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Hominin cranial base evolution and genes implicated in basioccipital development: Role of Pax7, Fgfr3 and Disp1 in basioccipital development and integration

ProQuest Dissertations and Theses, 2009
Dissertation
Author: Lisa Nevell
Abstract:
Cranial base morphology features in some hominin species diagnoses. One of the unsolved puzzles of the hominin cranial base is the evolutionary history of the apparent convergence seen in the cranial base morphology of two hominin subclades, Homo and Paranthropus. Both subclades apparently share the same suite of cranial base characters, namely, a reduction in anteroposterior length of the cranial base, more coronally-orientated long axes of the petrous component of the temporal bones of the cranial base, and a more centrally-located foramen magnum. Did the two subclades inherit this morphology from a recent common ancestor, or is the morphology homoplasic in the two subclades? This thesis had two main aims. The first was to provide a better comparative context for the study of hominin cranial base evolution; this forms Chapter 2 of the thesis. The second was to use animal models to investigate (A) the extent to which the cranial base is affected by morphological integration, and (B) whether three genes that have been implicated in one way, or another, in the development of the cranial base, affect its development in ways that are analogous to the differences between the cranial base of modern humans and our close living relatives, chimpanzees, bonobos and gorillas. Disruption of Disp1, Pax7, or FGFr3 each resulted in an increase on the length of the basioccipital bone in newborn mice. The basioccipital responded in a highly integrated fashion to various perturbations of normal growth. Morphological integration may facilitate apparent homoplasy in the hominin cranial base.

Table of Contents Acknowledgements iii Abstract iv Table of Contents v List of Figures vi List of Tables ix Chapter 1: Introduction 1 Chapter 2: Cranial base evolution within the hominin clade 6 Chapter 3: A genetically defined role of Pax7 in patterning the basioccipital bone in mice 44 Chapter 4: Basioccipital development and morphological integration 98 Chapter 5: Discussion and Conclusions 127 Bibliography 143

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List of Figures Chapter 2: Figure 1: Cranial base morphology in Homo sapiens and Pan troglodytes 9 Figure 2: Proposed hominin phylogeny based on cranial and dental characters 15 Figure 3: Proposed hominin phylogeny based on marginally less parsimonious trees 17 Figure 4: Hominin cranial base morphological grades 40

Chapter 3: Figure 5: Morphology of the ventral neonatal mouse c ranial base 45 Figure 6: Schematic representation of muscle development 50 Figure 7: Anatomical landmarks describing the shape of the basioccipital 54 Figure 8: Regression analysis statistical power and sample size 56 Figure 9: Pax7-deficient

mice differ from the wildtype 63 Figure 10a: Pax7 lacz/lacz basioccipital bone is larger than Pax7 wt/wt

67 Figure 10b: Pax7 lacz/lacz basioccipital bone is longer than Pax7 wt/wt

68 Figure 11: Pax7-deficient

mice differ from the wildtype with respect to basioccipital shape 70 Figure 12: Pax7-deficient

phenotype cannot be attributed to the effects of size 72 Figure 13: Pax7 basioccipital morphology through post-natal development 76 Figure 14: Correspondence point models of Pax7 WT/WT and Pax7 LacZ/LacZ

78

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Figure 15: Principal components results of correspondence point analysis comparing Pax7 WT/WT and Pax7 LacZ/LacZ

80 Figure 16: Principal components results of correspondence point analysis comparing Pax7 WT/WT and Pax7 LacZ/WT

81 Figure 17: Correspondence point comparison of the Pax7 WT/WT and Pax7 LacZ/LacZ

basioccipitals seen in superior view 83 Figure 18: Correspondence point comparison of the Pax7 WT/WT and Pax7 LacZ/LacZ

basioccipitals in posterior view 84 Figure 19: Correspondence point comparison of the Pax7 WT/WT and Pax7 LacZ/LacZ

basioccipitals in lateral view 85 Figure 20: Correspondence point comparison of Pax7 WT/WT and Pax7 LacZ/LacZ

basioccipitals in oblique view 86 Figure 21: Correspondence point comparison of Pax7 WT/WT and Pax7 LacZ/WT

basioccipitals in superior view 88 Figure 22: Correspondence point comparison of Pax7 WT/WT and Pax7 LacZ/WT

basioccipitals in posterior view 89 Figure 23: Correspondence point comparison of Pax7 WT/WT and Pax7 LacZ/WT

basioccipitals in lateral view 90 Figure 24: Correspondence point comparison of Pax7 WT/WT and Pax7 LacZ/WT

basioccipitals in oblique view 91

C hapter 4: Figure 25: Cranial base morphology in Homo sapiens and Pan troglodytes 101

