Karen L. Baab

The taxonomic implications of cranial shape variation in Homo erectus.

The taxonomic status of Homo erectus sensu lato has been a source of debate since the early 1980s, when a series of publications suggested that the early African fossils may represent a separate species, H. ergaster. To gain further resolution regarding this debate, 3D geometric morphometric data were used to quantify overall shape variation in the cranial vault within H. erectus using a new metric, the sum of squared pairwise Procrustes distances (SSD). Bootstrapping methods were used to compare the H. erectus SSD to a broad range of human and nonhuman primate samples in order to ascertain whether variation in H. erectus most clearly resembles that seen in one or more species. The reference taxa included relevant phylogenetic, ecological, and temporal analogs including humans, apes, and both extant and extinct papionin monkeys. The mean cranial shapes of different temporogeographic subsets of H. erectus fossils were then tested for significance using exact randomization tests and compared to the distances between regional groups of modern humans and subspecies/species of the ape and papionin monkey taxa. To gauge the influence of sexual dimorphism on levels of variation, comparisons were also made between the mean cranial shapes of single-sex samples for the reference taxa. Results indicate that variation in H. erectus is most comparable to single species of papionin monkeys and the genus Pan, which included two species. However, H. erectus encompasses a limited range of variation given its extensive geographic and temporal range, leading to the conclusion that only one species should be recognized. In addition, there are significant differences between the African/Georgian and Asian H. erectus samples, but not between H. ergaster (Georgia+Africa, excluding OH 9 and Daka) and H. erectus sensu stricto. This finding is in line with expectations for intraspecific variation in a long-lived species with a wide, but probably discontinuous, geographic distribution.

Size, shape, and asymmetry in fossil hominins: the status of the LB1 cranium based on 3D morphometric analyses.

The unique set of morphological characteristics of the Liang Bua hominins (Homo floresiensis) has been attributed to explanations as diverse as insular dwarfism and pathological microcephaly. This study examined the relationship between cranial size and shape across a range of hominin and African ape species to test whether or not cranial morphology of LB1 is consistent with the basic pattern of static allometry present in these various taxa. Correlations between size and 3D cranial shape were explored using principal components analysis in shape space and in Procrustes form space. Additionally, patterns of static allometry within both modern humans and Plio-Pleistocene hominins were used to simulate the expected cranial shapes of each group at the size of LB1. These hypothetical specimens were compared to LB1 both visually and statistically. Results of most analyses indicated that LB1 best fits predictions for a small specimen of fossil Homo but not for a small modern human. This was especially true for analyses of neurocranial landmarks. Results from the whole cranium were less clear about the specific affinities of LB1, but, importantly, demonstrated that aspects of facial morphology associated with smaller size converge on modern human morphology. This suggests that facial similarities between LB1 and anatomically modern humans may not be indicative of a close relationship. Landmark data collected from this study were also used to test the degree of cranial asymmetry in LB1. These comparisons indicated that the cranium is fairly asymmetrical, but within the range of asymmetry exhibited by modern humans and all extant African ape species. Compared to other fossil specimens, the degree of asymmetry in LB1 is moderate and readily explained by the taphonomic processes to which all fossils are subject. Taken together, these findings suggest that H. floresiensis was most likely the diminutive descendant of a species of archaic Homo, although the details of this evolutionary history remain obscure.

Homo floresiensis Contextualized: A Geometric Morphometric Comparative Analysis of Fossil and Pathological Human Samples

The origin of hominins found on the remote Indonesian island of Flores remains highly contentious. These specimens may represent a new hominin species, Homo floresiensis, descended from a local population of Homo erectus or from an earlier (pre-H. erectus) migration of a small-bodied and small-brained hominin out of Africa. Alternatively, some workers suggest that some or all of the specimens recovered from Liang Bua are pathological members of a small-bodied modern human population. Pathological conditions proposed to explain their documented anatomical features include microcephaly, myxoedematous endemic hypothyroidism (“cretinism”) and Laron syndrome (primary growth hormone insensitivity). This study evaluates evolutionary and pathological hypotheses through comparative analysis of cranial morphology. Geometric morphometric analyses of landmark data show that the sole Flores cranium (LB1) is clearly distinct from healthy modern humans and from those exhibiting hypothyroidism and Laron syndrome. Modern human microcephalic specimens converge, to some extent, on crania of extinct species of Homo. However in the features that distinguish these two groups, LB1 consistently groups with fossil hominins and is most similar to H. erectus. Our study provides further support for recognizing the Flores hominins as a distinct species, H. floresiensis, whose affinities lie with archaic Homo.

