Human Origins: Morphology of the Orrorin tugenensis Femur

This is a general synopsis of the findings examined in the article "External and Internal Morphology of the BAR 1002'00 Orrorin tugenensis Femur".

The article “External and Internal Morphology of the BAR 1002'00 Orrorin tugenensis Femur” was written by K. Galik; associated with the Orthopedic Biomechanics Laboratory, Allegheny General Hospital, Pittsburgh, PA. B. Senut who is associated with the Département Histoire de la Terre, Muséum national d'Histoire naturelle. M. Pickford, the Chaire de Paléoanthropologie et de Préhistoire, Collège de France. D. Gommery who is associated with UPR 2147 CNRS. J. Treil from the Service du Radiologie, UMR 8555 du CNRS et Service de Radiologie, Clinique Pasteur. Lastly, A. J. Kuperavage and R. B. Eckhardt from the Laboratory of Comparative Morphology and Mechanics, Department of Kinesiology, Pennsylvania State University. The article was written September 3, 2004 and it focuses on three proximal femurs, primarily BAR 1002'00 of Orrorin tugenensis, which has a thinner cortex superiorly than inferiorly. The three femurs examined were originally found at the Lukeino Formation in Kenya's Tugen Hills in the Baringo District, Kenya in 2001. The purpose of this article, albeit it is somewhat controversial, is to support the idea that Orrorin tugenensis practiced bipedalism. The authors are essentially convinced, according to the femurs discovered, that Orrorin tugenensis was a biped considering the head of the femur had a spherical orientation and was rotated anteriorly. The article essentially explains Brigitte Senut and Martin Pickford’s discovery of Late Miocene fossils from the Lukeino Formation in Kenya's Tugen Hills. Although there were 20 fossils discovered to date, the three proximal femurs are of main focus in this article. The primary fossil investigated was of BAR 1002′00, a femur comparable in size to Pan troglodytes. The issue and primary focus was that the cortex is markedly thinner superiorly than inferiorly compared to the equal cortical thicknesses observed in extant African apes, approaching the condition in later hominids, and indicating that Orrorin tugenensis was bipedal².

The point that Eckhardt, Galik, Gommery, Kuperavage, Pickford, Senut and Treil are trying to make is that the anatomical correlates in the BAR 1002′00 femur would support its bipedal status. Several attributes were observed that are characteristic of the Plio-Pleistocene through later hominids and distinguishing them from the African Apes: a shallow trochanteris fossa, an obturator exterius groove, and long femoral neck. Furthermore, there is no evidence of deep penetration of the trochanteris fossa into the shaft, in contrast with the usual morphology observed in Pan ². Consequently, the length of the femoral neck in the BAR 1002′00 sample indicates a reorientation of the anterior gluteal muscles, which is strongly indicative of habitual hominid locomotion. “In BAR 1002′00, the femoral neck length exceeds that of Miocene hominoid fossils attributed to Afropithecus, Dryopithecus, Kenyapithecus, Nacholapithecus, Oreopithecus, and Ugandapithecus. Among Plio-Pleistocene hominids, a long femoral neck recently has been reconstructed for the partial MAK-VP-1/1 femur”². The CT scans did indicate that the cortex was decidedly thinner superiorly than inferiorly, differing from the approximately equal cortical thicknesses observed in extant African apes. However, it is important to take note that bipedalism places structural integrity where gravity would most affect a bipedal organism, at the superior end of the femur. This is a decisive trait of later hominids. The dimensions of bone in the superior margin of femoral neck actually exceed those of the inferior margin often associated with Pan troglodytes.

There is a problem, however. According to a questions submitted to Science Magazine about Orrorin tugenensis by James Ohman, Owen Lovejoy, and Tim White, with answers by Robert B. Eckhardt, Karol Galik, Adam J. Kuperavage, the femoral neck of sample BAR 1002′00 was actually broken and glued back together “at the exact location most needed for an accurate analysis”. Therefore measurements of actual cortical thickness should have been made prior to the bone being glued back together. Although computerized tomography scans of the neck-shaft junction of BAR 1002′00, and the cortical bone, support the idea that Orrorin tugenensis walked with an up-right posture and habitual bipedal locomotion, there still seems to be an issue with the reconstruction of the sample. “We concur that the femur's external morphology suggests some form of bipedalism. However, the original scans appear to show a distinct superior cortex distinctive from Australopithecus and humans, with the cortex distribution being more primitive than that seen in any other hominid” ³. This means that the BAR 1002′00 femur has such derived characteristics as to exclude Australopithecus from direct human ancestry.

Critical adaptive signatures such a longer femoral neck and cortical thickness supports our lineage, as well as provides a map of the internal distribution of cortical bone in the most ancient femora pertinent to reconstructing hominid origins². Furthermore, the internal distribution of cortical bone in its femoral neck constitutes direct evidence for frequent bipedal posture and locomotion in this Late Miocene ancestor. “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” ². This means then, according to my understanding of the article, that Orrorin tugenensis would mark the earliest evidence for habitual locomotion in the human fossil record and would shed light on the evolutionary causes of the shift to bipedalism.