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Figure 26: Hominin grades and cranial base morphology 102 F igure 27: Hominin phylogeny based on cranial and dental characters 103 Figure 28: Hominin phylogeny based on cranial base characters 103 Figure 29: Anatomical landmarks describing the shape of the basioccipital 111 Figure 30: Pax7-deficient

mice differ from the wildtype in size 116 Figure 31: Pax7-deficient

mice differ from the wildtype in shape 117 Figure 32: Fgfr3-deficient

mice differ from the wildtype 118 Figure 33: Disp1-deficient

mice differ from the wildtype 119 Figure 34: Mantel Test results for significance of morphological integration 120

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List of Tables Chapter 2: Table 1: Specimens included in the hypodigms of early hominin species 11 Table 2: Eight synapomorphies differentiate Pan-Homo LCA from extant hominoids 31 Table 3: Predicted cranial base and cranial base-related morphology of the Pan-Homo LCA compared with the predominant character states seen in H. sapiens and P. troglodytes 32 Table 4: Comparison between the Pan-Homo LCA and a hypothetical stem hominin 37

Chapter 3: Table 5: Selected variables representing the basiocc ipital bone length width and height compared between Pax7 lacz/lacz and Pax7 wt/wt

mice 65 Table 6: Pax7 lacz/lacz basioccipital bone is longer than Pax7 wt/wt

69 Table 7: A) Correlation between the overall size of the basioccipital and selected variables. B) Relationship between the overall size of the basioccipital and a ratio between parasagittal length and height 74

Chapter 4: T able 8: Length of the basioccipital in the midline among newborn mice 115

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Table 9: Morphological integration within effected r egions 121 Table 10: Degree of similarity between covariance st ructures 122

1 Chapter 1: Introduction The word cranium comes from the Greek word kranion, which means ‘brain case’. The three major components of the cranium are A) the part that covers the top and sides of the brain, called the cranial vault (or the calotte), B) the part that covers the front of the brain, called the face, and C) the part beneath the brain, called the cranial base, or basicranium. Another way of dividing up the cranium recognizes two components, the neurocranium and the viscerocranium. The neurocranium is the combination of the calotte and the basicranium; the viscerocranium is equivalent to the face. The cranial base has an internal or endocranial surface, and an external surface. Its internal, or endocranial, surface is divided into three hollowed areas called cranial fossae, each of which is occupied by major components of the brain. The anterior cranial fossa lies beneath the frontal lobes of the cerebral cortex, the middle beneath the temporal lobes of the cerebral cortex, and the posterior beneath the cerebellum. The single un-paired bones that contribute to the cranial base are, from front to back, all or parts of the ethmoid, sphenoid and occipital bones. Only one paired bone, the temporal, contributes to the cranial base. The features of the external surface of the cranial base are those that can be seen when viewing the cranium from below; this view of the cranium is called the norma basilaris.

2 Most of the bones contributing to the cranial base develop via a process called endochondral ossification. In the second month of modern human intrauterine life hyaline cartilage appears beneath the developing brain. This hyaline cartilage is in the form of pairs of symmetrical elements. From posterior to anterior they are four pairs of occipital cartilages that give rise to the adult occipital bone except for the occipital squame, a pair of parachordal cartilages that lie either side of the anterior end of the notochord and which eventually contribute to the body of the sphenoid, a pair of otic cartilages that surround the inner ear and that later develop into the petrous component of the temporal bone, a pair of hypophyseal cartilages that surround the pituitary and eventually also contribute to the body of the sphenoid, two pairs of laterally-situated sphenoid cartilages, the orbitosphenoids that form the lesser wings of the sphenoid and the alisphenoids that form the greater wings of the sphenoid, two pairs of trabecular cartilages that contribute to the ethmoid and the nasal skeleton, and finally a pair of components that contribute to the presphenoid. All the cartilages anterior to, and including, the orbitosphenoids derive from neural crest cells, whereas the paired cartilaginous elements posterior to, and including, the hypophyseal cartilages derive primarily from mesoderm in the form of somites. Bone f ormation within the chondrocranium begins at its caudal (i.e., the posterior or inferior) end. Endochondral bone formation involves many ossification centers,

