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The talus is more robust in bipeds in order to compensates for greater weight transmission during the foot strike. Chimpanzees have relatively small tali compared to the lager talus seen in humans. Australopithecus has a robust talus like that seen in modern humans.
The modern human thoracic vertebral section exhibits a convex or outwardly bulging curvature. This is one of the curves in the spine that positions the torso directly over the hip joints, enabling an erect posture for bipedal locomotion. Chimpanzees differ from modern humans in that they lack the convex curvature of the thoracic region.
Modern human thoracic vertebrae are characterized by short centra (bodies) and inferiorly-directed spinous process, which are positioned higher up on the centra. Chimpanzees have longer spinous and transverse processes when compared to modern humans. The chimpanzee spinous process projects dorsally compared to the downward direction of modern humans.
The inferior articular processes in human thoracic vertebrae are larger than the superior articular processes; the superior articular process in chimpanzees is more pronounced than the inferior articular process.
Lucy's thoracic vertebrae are a combination of chimpanzee and modern human features. Lucy's thoracic vertebral section exhibits the human-like convex curvature. Her thoracic vertebral bodies are intermediate in size between chimpanzees and humans. Her vertebrae have long spinous and transverse processes and the superior and inferior articular facets appear almost equal in size. The direction of the Lucy's spinous process is unclear. While not identical to modern humans, Lucy's thoracic vertebrae are more similar to modern humans than chimpanzees.
The first rib of modern humans and chimpanzees are distinctly different from one another. The modern human first rib has a univertebral articulation. meaning it only articulates with the first thoracic vertebrae (T1). In chimpanzees, the first rib has a bivertebral articulation, meaning that it articulates with both the last cervical (C7) and the first thoracic vertebrae (T1).
The morphological understanding of Lucy's first rib remains controversial. While some anthropologists suggest that Lucy's first rib is bivertebral, others argue Lucy has only a single vertebral facet similar to modern humans. Dual articulation of the first rib would correlate with some of Lucy's other Chimpanzee-like features of the upper limb. However, if Lucy's first rib is indeed univertebral, like modern humans, then this articulation would be an adaptation for upright posture and changing shoulder girdle morphology.
The chimpanzee pelvis is taller relative to its width and the iliac ala are more flat than seen in modern humans, lying roughly parallel with the plane of the back. In modern humans, the iliac alae are curved forward and flare more to the sides of the body. Lucy has the broader hips and the flaring iliac alae, which are all hallmarks of bipedal locomotion. However, the pelvis is wider in modern humans than in Lucy. The more widely spaced hip joints and the broader pelvic outlet is a critical adaptation that allows enough space for the larger brained babies of modern humans to pass through the birth canal. During the widening of the pelvic outlet, the hip joints (acetabulum) are moved further away from the body's center of gravity. This exerts more force on the femoral head during locomotion. An increase in the size of the femoral head helps to counteract these forces.
Lucy's ulna, like her radius, is more similar to a chimpanzee than a modern human. The ulnar tuberosity, of both Lucy and the chimpanzee, is relatively distal in comparison to the human ulna. This ape-like feature is indicative of the ability and proclivity for climbing trees.
Lucy's radius is more similar to a chimpanzee than a modern human. The neck of Lucy's radius is long and narrow, a feature typical of a chimpanzee's upper limb. The radius in modern humans has a relatively shorter neck compared to arboreal quadrupeds. Ape-like traits in the upper limb are an indication that Lucy was adept at arboreal activity as well as moving bipedally when on the ground.
In primates, the degree of phalangeal shaft curvature is directly related to the frequency of arboreal behavior. Curved fingers and toes aid in the ability to grasp onto a curved branch. Species that spend a great deal of time in suspensory behaviors have more curved phalangeal shafts, while species not habitually suspensory have relatively flat phalangeal shafts. When comparing human phalanges with those of chimpanzees, the shaft of the human phalanx is relatively straighter than a chimpanzee. Lucy shows an intermediate curvature between those of modern humans and chimpanzees, which suggest that Lucy was still engaged in some amount of arboreal behaviors.
Primates that move by quadrupedal locomotion have arms (forelimbs) and legs (hindlimbs) of equal length, while bipedal humans have longer legs than arms. In comparison, Lucy maintains relatively longer arms to her legs.
Lucy's intermembral index (i.e., radius length plus humerus length, divided by femur length plus tibia length) is 84.6. The mean of this measurement in humans is 71.8, indicating humans have shorter arms relative to their legs. The common Chimpanzee measurement is 106. In other words, Lucy appears to have intermediate measurements between modern humans and chimpanzees.
In humans, the lumbar vertebrae are much larger than those seen in the great apes. In addition, modern humans have five lumbar vertebrae while the apes typically have only four. An increase in the number of vertebrae enhances flexibility of the trunk, reducing the distance the hips must swivel forward during bipedal locomotion, and producing a more efficient stride. In addition, the human spinal column demonstrates a very distinct curve, known as the lumbar curvature. This helps to bring the body's center of gravity closer to the midline and above the feet. Also, the relatively larger size of the human lumbar vertebrae to that of apes indicates an adaptation to the greater amount of stress transferred through the trunk and into the hips.
