Karen L Baab

Midwestern University · Department of Anatomy at the AZCOM

Objectives: Modular architecture of traits in complex organisms can be important for morphological evolution at micro- and sometimes macroevolutionary scales as it may influence the tempo and direction of changes to groups of traits that are essential for particular functions, including food acquisition and processing. We tested several distinct hy...

Quantifying and characterizing the pattern of trait covariances is crucial for understanding how population-level patterns of integration might constrain or facilitate craniofacial evolution related to the feeding system. This study addresses an important gap in our knowledge by investigating magnitudes and patterns of morphological integration of...

Laryngeal morphotypes have been hypothesized related to both phonation and to laryngeal pathologies. Morphotypes have not been validated or demonstrated quantitatively and sources of shape and size variation are incompletely understood but are critical for the explanation of behavioral changes (e.g., changes of physical properties of a voice) and f...

The nearly complete cranium DAN5/P1 was found at Gona (Afar, Ethiopia), dated to 1.5-1.6 Ma, and assigned to the species Homo erectus. Its size is, nonetheless, particularly small for the known range of variation of this taxon, and the cranial capacity has been estimated as 598 cc. In this study, we analyzed a reconstruction of its endocranial cast...

Laryngeal morphotypes have been hypothesized related to both phonation and to laryngeal pathologies. Morphotypes have not been validated or demonstrated quantitatively and sources of shape and size variation are incompletely understood but could be critical for the explanation of behavioral changes (e.g., changes of physical properties of a voice)...

Prior research has shown that morphological distances between modern human populations calculated using the temporal and parietal bones are significantly correlated with genetic distances, presumably due to genetic drift. An implication is that temporal and parietal morphology may provide insights about population history and relationships in fossi...

Objectives Intraspecific shape variation in the recent Homo sapiens bony labyrinth has been assessed for association with sexual dimorphism, body size, and genetic differences, but has not been fully assessed for association with extrinsic factors, such as subsistence strategy and climate. While the skull overall is known to vary with these variabl...

Two Early Pleistocene fossils from Gona, Ethiopia, were originally assigned to Homo erectus, and their differences in size and robusticity were attributed to either sexual dimorphism or anagenetic evolution. In the current study, we both revisit the taxonomic affinities of these fossils and assess whether morphological differences between them refl...

Based on ontogenetic data of endocranial shape, it has been proposed that a younger than previously assumed developmental status of the 1.5-Myr-old KNM-ER 42700 calvaria could explain why the calvaria of this fossil does not conform to the shape of other Homo erectus individuals. Here, we investigate (ecto)neurocranial ontogeny in H. erectus and as...

Homo erectus is the first hominin species with a truly cosmopolitan distribution and resembles recent humans in its broad spatial distribution. The microevolutionary events associated with dispersal and local adaptation may have produced similar population structure in both species. Understanding the evolutionary population dynamics of H. erectus h...

Diversification of animal vocalizations plays a key role in behavioral evolution and speciation. Vocal organ morphology represents an important source of acoustic variation, yet its small size, complex shape, and absence of homologous landmarks pose major challenges to comparative analyses. Here, we use a geometric morphometric approach based on ge...

Purpose The larynx plays a role in swallowing, respiration, and voice production. All three functions change during ontogeny. We investigated ontogenetic shape changes using a mouse model to inform our understanding of how laryngeal form and function are integrated. We understand the characterization of developmental changes to larynx anatomy as a...

There is abundant theoretical and empirical evidence for the influence of variational properties of populations on microevolution, and more limited support for their lasting impact during macroevolution. This study applies evolutionary quantitative genetic approaches to assess the long‐term impact of within‐population phenotypic variation and covar...

Significance In humans, patterns of cranial variation mirror genetic diversity globally, implicating population history as a key driver of cranial disparity. Here, we demonstrate that the magnitude of genetic diversity within 12 extant ape taxa explains a large proportion of cranial shape variation. Taxa that are more genetically diverse tend to be...