3 and results in most of the occipital bone, the petrous part of the temporal bone, and the sphenoid and ethmoid bones. Comparative studies of the cranial base in primates in general, and in higher primates in particular, have usually either considered its sagittal or parasagittal morphology. The former studies have focused on the lengths and angular relationships of the midline structures that form the floors of the anterior and the middle cranial fossae; relatively few have included the posterior cranial fossa. The studies that have considered the parasagittal morphology of the cranial base have tended to focus on the distances between landmarks (e.g., major nervous or vascular foramina), or on the distances between coronal planes defined by pairs of landmarks, or on the angles subtended to the midline by the long axes of structures like the petrous component of the temporal bone. Some parts of the cranial base such as the petrous component of the temporal bone are relatively well represented in the hominin fossil record because of their hardness and durability (N.B., L. petrous = rock-like; it has the same root as ‘petroleum’ which means literally ‘rock oil’), but well-preserved examples of the cranial base in the early hominin fossil record are relatively rare. Nonetheless, researchers have investigated what fossil evidence there is of the cranial base, and cranial base morphology features in some hominin species diagnoses. Most attention has been paid to the cranial base morphology that differs between modern humans and chimpanzees and bonobos. These include differences in the

4 anteroposterior length of the cranial base, in the angle subtended by the long axes of the floors of the anterior and middle cranial fossae (as captured by one or other versions of the external cranial base angle), in the morphology and angular relationships of the petrous component of the temporal bone, in the location of the foramen magnum, and in the orientation of the plane of the foramen magnum. One of the unsolved puzzles of the hominin cranial base is the evolutionary history of the apparent convergence seen in the cranial base morphology of two hominin subclades, Homo and Paranthropus. Both subclades apparently share the same suite of cranial base characters, namely, a reduction in anteroposterior length of the cranial base, more coronally-orientated long axes of the petrous component of the temporal bones of the cranial base, and a more centrally-located foramen magnum. Did the two subclades inherit this morphology from a recent common ancestor, or is the morphology homoplasic in the two subclades? This thesis had two main aims. The first was to provide a better comparative context for the study of hominin cranial base evolution; this forms Chapter 2 of the thesis. The second was to use animal models to investigate A) the extent to which the cranial base is affected by morphological integration, and B) whether three genes that have been implicated in one way, or another, in the development of the cranial base, affect its development in ways that are analogous to the differences between the cranial base of modern humans and our close living

5 relatives, chimpanzees, bonobos and gorillas. The studies related to these questions form Chapters 3 and 4 of the thesis. Chapter 5 summarizes the results and suggests ways in which these topics might be pursued in future research.

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Chapter 2: Cranial base evolution within the hominin clade

Abstract The base of the cranium (i.e., the basioccipital, the sphenoid and the temporal bones) is of particular interest because it undergoes significant morphological change within the hominin clade, and because basicranial morphology features in several hominin species diagnoses (Wood and Richmond, 2000). We use a parsimony analysis of published cranial and dental data (Strait and Grine, 2004) to predict the cranial base morphology expected in the hypothetical last common ancestor (LCA) of the Pan-Homo clade. We also predict the primitive condition of the cranial base for the hominin clade, and document the evolution of the cranial base within the major subclades within the hominin clade. This analysis suggests that cranial base morphology has continued to evolve in the hominin clade, both before and after the emergence of the genus Homo. This analysis indicates a number of homoplastic cranial base traits between two subclades; one of these subclades includes the genus Homo and the other subclade includes the genus Paranthropus. Both subclades apparently share the same suite of cranial base characters, namely, a reduction in anteroposterior length of the cranial base, more coronally-orientated long axes of the petrous component of the temporal bones, and a more centrally-located foramen magnum.

7 Introduction The base of the cranium (i.e., the basioccipital, the sphenoid and the temporal bones) undergoes significant morphological change within the hominin lineage, and basicranial distinctions feature in several hominin species diagnoses (Wood and Richmond, 2000). The cranial base is relatively well represented in the hominin fossil record. One of the reasons is that one of its components, the petrous part of the temporal bone, is apart from the teeth the densest part of the cranial skeleton. Paradoxically, it is better represented in the early part of the hominin fossil record than in parts of the Homo subclade. This is because the cranial base is damaged in many of the Homo erectus specimens recovered from the Indonesian sites on the island of Java. This damage is almost certainly anthropogenic and is linked with the extraction of the brain of the deceased. The cranial base is the only part of the skeleton where so many important functions (e.g., respiration, feeding and ingestion, posture, and balance) converge. This has led to the hypothesis that the cranial base must be a highly integrated structure, for modifications that might benefit one of these functions may well be detrimental to another (Lieberman et al. 2000). However, despite all these reasons to study it, compared with the face and the cranial vault the cranial base has been relatively neglected by paleoanthropologists. This has changed since imaging methods have enabled researchers to non-destructively access information about the structure of the

