A morphometric analysis of the hominid iliac crest

Variations in hominoid pelvic morphology reflect modifications in the evolution of ape and human locomotion. Numerous studies have addressed the evolutionary significance of changes in the ilium from pongid to hominid. Of considerable interest has been the orientation and degree of lateral flare in the ilia and its role in bipedalism.

The iliac crest forms a distinctive S-shaped curve in modern humans reflecting biomechanical demands of bipedalism. The degree of curvature is greatly reduced in apes while early fossil hominids display a condition that is neither identical to apes nor to humans. Traditional morphometric analyses of evolutionary trends in iliac shape have had limited success because they have not succeeded in quantifying the entire form. In contrast, this thesis discusses the use of the convex hull as a methodology that more completely accounts for variation in the form of the hominoid iliac crest. In applying this methodology, this study has quantified the shape differences in the iliac crest between living pongids and hominids and applied these differences to fossil hominids in the attempt to identify evolutionary trends in development.

Photographic and video captured images of the superior aspect of the iliac crest are digitized for samples of 45 Pan, 12 Gorilla, casts of 4 fossil hominids (Sts 14, A.L. 288-1, KNM-WT 15000, and Kebara 2) and 43 modern humans. An additional image of the fossil KNM-ER 3228 was obtained through the generosity of Dr. Alan Walker of Johns Hopkins University. The images are first smoothed by Elliptical Fourier Analysis and then undergo shape analysis by constructing a convex hull (CH). Vertices of the CH are determined using a polygonal edge approximation and the resulting concavities reflect the magnitude and number of curvatures in the iliac crest. The vertices of the CH are then used to determine shape differences by three statistical applications; MANOVA, Posterior probability, and Tukey's Studentized Range Test.

Results indicate that differences in the shape of the iliac crest can be repeatedly quantified. Further significant differences exist between the iliac crest of hominids and pongids. Fossil hominids exhibit varying degrees of shape differences when compared to living groups. Evolutionary trends of development in the iliac crest can be hypothesized from the results.

Bipedalism of early hominids, as suggested by bony anatomy, indicates a form of locomotion similar to, but somewhat different from, modern humans. The CH is used here to quantify the shape differences among hominoid iliac crests in order to test the hypothesis that australopithecines represented an intermediate morphology through which later hominids evolved.