A biped? The answer was a suspensor or a climber

20 Nov 2012
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The observation of modern bones and fossils by computed tomography techniques has shown, study after study, to be an inexhaustible source of data used to confirm theories and hypotheses, or counter them. The latter is the case of a work led by researchers at the ICP, which studies the thickness of the cortical bone in the femoral neck of the fossil hominid Hispanopithecus laietanus, from the remains recovered at the site of Can Llobateres in Sabadell. The homogeneous distribution of this thickness has corroborated the locomotor behavior that had been proposed for the species, and also provides the first evidence in the fossil record of this distribution pattern, indicating a suspensor and/or climber locomotion.

Although distinguishing between a bipedal primate of one who is not may seem patently obvious when we look at the extant species, this distinction is much less trivial when we try to find the first bipedal primate, or when we try to understand the type of locomotion of extinct primitive hominids. For several decades, the thickness of the cortical bone in the femoral neck has been one of the key characteristics to infer bipedalism among extinct hominid species. The observed differences in the distribution of the thickness between the great apes (gorillas, chimpanzees and orangutans) and humans, led some American researchers to propose this characteristic as an indicator of bipedalism. Humans, bipeds par excellence, have a thicker cortex at the inferior part of the femoral neck than in the superior part. Furthermore, great apes, climbers and suspensors, show a more homogenous distribution of the cortical bone.

Cortical bone in the femoral neck in a chimpanze (left) and a human (right).

Cortical bone in the femoral neck in a chimpanze (left) and a human (right). Lovejoy CO. 1988. Evolution of human walking. Sci Am 259:118-125.

Some years later, it was discovered that the pattern found in humans was not unique among primates, as quadrupedal species also had thicker cortical bone thickness in the inferior part of the femoral neck. This is explained because both in quadrupedal and bipedal behaviors the body weight rests vertically and unidirectionally on the hip joint, resulting in a cortical bone thickness thicker at the inferior half of the femoral neck. However, in the case of great apes, the load on the joint does not have a preferred direction, so that the distribution of the cortical thickness at the femoral neck displays a more homogeneous structure.

Precisely, confirming this hypothesis, researchers at the ICP published this summer a paper in the American Journal of Physical Anthropology in which they measured the thickness of the cortical bone of the femoral neck of the fossil hominid Hispanopithecus laietanus,  from the Catalan Miocene about 9.6 million years ago. Recent research has attributed to this hominid a mosaic locomotor repertoire that includes adaptations to below-branch suspension as well as other orthograde behaviors such as vertical climbing. As expected, H. laietanus had a more homogeneous cortical bone thickness than other bipedal hominids. Furthermore, compared to other primates, the differences between the superior and inferior cortex thicknesses have been shown to be unable to distinguish between suspensory or climbing behaviors. Thus, the discussion on this hominid locomotor adaptations still remains open.

 

Femur of Hispanopithecus laietanus, where the distribution of the cortical bone thickness in the femoral neck is shown.

Femur of Hispanopithecus laietanus, where the distribution of the cortical bone thickness in the femoral neck is shown. Marta Pina. ICP

As explained by Marta Pina, predoctoral researcher at the ICP and first author of this paper,

While we have seen that the thickness of the femoral neck of H. laietanus, and its comparison with other primates, does not allow us to distinguish whether it had a below-branch suspensory or vertical climbing locomotion, this work represents the first cite in the fossil record of a hominid with a homogeneous distribution of the cortical bone of the femoral neck. The pattern, similar to that of great apes, adds to the thesis that the study of the thickness of the cortical bone of the primates femoral neck allows the identification of adaptations to below-branch suspensory and / or vertical climbing behaviors.

+ info Pina, M., Alba, D.M., Almécija, S., Fortuny, J. & Moyà-Solà, S. (2012). Paleobiological inferences on the locomotor repertoire of extinct hominoids based on femoral neck cortical thickness: the fossil great ape Hispanopithecus laietanus as a testcase study. American Journal of Physical Anthropology 149:142–148

+ info Pina, M., Alba, D.M., Almécija, S. & Moyà-Solà, S. (2011). Is the cortical thickness of the femoral neck a diagnostic trait for inferring bipedalism? Paleontol Evol Memòria especial núm 5:313-317.

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