Femoral neck structure and function in early hominins

Abstract

All early (Pliocene-Early Pleistocene) hominins exhibit some differences in proximal femoral morphology from modern humans, including a long femoral neck and a low neck-shaft angle. In addition, australopiths (Au. afarensis, Au. africanus, Au. boisei, Paranthropus boisei), but not early Homo, have an "anteroposteriorly compressed" femoral neck and a small femoral head relative to femoral shaft breadth. Superoinferior asymmetry of cortical bone in the femoral neck has been claimed to be human-like in australopiths. In this study, we measured superior and inferior cortical thicknesses at the middle and base of the femoral neck using computed tomography in six Au. africanus and two P. robustus specimens. Cortical asymmetry in the fossils is closer overall to that of modern humans than to apes, although many values are intermediate between humans and apes, or even more ape-like in the midneck. Comparisons of external femoral neck and head dimensions were carried out for a more comprehensive sample of South and East African australopiths (n = 17) and two early Homo specimens. These show that compared with modern humans, femoral neck superoinferior, but not anteroposterior breadth, is larger relative to femoral head breadth in australopiths, but not in early Homo. Both internal and external characteristics of the australopith femoral neck indicate adaptation to relatively increased superoinferior bending loads, compared with both modern humans and early Homo. These observations, and a relatively small femoral head, are consistent with a slightly altered gait pattern in australopiths, involving more lateral deviation of the body center of mass over the stance limb.

The femur of Orrorin tugenensis exhibits morphometric affinities with both Miocene apes and later hominins

“…A more recent re-analysis of the original BAR 1002 0 00 CT data, based on a limited comparative sample of chimpanzees and humans, led to conclude that the skewness of femoral cortical bone distribution at the midneck in BAR 1002 0 00 is more similar to that of chimpanzees than to that of humans, although less so when approaching the neck-shaft junction 12 . Interestingly, a recent study comparing the cortical neck distribution at the midneck and closer to the shaft has found a similar pattern in South African australopith femora 50 . Overall, differences in internal and external morphology in Orrorin specifically and australopiths in general relative to modern humans provide evidence of an altered gait due to the lack of a fully-developed, modern human-like abductor apparatus 6,12,50 .…”

“…Interestingly, a recent study comparing the cortical neck distribution at the midneck and closer to the shaft has found a similar pattern in South African australopith femora 50 . Overall, differences in internal and external morphology in Orrorin specifically and australopiths in general relative to modern humans provide evidence of an altered gait due to the lack of a fully-developed, modern human-like abductor apparatus 6,12,50 . A different mode of bipedalism (from that of modern humans) practised by Orrorin (and Ar.…”

Limb Bone Structural Proportions and Locomotor Behavior in A.L. 288-1 ("Lucy")

“…Reconstructed length' in the femur was 263 mm and in the humerus 236 mm. Cross sections were obtained at percentages of these lengths, at 20%, 35%, 40% (humerus only), 50%, 65%, and 80%, measured from the distal end [43], and through the mid- and base of the femoral neck, transverse to the neck axis [14]. Only the 20% and 80% sections were measurable for the left humerus.…”

“…Yellow lines through femoral neck sections indicate planes where superior and inferior cortical thicknesses were measured (as in [14]). Inset at lower right shows physical section (left) and CT image at an adjacent location (right) for a natural break in the femur at about 75% of length', indicated with an orange arrow in the main figure.…”

“…Measurement definitions follow [43]. Femoral neck superior and inferior cortical thicknesses were measured from CT images as described previously [14] along axes illustrated in Fig 2. Measurement of femoral biomechanical neck length followed [65].…”