Lucy's vertebrae articulate to form a distinct lumbar curve, as is seen in modern humans. The fragmentary remains also indicate that Australopithecus most likely had five or six lumbar vertebrae.
The size of the sacroiliac and lumbosacral joint surfaces are related to the amount of weight transmitted through the pelvis during locomotion. As humans walk, the pelvis must cope with the stress of weight transfer, while quadrupedal chimpanzees are able to alleviate some of this stress through the support of their upper limbs. Thus, the surface size of the sacroiliac and the lumbosacral joints in humans are much larger than that seen in chimpanzees. Lucy has comparatively larger sacroiliac and lumbosacral joint surfaces compared to chimpanzees, as expected of bipeds.
Additionally, the angle of curvature of the sacrum reflects that of the lumbar curvature, and is more pronounced in humans while the chimpanzee sacrum is straight. Lucy's sacrum shows some curvature, but is not as pronounced as in modern humans.
The condyles of the distal femur are smaller and more round in primates, but are more elliptical and larger in humans. Lucy's condyles are large as seen in bipeds. The relative size of the condyle is an adaptation which supports the leg during locomotion. The larger condyles help to support the leg due to the increased weight transfers of bipedal locomotion.
When held in the anatomical position, the femur of an ape will stand almost vertical within a horizontal plane. In humans, the femur will form a bicondylar angle (i.e., the angle at which the femur lies to the midline of the body), and brings the knees closer together (valgus knee). Because of this angle, the feet fall directly below the center of gravity, balancing the body over the stance leg during the stride cycle, thereby stabilizing bipedal locomotion. Lucy's femur exhibits a bicondylar angle.
The size of the femoral head is related to body mass. The large femoral head of modern humans reflects their larger body size. Comparing Lucy with modern humans, Lucy has a smaller femoral head and a relatively longer femoral neck. Reduction of the femoral neck length increased the amount of stress the femur was able to support, and may have been a specialization due to a change in bipedal posture or an adaptation for the larger body size of later humans.
The chimpanzee pelvis is taller relative to its width and the iliac ala are more flat than seen in modern humans, lying roughly parallel with the plane of the back. In modern humans, the iliac alae are curved forward and flare more to the sides of the body. Lucy has the broader hips and the flaring iliac alae, all hallmarks of bipedal locomotion. However, the pelvis is wider in modern humans that in Lucy. The more widely spaced hip joints and the broader pelvic outlet is a critical adaptation that allows enough space for the larger brained babies of modern humans to pass through the birth canal. During the widening of the pelvic outlet, the hip joints (acetabulum) are moved further away from the body's center of gravity. This exerts more force on the femoral head during locomotion. An increase in the size of the femoral head helps to counteract these forces.
In bipeds, there is a right angle between the tibial shaft and its proximal surface. In modern humans, the medial and lateral proximal articular condyles are more similar in size, more concave, and more elliptical in shape compared to quadrupeds which tend to be more spherical and more convex. The size of the lateral proximal condyle is larger in bipeds to adapt to increased weight transfer from the femur to the foot. The elliptical shape helps lock the knee in place and create a straighter forward leg movement.
The articular surface at the distal end of the tibia is oriented more inferiorly than the antero-inferior orientation seen in quadrupeds, resulting in a straighter foot path during walking.
Chimpanzees retain smaller lateral proximal condyles and the shaft lies at more of an angle to the proximal surface of the tibia. The australopithecine tibia has a nearly angle between the tibia's shaft and proximal surface, and the distal end is oriented more inferiorly, as seen in modern humans.
The most notable difference in the teeth of modern humans and modern chimpanzees is that humans have smaller canines. Compared to chimpanzees, the canines of A. afarensis are also relatively smaller, but are still relatively large compared to modern humans.
A. afarensis incisors are more similar to the size and shape of modern chimpanzees. Molars in A. afarensis are relatively and absolutely larger than molars in either chimpanzees or modern humans, and are wider bucco-lingually.
Tooth enamel is thicker in A. afarensis than in either modern species.
The glenoid cavity of Lucy's scapula, the surface of the scapula that articulates with the humerus to form the shoulder joint, faces more cranially/superiorly than in modern humans. A more superior orientation of the glenoid cavity is similar to the Chimpanzee morphology, and is a feature seen in many arboreal quadrupeds. Lucy's retention of this trait suggests that she may have continued to practice suspensory behaviors while also being bipedal.
This page compares elements of Lucy's skeleton against the same elements of a chimpanzee and a modern human. To compare chimpanzees to modern humans click on the eSkeletons link below.
Click Here for full instructions on using the Compare Lucy page.