Sinanthropus

Recent articles by Holliday et al. (2010) and Harmom (2009) comparing the morphology of the proximal femur in extant African hominids (Gorilla, Pan and Homo) and early hominans are aimed at distinguishing them from one another, allowing for at least “genus level” identification of fossil specimens. Both studies use multivariate statistical analysis to identify character states that discriminate various early hominans, such as Australopithecus, Paranthropus and early Homo, from the extant African apes. This is a very important consideration as more and more proximal femora crop up in the fossil record.

For instance, Holliday et al. (see a useful review by Afarensis) state that, relatively speaking, the following features separate hominan from non-human hominid femora:

1. Large femoral heads
2. Long femoral necks
3. Less cranially projecting greater trochanters, and
4. Less posteriorly-positioned lesser trochanters

Holliday et al. attribute these differences to adaptations for bipedalism. They state that, “Importantly, most of the features that distinguish Homo (or for that matter, all hominins) from the African apes are related to bipedalism.” These are enumerated and analyzed as follows:

1. The long femoral neck increases the moment arm for the gluteal abductors (tensor fasciae latae, gluteus medius, and gluteus minimus).
2. The small size (i.e., reduced height) of the greater trochanter is argued to reflect a reduction in size of the gluteus medius and gluteus minimus muscles.
3. The reduction in height of the greater trochanter relative to the position of the superior margin of the head is an ontogenetic consequence of the developing bicondylar angle
4. A ‘‘lower’’ greater trochanter tends to reduce bending on the (longer) hominin femoral neck (provided the iliac origin of the gluteal abductors were lowered, as well) – but the exact functional significance of this trait (if any) remains equivocal).

Based on the above criteria any hominid (s.l.) possessing a relatively large femoral head, long femoral neck, less projecting greater trochanter and less posteriorly positioned lesser trochanter should be considered an erectly bipedal hominan. Moreover, relative to recent humans, all the early hominans are characterized by long and antero-posteriorly thin necks, shorter greater trochanters, a more lateral position of the lesser trochanter, and medio-lateral expansion of the proximal shaft.

In addition, Harmon (2009) states that, “there is a general consensus that Australopithecus and Paranthropus femora are longer-necked than those of modern humans, and that these early hominins have smaller femoral heads and antero-posteriorly flattened necks as well. The suite of features that characterize australopith femora make them unique among hominoids, and “bipedal femora as a group differ from non-bipeds in having an inferiorly positioned greater trochanter and a NSA(neck/shaft angle) value that does not overlap with those of orangutans.”

How does the  Lufengpithecus femur (PA 1276) stand up (npi) in comparison to other hominids? Xu and Lu (2008) unequivocally state that, the length of the neck in PA 1276 is the same as in KNM-ER 738, i.e. “the neck is long, the distance from the intertrochanteric crest to the lip of the head being about 46 mm." The size of the head is also nearly identical (33.4 mm x 33.4 in PA 1276 vs. 32.8 mm x 33.6 mm in KNM-ER 738). Visual inspection also confirms the other two criteria mentioned by Holliday et al. as indicative of bipedalism in early hominans. Thus as Xu and Lu state, "the head, neck and shaft of the Lufengpithecus femur possess the characteristics and the robustness seen in the late Australopithecus femur KNM-ER 738. And the fovae capitis of both femora have an eccentric placement in the articular surface of (the) head. In sum, we can infer that the two kinds of femora possess equivalent loading function.” They go on to state that "In Lufengpithecus, Australopitehcus and early Homo the values of the sagittal diameters of the femoral necks fall in the upper parts of the vertical diameters of the femoral necks. In relation to the values mentioned above , the diameters of the area of transition between femoral heads and necks are also increased. These kinds of constructional characteristics were attributed to the adaptation to bipedal walking. The loading of weight at the femoral necks is heavier than in the quadruped, because their trunks were oriented vertically. In order to increase the loaded stress of the femoral neck, the neck has necessarily become larger."

So, based on the criteria recently established to identify hominan proximal femora in the fossil record, Lufenpithecus fits the bill.

Literature cited:

Holliday, Hutchinson, Morrow, and Livesay (2010) Geometric Morphometric analysis of hominid proximal femora: Taxonomic and phylogenetic considerations. Homo – Journal of Comparative Human Biology 61:3-15.

Harmon, E.H. (2009) The shape of the early hominin femur. Am. J. Phys. Anthropol. 139:154–171.

Xu Qinghua and Lu Qingwu (2008) Lufengpithecus lufengensis - An Early Member of Hominidae. Science Press, Beijing.