A Multivariate Analysis of Orrorin tugenensis and the Ancestry of Bipedalism

Tomorrow’s issue of the Science will host a reinvestigation of the famous (or infamous?) Orrorin tugenensis. The study, “Orrorin tugenensis Femoral Morphology and the Evolution of Hominin Bipedalism,” comes from William Jungers and Brian Richmond. Their shtick is that their results indicate Orrorin’s bipedality was like that of early Australopithecus.

This conclusion, albeit not too novel, directly challenges Brigitte Senut et al., who published the anouncement of Orrorin tugenensis in 2001. In that paper, “First hominid from the Miocene (Lukeino Formation, Kenya),” Senut and crew lay the foundation that Orrorin is an ancestor of modern humans because proximal femur is really different from Lucy’s, and the overall proportions of the head of the femur to the shaft resemble that of humans and not other early human ancestors. Orrorin is really old, like 6 million years old.

Of course, that was an outrageous claim. No one really doubted the bipedality… But looking at the bone, it really looked like Australopithecus. It was the same size as a chimpanzees too. Femora of Homo are longer. Furthermore, the other associated Orrorin fossils, like the canines, were like chimpanzees. So it is no surprise she got a lot of flak from people. It reeked of bias, as if Senut had this idea that there’s no way Australopithecus coulda been ancestral to humans and the first fossil she found that showed otherwise would be her cash cow. She even named it after the Tugen word for “original man.”

Criticism flocked, and Senut dug herself in a deeper hole when she was a part of the team that analyzed the internal morphology of one of the Orrorin femora with computed tomography (CT). I remember reading the 2004 paper, “External and Internal Morphology of the BAR 1002’00 Orrorin tugenensis Femur.” According to this paper, the CT scans of BAR 1002’00 revealed that the top of neck of the femur was thinner than the bottom of the neck of the femur. This indicates more structural integrity on the bottom, where gravity would most affect a bipedal organism. This trait, “approached the condition in later hominids.”

This fancy CT study didn’t do much convincing. The most prominent critique came from Ohman et al., who slammed Senut for originally gluing the fractured fossil right at the very position where one coulda made an accurate analysis of the cortical thickness without having to do crazy high tech obfuscation. Ohman and crew also argued that the fossilization process thickened the cortices, and that a simple X-ray woulda been more informative than very pixelated CT images. The response to Ohman et al. was pitifully, resorting whining. Everything remained quiet for about three years. Sunet and Pickford as well as some Japanese colleagues published a paper investigating body mass, and stature estimates of Orrorin last year. But it didn’t make that much of a buzz.

So in summary, it is agreed by many that Orrorin was bipedal, but just the degree it diverged in relation to other early hominids hasn’t widely accepted. Unfortunate for Senut, that was just one lemon she couldn’t hustle.

Fast forward to today, where Jungers and Richmond say their findings indicate that the Orrorin belongs to very early human ancestors, and that upright walking is one of the first human characteristics to appear in our lineage, right after the split between human and chimpanzee lineages.

How they went about doing it was by a multivariate analysis of measurement from the outside of the femur. The outside folks. Where would there be the most restructuring of femur for bipedalism? On the outside of the femur or on the inside? Think about it for a second… If you’re still confused a bit, just ask a structural engineer, the thickness of a hallow structure would need to increase as it bears more weight. As early humans became more bipedal, more weight was distributed on the femora compared to quadrupedal locomotion, where weight is distributed between four limbs.

That being said, I’m really curious to read just what they found about the measurements of the outside of the Orrorin femur. Why didn’t they just do an X-ray? A simple 2 view X-ray costs $250 or so.

One last thing, this National Geographic News article quotes Ian Tattersall, curator of the division of anthropology at the American Museum of Natural History, saying,

“If you were going to predict what an early hominid would look like six million years ago, you’d say [it looks] much more like the Australopithicines than like Homo… “

Seems like Tattersall is flipping the stance he took on Orrorin‘s place in the ancestry of humans. In 2oo2, he’s quoted in an Ann Gibbon’s piece, “In Search of the First Hominids,”

“As a working hypothesis, I think [Senut et al.] are correct, although they don’t have the most diagnostic set of fragments.”

Galik, K. (2004). External and Internal Morphology of the BAR 1002’00 Orrorin tugenensis Femur. Science, 305(5689), 1450-1453. DOI: 10.1126/science.1098807

Gibbons, A. (2002). BECOMING HUMAN: In Search of the First Hominids. Science, 295(5558), 1214-1219. DOI: 10.1126/science.295.5558.1214

Nakatsukasa, M., Pickford, M., Egi, N., Senut, B. (2007). Femur length, body mass, and stature estimates of Orrorin tugenensis, a 6-Ma hominid from Kenya. Primates, 48(3), 171-178. DOI: 10.1007/s10329-007-0040-7

Ohman, J.C. (2005). Questions About Orrorin Femur. Science, 307(5711), 845b-845b. DOI: 10.1126/science.307.5711.845b

Senut, B. (2001). First hominid from the Miocene (Lukeino Formation, Kenya)Premier hominidé du Miocène (formation de Lukeino, Kenya).. Comptes Rendus de l’Académie des Sciences – Series IIA – Earth and Planetary Science, 332(2), 137-144. DOI: 10.1016/S1251-8050(01)01529-4