The systemic robusticity hypothesis links the thickness of cortical bone in both the cranium and limb bones. This hypothesis posits that thick cortical bone is in part a systemic response to circulating hormones, such as growth hormone and thyroid hormone, possibly related to physical activity or cold climates. Although this hypothesis has gained p...

Homo erectus is among the best-represented fossil hominin species, with a particularly rich record in Indonesia. Understanding variation within this sample and relative to other groups of H. erectus in China, Georgia, and Africa is crucial for answering questions about H. erectus migration, local adaptation, and evolutionary history. Neurocranial s...

The evolution of the modern human ( Homo sapiens ) cranium is characterized by a reduction in the size of the feeding system, including reductions in the size of the facial skeleton, postcanine teeth, and the muscles involved in biting and chewing. The conventional view hypothesizes that gracilization of the human feeding system is related to a shi...

Supplementary Methods.

Strain and strain energy density results from simulated premolar bites. Maximum principal strain (MaxPrin), minimum principal strain (MinPrin), strain mode (Mode), maximum shear strain (Shear), von Mises strain, and strain energy density (SED) generated during simulated premolar (P3) biting in the ALL-HUM variants of “extreme” and “average” modern...

Principal strain orientations recorded during validation analysis: Sites 1, 2, and 3. Purple lines represent the orientation of minimum principal strain (compression), which is 90° to orientation of maximum principal strain. Black circles represent location of strain gages at the dorsal interorbital (site 1), working-side dorsal orbital (site 2), a...

Principal strain orientations recorded during validation analysis: Sites 4, 6, and 13. Blue lines represent the orientation of maximum principal strain (tension). Purple lines represent the orientation of minimum principal strain (compression), which is 90° to orientation of maximum principal strain. Black circles represent location of strain gages...

Cortical bone mechanical properties collected from two cadaveric human specimens. E3 and v23 refer to the elastic (Young’s) modulus and Poisson’s ratio in the axis of maximum stiffness, respectively. For modulus, factor and temperature data were used to distribute regionally variation mechanical properties throughout each of the ALL-HUM models (see...

Strain and strain energy density results from simulated molar bites. Maximum principal strain (MaxPrin), minimum principal strain (MinPrin), strain mode (Mode), maximum shear strain (Shear), von Mises strain, and strain energy density (SED) generated during simulated molar (M2) biting in the ALL-HUM variants of “extreme” and “average” modern human...

In vitro loading of human cranium. Illustration of the loading apparatus constructed for the current analysis within the INSTRON loading machine during loading of the left P3.

Principal strain orientations recorded during validation analysis: Sites 9 and 11. Blue lines represent the orientation of maximum principal strain (tension). Purple lines represent the orientation of minimum principal strain (compression), which is 90° to orientation of maximum principal strain. Black circles represent location of strain gages at...

The GRGL finite element model showing constraints and muscle loads applied following Wroe et al. (2010). We compared two variants of this “beamed” model to our original “boneloaded” model, one that only included muscle beams for the anterior temporalis, superficial masseter, deep masseter, and medial pterygoid muscles (A) and a second that also inc...

Results of sensitivity analysis: color maps of von Mises strain magnitudes. Panels show strain distributions during premolar (P3) biting in the (A) original “boneloaded” ALL-HUM model, (B) “beamed” model lacking a posterior temporalis, and (C) “beamed” model including a posterior temporalis. Scales are set to range from 0–300 μɛ White regions excee...

Beam forces used in sensitivity analysis. Total muscle forces, beam count, and force per beam for each muscle group assigned to the GRGL model in the sensitivity analysis. Forces are in Newtons (N).

Transparent view of the model under in vitro validation. The surface model is shown in the position it was constrained during muscle loading, as in Fig. S1.

Principal strain orientations recorded during validation analysis: Sites 8, 10, and 12 strain. Blue lines represent the orientation of maximum principal strain (tension). Purple lines represent the orientation of minimum principal strain (compression), which is 90° to orientation of maximum principal strain. Black circles represent location of stra...

