Geometric morphometric analyses of hominid proximal femora: Taxonomic and phylogenetic considerations
Introduction
The proximal femur is an anatomical complex that has been argued to exhibit shape differences between the genus Homo and other hominin genera (e.g., Australopithecus, Paranthropus, Orrorin; Aiello and Dean, 1990; Galik et al., 2004; Harmon, 2009; Lovejoy, 1975, Lovejoy, 1988; Lovejoy et al., 1973; Napier, 1964; Pickford et al., 2002; Richmond and Jungers, 2008; Robinson, 1972; Senut, 2006; Walker, 1973). First, as seen in Fig. 1, it has been demonstrated that the femoral heads of the australopiths (including members of the genera Orrorin and Paranthropus) tend to be smaller in absolute size than those of Homo (Harmon, 2009; Jungers, 1988, Jungers, 1991; Kennedy, 1983a; Lovejoy et al., 1973; Napier, 1964; Pickford et al., 2002; Richmond and Jungers, 2008; Robinson, 1972; Ruff, 1988; Senut et al., 2001; Walker, 1973). Second, Orrorin, Australopithecus, and Paranthropus evince longer femoral necks than Homo, even when corrected for the size of the femoral head (Galik et al., 2004; Harmon, 2009; Lovejoy et al., 1973; Napier, 1964; Pickford et al., 2002; Richmond and Jungers, 2008; Robinson, 1972; Senut, 2006; Senut et al., 2001; Walker, 1973). Third, it has been noted that there is a more lateral flare to the greater trochanter in the genus Homo (Kennedy, 1983a; Lovejoy, 1975, Lovejoy, 1988; Lovejoy et al., 1973; Walker, 1973). Based on external dimensions, australopith femora are also said to exhibit anteroposteriorly compressed, or “narrow” femoral necks (Harmon, 2009; Reed et al., 1993; Richmond and Jungers, 2008; Walker, 1973). Early (non-Homo) hominin femora have also been shown to be characterized by more medio-laterally expanded proximal shafts than those of Early Pleistocene or recent Homo specimens (Richmond and Jungers, 2008). Finally, the australopiths are said to have on average lower femoral neck–shaft angles than H. sapiens (Harmon, 2009; Lovejoy, 1975; Walker, 1973). Most, if not all of these differences are likely to have functional and/or locomotor implications (Harmon, 2009; Lovejoy, 1975, Lovejoy, 1988; Lovejoy et al., 1973, Lovejoy et al., 2002; Richmond and Jungers, 2008), and while these features are macroscopically evident, how might they best be quantified? Standard one- and two-dimensional osteometrics may be of little use in quantifying the morphological relationship between femoral neck length and lateral flare, since (depending on how each is measured) these measurements could overlap significantly – a fact that would not become readily apparent from an analysis of these measurements themselves. Fortunately, an alternative to standard osteometrics for examining shape exists in those methods that are collectively known as geometric morphometrics. In fact, in recent years, the application of geometric morphometrics to paleoanthropological questions has become much more prevalent (e.g., Bookstein et al., 1999; Dean et al., 1998; Delson et al., 2001; Gunz and Harvati, 2007; Harmon, 2007, Harmon, 2009; Harvati, 2003a, Harvati, 2003b; Lague, 2002; Ponce de León and Zollikofer, 2001; Yaroch, 1996). The field of morphometrics itself is widely regarded as the study of variation and change in biological form (Bookstein, 1991). From its infancy, geometric morphometrics has been used to quantify biological form via the use of landmark data (Thompson, 1917), and since its data are three-dimensional coordinate points that capture the shape of the object of study, it is arguably the best means by which to quantify morphological shape (Rohlf and Marcus, 1993).
The current study uses 3D morphometrics to quantify proximal femoral shape differences among extant African hominids (sensu lato) and fossil hominins in light of the following hypotheses: (1) extant African hominid (sensu lato) taxa are distinguishable from each other based on proximal femoral shape, and (2) the proximal femora of Homo will be distinguishable in shape from those of Australopithecus or Paranthropus. It is important to note that there has been some variation in findings with regard to these hypotheses; a recent 3-D morphometric studies of proximal femoral data (Harmon, 2007, Harmon, 2009) found considerable support for the first hypothesis and qualified support for the second, while a recent multivariate analysis of 2-D proximal femoral data (Richmond and Jungers, 2008) failed to find support for the first hypothesis, but reported considerable support for the second. Here, we use a different set of landmarks than those used by Harmon, 2007, Harmon, 2009 in an attempt to further examine morphological patterning in the proximal femora of hominids.
Section snippets
Materials and methods
Twenty proximal femoral anatomical landmarks and semi-landmarks (Table 1, Fig. 2) were digitized using a digitizing arm (Microscribe® 3D, Immersion Technologies, San Jose, CA, USA) on a sample of femora that included those of 82 recent humans, 16 chimpanzees, 20 gorillas, and casts of six fossil hominins (A.L. 288-1; KNM-ER 1472; KNM-ER 1481; KNM-WT 15000; SK 82; SK 97). These landmarks and semi-landmarks were chosen to elucidate the overall shape of the proximal femur, including the head,
Results
Fig. 3 is a scatter plot of principal component 1 (PC 1) and principal component 2 (PC 2) scores calculated from the Procrustes shape data. PC 1 accounts for 20.4% of the total variance, and separates those individuals on the left, who have larger femoral heads, longer femoral necks, less cranially projecting greater trochanters, and less posteriorly-positioned lesser trochanters, from those individuals on the right, who have smaller femoral heads, shorter femoral necks, more cranially
Discussion and conclusions
The current study investigated two main hypotheses. The first hypothesis, that proximal femoral shape could be used to distinguish extant African hominid (sensu lato) taxa from each other, is confirmed by the current data. As was found by Harmon (2007), we were able to discriminate Pan, Gorilla, and recent Homo from each other in multivariate Procrustes shape space. Specifically, Homo differed from Pan and Gorilla in that it was characterized by a larger femoral head and longer femoral necks,
Acknowledgments
We thank Prof. R. Eckhardt and Prof. M. Henneberg for helpful comments on this manuscript.
References (60)
- et al.
On the phylogenetic position of the pre-Neanderthal specimen from Reilingen, Germany
J. Hum. Evol.
(1998) - et al.
Phenetic affinities among early Homo crania from East and South Africa
J. Hum. Evol.
(1996) - et al.
The Neanderthal “chignon”: variation, integration, and homology
J. Hum. Evol.
(2007) Relative joint size and hominoid locomotor adaptations with implications for the evolution of hominid bipedalism
J. Hum. Evol.
(1988)Some aspects of femoral morphology in Homo erectus
J. Hum. Evol.
(1983)Another look at shape variation in the distal femur of Australopithecus afarensis: implications for taxonomic and functional diversity at Hadar
J. Hum. Evol.
(2002)- et al.
Temporal trends and metric variation in the mandibles and dentition of Australopithecus afarensis
J. Hum. Evol.
(2000) - et al.
Bipedalism in Orrorin tugenensis revealed by its femora
C. R. Acad. Sci. Paris Palevol.
(2002) Hindlimb articular surface allometry in Hominoidea and Macaca, with comparisons to diaphyseal scaling
J. Hum. Evol.
(1988)Bipédie et climat
C. R. Acad. Sci. Paris Palevol.
(2006)