In “Late Miocene hominids from the Middle Awash, Ethiopia” Selasse explains that the primitive dental anatomy and postcranial characteristics indicates that Ardipithecus was phylogenetically close to the common ancestor of chimpanzees and humans ⁴. These new findings raise additional questions about the claimed hominid status of Orrorin tugenensis. Why exactly? I mentioned before that the examination of the femoral neck led to the assumption that Orrorin tugenensis was a habitual biped. Selasse on the other hand contends that the “locomotor anatomy of Orrorin remains uncertain at this time because its description lacked comment on characters directly diagnostic of bipedalism, such as the presence of an obturator externus groove or an asymmetrical distribution of cortex in the femoral neck” ⁴. I don’t fully agree with this statement as the distribution of cortical bone and the obturator externus groove were both mentioned in the article “External and Internal Morphology of the BAR 1002'00 Orrorin tugenensis Femur”. The authors explain that the obturator externus groove, present in BAR 1002’00, resulted from bone remodeling to accommodate the direct contact of the obturator externus tendon with the dorsal surface of the femoral neck at full extension of the femur ²  Furthermore, a sample of 155 African hominoids, contrary to BAR 1002’00, did not include a single example of an obturator externus groove. This doesn’t discount Orrorin from representing the last common ancestor; it might just mean “it represents a previously unknown African hominoid with no living descendants, or an exclusive precursor of chimpanzees, gorillas or humans” ⁴.

In “A New Hominid from the Upper Miocene of Chad, Central Africa” the authors explain that the discoveries of Ardipithicus ramidus, Ardipithicus kadabba and Orrorin tugenensis have extended the human lineage well back into the Miocene, however, the discovery of Australopithecus bahrelghazali in Chad, demonstrated a considerably wider geographic range for early hominids than conventionally expected ¹. In “External and Internal Morphology of the BAR 1002'00 Orrorin tugenensis Femur”, the authors were not speaking primarily of geographical boundaries of Orrorin, but the actual morphological traits observed in the samples discovered. Also, dental measurements are looked at in depth by Burnet ¹, which compare and contrast the upper and lower dentition of lineages, including Orrorin tugenensis. Consequently, “A New Hominid from the Upper Miocene of Chad, Central Africa” looks at the primitive and derived characters evident in Sahelanthropus, which indicates its phylogenetic position as a hominid close to the last common ancestor of humans and chimpanzees, similar to Orrorin tugenensis ¹.

Based upon the paleoanthropological research and articles I have read so far, I am convinced that when paleoanthropologists attempt to reconstruct the lifeway's of early bipedal species, it is not always based upon the actual fossil evidence. Inferences about the missing pieces, and thinning of the cortical bone, as we saw in the BAR 1002'00 Orrorin tugenensis femur sample, are reconstructed based upon assumptions and cross comparisons. We also see these assumptions used in the Australopithecus garhi forearm sample BOU-12/1, where the top and bottom of the radius and ulna were essentially filled in, based on what, we don’t know. Although there is a degree of changeability in the species data due to new discoveries and estimations based on comparable samples, we cannot discount the significance of the fossil samples we do have from the early hominines. They provide us a glimpse into the human lineage and also create phylogenic possibilities. These possibilities help to explain the anagenesis within a lineage and the cladogenesis, which results in the splitting of a lineage. Consequently, the organization of these traits, based almost entirely on morphology, allows us to better understand evolutionary history.

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Literature Cited

1. Brunet, M., Franck, G., Pilbeam, D., Hassane, T.K., Likius, A, Ahounta, D., Beauvilain, A.,Blondel, C., Bocherens, H., Boisserie, J.R., De Bonis, L., Coppens, Y., Dejax, J., Denys, C., Duringer, P., Eisenmann, V., Fanone, G., Fronty, P., Geraads, d., Lehmann, T., Lihoreau, F., Louchart, A., Mahamat, A., Merceron, G., Mouchelin, G., Otero, O., Campomanes, P.P., Ponce De Leon, M., Rage, J.C., Sapanet, M., Schuster, M., Sudre, J., Tassy, P., Valentin, X., Vignaud, P., Viriot, L., Zazzo, A., & Christoph Zollikofer. A New Hominid from the Upper Miocene of Chad, Central Africa. 2002. Nature 418:145-151.
2. Eckhardt, R. B., Galik, K., Gommery, D., Kuperavage, A. J., Pickford, M., Senut, B., & Treil, J. 2004. External and Internal Morphology of the BAR 1002'00 Orrorin tugenensis Femur. American Association for the Advancement of Science 305:1450-1453.
3. Ohman, James C., Owen Lovejoy C., White, Tim D., Eckhardt, Robert B., Galik, Karol & Adam J. Kuperavage. 2005. Questions about Orrorin Femur. American Association for the Advancement of Science 305: 845.
4. Selassie, Yohannes Haile. Late Miocene hominids from the Middle Awash, Ethiopia. 2001. Nature 412: 178-181.