Principal strain orientations recorded during validation analysis: Sites 5, 7, and 14. Blue lines represent the orientation of maximum principal strain (tension). Purple lines represent the orientation of minimum principal strain (compression), which is 90° to orientation of maximum principal strain. Black circles represent location of strain gages...

Results of sensitivity analysis: line plot of von Mises strain. Plot shows the microstrain generated during simulated premolar (P3) biting, recorded from 14 identical brick elements across the craniofacial skeletons of our original “boneloaded” model, a “beamed” variant with muscle forces and constraints modeled following Wroe et al. (2010), and a...

The evolution of the modern human (Homo sapiens) cranium is characterized by a reduction in the size of the feeding system, including reductions in the size of the facial skeleton, postcanine teeth, and the muscles involved in biting and chewing. The conventional view hypothesizes that gracilization of the human feeding system is related to a shift...

The Liang Bua hominins from Flores, Indonesia, have been the subject of intense scrutiny and debate since their initial description and classification in 2004. These remains have been assigned to a new species, Homo floresiensis, with the partial skeleton LB1 as the type specimen. The Liang Bua hominins are notable for their short stature, small en...

CT and MRI data of DS patients. (DOCX)

Manual phalanx dimensions of average-sized modern humans and the Liang Bua hominins. (DOCX)

Results of additional analyses. (DOCX)

Box plot of humerus: femur and upper arm: thigh ratios in euploid and DS samples and LB1. (TIF)

Comparison of the LB1 endocast to DS and euploid virtual endocasts. (TIF)

Radiographs of LB6 mandible. (TIF)

Dental traits common in DS and their presence / absence in LB1 and LB6. (DOCX)

The evolution of the modern human (Homo sapiens) cranium is characterized by a reduction in the size of the feeding system, including reductions in the size of the facial skeleton, postcanine teeth, and the muscles involved in biting and chewing. The conventional view hypothesizes that gracilization of the human feeding system is related to a shift...

Two main evolutionary scenarios have been proposed to explain the presence of the small-bodied and small-brained Homo floresiensis species on the remote Indonesian island of Flores in the Late Pleistocene. According to these two scenarios, H. floresiensis was a dwarfed descendent of H. erectus or a late-surviving remnant of a older lineage, perhaps...

Pithecanthropus (now Homo) erectus was first recognized as a species by Eugène Dubois in the 1890s from fossils at the Indonesian site of Trinil. Additional finds from Indonesia and then China expanded the morphological, geographic, and temporal bounds of this species, but it was not until 1960 that H. erectus was recognized in Africa. Since that t...

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...

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 work...

Study of morphological form is fundamental to the discipline of paleoanthropology. The size and shape of our ancestors' anatomical features have long been the focus of research on hominin systematics, phylogeny, functional morphology, ontogeny, variation, and evolutionary change. Early physical anthropologists relied on both qualitative description...

A strong focus on the morphological ­differences between African and Asian H. erectus has generally overshadowed variation among populations of Asian H. erectus. This study explored variation in Asian H. erectus using 3D geometric morphometric methods, examining the shape of the neurocranium, frontal bone, occipital bone and temporal base. Analyses...

Until quite recently modern humans shared the earth with now extinct relatives—Neanderthals and others—in Europe and Asia. Homo floresiensis—dubbed the “hobbits” in the popular press—is the most recent, and the most surprising, addition to the human family tree. Were the tiny hobbit people of Flores dwarfed descendents of known species, modern huma...

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 morpholog...

Variation in cranial robusticity among modern human populations is widely acknowledged but not well-understood. While the use of "robust" cranial traits in hominin systematics and phylogeny suggests that these characters are strongly heritable, this hypothesis has not been tested. Alternatively, cranial robusticity may be a response to differences...

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...

Homo erectus is frequently differentiated from other taxa on the basis of cranial size, shape and the robusticity of cranial superstructures. However, the influence of size, and especially shape, on the expression of robusticity is not well understood. We collected data from 140 recent human crania representing seven regional populations with vary...