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Missing link discovered - hominids walked in 6 million BC

  1. Sep 4, 2004 #1
    A new article in Science (Vol 305, Issue 5689, 1450-1453, 3 September 2004) magazine claims to have produced data showing hominids walked 6 million years ago. This date is 3 million years earlier than the currently accepted date of first walking of 3 million B.C. Apparently, the marked thinness of the upper part of the femur indicates that the skeleton had morphed in accommodation to the special stresses of walking.

    From the article:

    • The external attributes of bipedal locomotion summarized here, and more extensively elsewhere for the Tugen Hills femurs (3), have developmentally determined internal structural correlates. In particular, femoral neck length has functional importance due to the force-transmission pattern through the hip joint resulting from the mass of the trunk and head superior to it. Biomechanically, the structure of the femoral neck approximates a cantilevered beam. Among apes that assume a variety of postures during locomotion but are primarily quadrupedal, the femoral neck is relatively short. A cross section in the region of the neck-shaft junction (which biomechanically is the most relevant section) includes a central marrow cavity surrounded by trabecular bone extending outward toward the surface, which in turn is characterized by a relatively uniform dense ring of cortical bone. The outer cortical bone is thickened inferiorly due to compressive forces. Also, because cortical bone is weaker under tension than under compression, bone in the superior margin of the femoral neck is thickened as well, attaining dimensions that actually exceed those of the inferior margin in more than half (52%) of Pan troglodytes observed (9).

      In contrast, among past and present hominids the femoral neck exhibits cortical bone that is much thinner superiorly than inferiorly (8, 9). This reduction in superior cortical thickness is chiefly due to the altered functional demands of hominid bipedal locomotion, which over time has modified the action of the abductor muscles (gluteus medius and minimus). In hominids, these muscles are aligned approximately parallel to the femoral neck so that their contraction compresses the bone, balancing the tension that produced the thickening of the superior cortex in apes. The result is that in hominids, cortical thickening is greatest along the inferior margin of the neck-shaft junction. At this point of highest bending stress, in extant humans, superior cortical thickness approximates one-quarter of inferior cortical thickness or less; in extant African apes, superior and inferior cortical thicknesses approximate a 1:1 ratio (9, 10)....

      Our results show that the internal distribution of cortical bone in its femoral neck constitutes direct evidence for frequent bipedal posture and locomotion in this Late Miocene ancestor. In known features, external and internal, BAR1002'00 exhibits a total morphological pattern distinct from African apes, diagnostic of bipedal locomotion, and appropriate for a population standing at the dawn of the human lineage.

    (Note: the terms superior and inferior as used in the article quoted above can be taken to mean upper and lower.)
    Last edited: Sep 4, 2004
  2. jcsd
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