A re-evaluation of the taxonomic affinities of the early Homo cranium KNM-ER 42700

Spoor et al. (2007) described two recently discovered early Homo fossils from Ileret, Kenya. They allocated the small and relatively well-preserved calvaria KNM-ER 42700 to Homo erectus, while the partial maxilla (KNM-ER 42703) was attributed to Homo habilis. The taxonomic attribution of KNM-ER 42700 was based on a series of six cranial features and corroborated by a principal components analysis (PCA) of ten size-corrected cranial dimensions. It is necessary to carefully evaluate this taxonomic assessment because the presence of a small-sized African H. erectus at w1.55 Ma has far-reaching implications for both the evolutionary history of H. erectus and its degree of sexual dimorphism. Importantly, the inclusion of KNM-ER 42700 in H. erectus significantly expands the range of variation within this species, as it lacks several key diagnostic traits of H. erectus (e.g., a projecting supraorbital torus). Spoor et al. (2007) attributed the absence of certain features, including sagittal angulation of the occipital bone and low supraorbital thickness, to the presence of allometric scaling in early Homo based on a series of bivariate plots of several linear dimensions against endocranial volume. Here I compare the shape of the KNM-ER 42700 cranium to H. erectus and other Homo taxa through PCA of three-dimensional (3D) neurocranial landmark data. Procrustes distances are then used to establish whether the differences observed between H. erectus and KNM-ER 42700 are most comparable to those seen in intraor interspecific comparisons. Finally, I explore cranial allometry in H. erectus by incorporating information about both size and shape into a single PCA (Mitteroecker et al., 2004) and then test

PHYLOGENETIC, ECOLOGICAL, AND ALLOMETRIC CORRELATES OF CRANIAL SHAPE IN MALAGASY LEMURIFORMS

Adaptive radiations provide important insights into many aspects of evolution, including the relationship between ecology and morphological diversification as well as between ecology and speciation. Many such radiations include divergence along a dietary axis, although other ecological variables may also drive diversification, including differences in diel activity patterns. This study examines the role of two key ecological variables, diet and activity patterns, in shaping the radiation of a diverse clade of primates, the Malagasy lemurs. When phylogeny was ignored, activity pattern and several dietary variables predicted a significant proportion of cranial shape variation. However, when phylogeny was taken into account, only typical diet accounted for a significant proportion of shape variation. One possible explanation for this discrepancy is that this radiation was characterized by a relatively small number of dietary shifts (and possibly changes in body size) that occurred in conjunction with the divergence of major clades. This pattern may be difficult to detect with the phylogenetic comparative methods used here, but may characterize not just lemurs but other mammals.

The impact of superimposition choice in geometric morphometric approaches to morphological integration.

Integration of skeletal structures has traditionally been studied using correlations among linear dimensions, but has more recently been investigated using geometric morphometric data in the form of covariances among landmarks (e.g., Baab et al., 2012).Within this latter field, the use of partial least squares (PLS), or singular warps analysis, has proven particularly useful for examining covariance among sets (blocks) of landmarks (Bookstein, 1997; Klingenberg and Zaklan, 2000; Rohlf and Corti, 2000; Bookstein et al., 2003; Bastir and Rosas, 2005; Gunz and Harvati, 2007; Kulemeyer et al., 2009; Singh et al., 2012). An important step that occurs prior to a PLS analysis is the superimposition of landmark configurations. However, with two distinct blocks of landmarks to compare, there are two basic approaches to superimposition: a single superimposition of all landmarks followed by partitioning into distinct blocks; or, independent superimpositions of the blocks of landmarks. Both approaches have important and predictable downstream effects for interpreting the results of the PLS analysis. While the importance of superimposition choice prior to PLS has been touched on previously, this commentary is outside of the standard anthropological literature (Klingenberg, 2009; Kulemeyer et al., 2009; McCane and Kean, 2011; Mitteroecker et al., 2012). In light of the growing popularity of PLS for the study of morphological integration within the field of anthropology, a brief discourse on this topic is both necessary and timely.