There is an interesting article in HOMO – Journal of Comparative Human Biology on the proximal femur. The article, Geometric morphometric analyses of hominid proximal femora: Taxonomic and phylogenetic considerations, looked at whether one can separate extant hominids into different taxa using geometric morphometrics and whether one could distinguish Homo from Australopithecus and Paranthropus.
The study digitized 20 landmarks on the proximal femur from recent humans (n=82), chimpanzees (n=16), gorillas (n=20), and casts of fossil hominins (n=6, included are A. L. 288-1, KNM-ER 1472, KNM-ER 1481, KNM-WT 15000, SK 82, and SK 97). The landmarks are pictured below:
The data was then analyzed using both Procustes analysis and principal components analysis. What did the researchers find? Here is a scatter plot of the first two principle components (I apologize for the poor quality I had trouble extracting them)::
Note that principal component one separates Homo and the fossil hominins from extant apes. Principal component two clearly separates chimps from gorillas. Biologically, low PC1 scores indicate a large head, long neck, and short greater trochanter. High scores indicate a small head, short neck, and tall greater trochanter. Low PC2 scores indicate cranially projecting greater trochanters with a deep trochanteric fossa. Below is a scatter plot of PC3 and PC4:
PC3 is governed primarily by neck shaft angle and doesn’t segrate any of the groups. PC4, governed by neck length and breadth, greater trochanter height, and lesser trochanter position, separates all early hominins from recent Homo.
To summarize, there are distinct differences in proximal morphology between chimps, gorillas and recent humans. There are also differences in proximal femur morphology between the apes and fossil hominins (i.e. Australopithecus, Paranthropus, and early Homo). There are some differences between fossil hominins and recent humans – although there is some overlap as well. At this point cluster analysis was used which succeeded in separating Australopithecus from Paranthropus and both of these from Homo. All of this has some interesting implications for our ability to assign isolated femora to the generic level. An additional implication concerns the impact on certain creationist arguments concerning the taxonomic status of the australopithecines. Specifically, the claim that australopithecines were just bipedal apes. This study demonstrates that there are specific morphological differences between the femora of apes and australopithecines – differences that are large enough to allow us to distinguish between the two.
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