in Femur due to assumption of Erect Posture

• In humans, the femur, or thighbone, has undergone several modifications due to the adoption of an erect posture, as opposed to the quadrupedal or semi-erect postures of other primates. These alterations allow the femur to support the weight of the upper body and facilitate bipedal movement.
• Human legs are longer than arms, unlike other primates. As a result, the femur is longer and sturdier, allowing it to withstand the pressure of the body's weight. In fact, the femur is the longest, largest, and strongest single bone in the human body.
• The condyles, or rounded projections at the end of the femur, are larger in humans than in other primates. This is because the leg joints need to be strong and large to support the body's weight.
• The ridges for muscle attachment on the human femur are more distinct and differentiated than those of non-human primates. A notable example is the linea aspera, a rough ridge running along the posterior surface of the diaphysis. This ridge is the attachment point for the extensor muscles responsible for maintaining an erect posture and enabling bipedal walking.
• The human femur angles outward from the knee, rather than standing upright as seen in great apes. This carrying angle helps to position the knee under the body and closer to the line of action of the body's weight.
• The neck-shaft angle, or collo-diaphysial angle, is larger in humans compared to apes and other primates. This obtuse angle enables better support for heavier weight compared to a smaller angle.
• The popliteal surface, located on the back of the knee, is more concave in humans than in apes. This feature may also play a role in maintaining an erect posture. 

Changes in Foot due to assumption of Erect Posture

The transition to bipedal walking and an erect posture in humans has led to several significant anatomical changes in the hind limbs, transforming them from grasping organs to locomotory organs capable of supporting the body's weight. Here are some key modifications:

• The big toe and its supporting metatarsal bones have aligned with the long axis of the foot, preventing the toe from protruding and causing difficulties during ground walking. As a result, all the toes now form a straight line.
• Since the foot no longer functions as a grasping organ, the big toe has lost its opposability and has become shorter in comparison to the thumb.
• The four smaller toes have also undergone shortening and degeneration, as they are no longer utilized for grasping.
• The big toe is no longer separated from the other toes, and all the metatarsal bones are connected by a common ligament.
• When walking, the body's weight is transferred from the heel to the big toe. To accommodate this, the foot has developed transverse and longitudinal arches to help absorb shock.
• The calcaneum (heel bone) has become elongated and strengthened to facilitate the transmission of weight.

Additionally, the talus bone has enlarged and become wedge-shaped to aid in weight transfer to the big toe during walking. These are some of the notable anatomical changes observed in the pelvic girdle, femur, and foot of humans due to the adoption of an erect posture.

Conclusion

The adoption of an erect posture and bipedal locomotion in humans has led to significant anatomical changes in the skeletal system, particularly in the pelvic girdle, femur, and foot. These modifications have transformed the human body to better support and distribute weight, allowing for more efficient and stable movement on two legs. The evolution of these features distinguishes humans from other primates and highlights the importance of anatomical adaptations in the overall evolution of human locomotion.

Frequently Asked Questions (FAQs) for Skeletal changes due to erect posture and its implications

What are the significant changes in the human pelvic girdle due to the assumption of an erect posture?

The human pelvic girdle has evolved to become flatter, wider, and lower, with the ilium becoming shorter and broader. The sacrum is also shorter and broader, and the acetabular region is larger and thicker to support the body's weight on the femur.

How has the human femur adapted to the erect posture?

The human femur is longer and sturdier, with larger condyles to support the body's weight. It also features more distinct ridges for muscle attachment, an outward angling from the knee, a larger neck-shaft angle, and a more concave popliteal surface.

What are the main anatomical changes in the human foot due to the adoption of an erect posture?

The big toe has aligned with the long axis of the foot, and all the toes now form a straight line. The big toe, as well as the four smaller toes, have become shorter, and the metatarsal bones are connected by a common ligament. The foot has developed transverse and longitudinal arches, and the calcaneum has become elongated and strengthened.

How do the anatomical changes in the human pelvic girdle, femur, and foot contribute to bipedal walking and an erect posture?

The adaptations in the pelvic girdle, femur, and foot allow humans to support the body's weight more efficiently, maintain balance, and facilitate bipedal walking. The flatter and wider pelvis helps distribute weight, the stronger femur supports the upper body, and the modified foot structure aids in shock absorption and weight transfer during walking.