8 bony labyrinth, and from these data inferences can be made about the form of the membranous labyrinth. Researchers have shown that even in a group as small as the extant higher primates, quite modest differences in the relative size of the semicircular canals are linked with differences in habitual posture and locomotor mode. These findings, and the use of CT and more recently micro-CT to extract information about the bony labyrinth from intact petrous bones (reviewed by Spoor et al., 2000) has rekindled interest in the cranial base, but the form of the bony labyrinth will not be considered in this contribution. Studies of the external morphology of the cranial base can be divided into those that have concentrated on the midline (or sagittal) morphology and those that focus on the cranial base as a whole. Traditional (as opposed to three- dimensional geometric morphometric) sagittal studies have mainly focused on the relative lengths and angular relationships of the components of the midline of the cranial base (Ross and Ravosa, 1993, Ross and Henneberg, 1995, Lieberman and McCarthy, 1999, Strait, 1999, McCarthy, 2001, Bookstein et al., 2003, Jeffery, 2005, Jeffery and Spoor, 2002, Jeffery and Spoor, 2004). Traditional studies of the cranial base as whole have concentrated on the gross morphology that can be seen not from the endocranial surface, but from below (this aspect of the cranium is known as the norma basilaris). These studies mostly used linear variables to compare the antero-posterior proportions of the parasagittal components of the cranial base, the distances between bilateral structures such as vascular or neural foramina to compare the relative widths of the components, and angular variables

9 to compare the orientation of the petrous bones and the tympanic components of the temporal bones (Dean and Wood, 1981, Dean and Wood, 1982, Dean and Wood, 1984, Lockwood et al., 2002, Bastir et al., 2004) (Figure 1).

Figure 1: Cranial base morphology in Homo sapiens and Pan troglodytes

Photo of the cranial base in norma basilaris (above) and in sagittal section (below). The cranial base is highlighted in sagittal section. Note the greater width of the sphenoid in H. sapiens. Sagittal section of Homo sapiens is adapted with permission from Bookstein et al. (2003), and the sagittal section of Pan troglodytes is adapted with permission from www.digimorph.org (2008).

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We use a parsimony analysis of published cranial and dental data (Strait and Grine, 2004) to predict the cranial base morphology expected in the hypothetical last common ancestor (LCA) of the Pan-Homo clade. We also predict the primitive condition of the cranial base for the hominin clade and document the evolution of the cranial base within the major subclades within the hominin clade.

Methods Fourteen fossil hominin taxa (Ardipithecus ramidus, Australopithecus anamensis, Kenyanthropus platyops, Australopithecus garhi, Sahelanthropus tchadensis, Australopithecus afarensis, Australopithecus africanus, Paranthropus aethiopicus, Paranthropus boisei, Paranthropus robustus, Homo habilis, Homo rudolfensis, Homo ergaster, and Homo sapiens) were included in these cladistic analyses (se also Table 1). We excluded three taxa (Ardipithecus kadabba, Orrorin tugenensis, Australopithecus bahrelghazali) from detailed consideration because there is no, or very limited, cranial base data available for these taxa. The extant hominoid samples include Homo sapiens, Pan troglodytes, Gorilla gorilla, Pongo pygmaeus and a mixed sample of Hylobates lar and Hylobates hoolock. Two more distant outgroups, Colobus guereza and a mixed sample of Papio anubis and Papio ursinus, were included in the study in order to determine character state polarity.

11 Table 1: Specimens included in the hypodigms of early hominin species Sahelanthropus tchadensis: TM 266-01-060-1

Ardipithecus ramidus: ARA-VP 6/1, 1/128, 1/500 KNM-TH 13150 KNM-LT 329

Australopithecus anamensis: KNM-KP 29181, 29283, 29286

Australopithecus afarensis: A.L. 33-125, 58-22, 128-23, 145-35, 162-28, 188-1, 198-1, 199-1, 200-1, 207-13, 266-1, 277-1, 288-1, 311-1, 333-1, 333-2, 333-45, 333-105, 333w-1, 333w-12, 333w-60, 400-1a, 417-1, 444-2 Garusi 1 KNM-ER 2602 LH 4 MAK-VP 1/12

Australopithecus garhi: BOU-VP 12/130

Australopithecus africanus: MLD 1, 2, 6, 9, 12, 22, 29, 34, 37/38, 40, 45 Sts 5, 7, 17, 20, 26, 36, 52a and b, 67, 71 Stw 13, 73, 252, 384, 404, 498, 505, 513 Taung 1 TM 1511, 1512

Kenyanthropus platyops: KNM-WT 38350, 40000

Paranthropus aethiopicus: KNM-WT 16005, 17000 L 55s-33, 338y-6, 860-2 Omo 18-1967-18, 44- 1970-2466, 57-4-1968-41

Paranthropus robustus: DNH 7 SK 6, 12, 13/14, 23, 34, 46, 47, 48, 49, 52, 55, 65, 79, 83, 848, 1586 SKW 5, 8, 11, 29, 2581, SKX 265, 4446, 5013 TM 1517

Paranthropus boisei: OH 5 KGA 10-506, 10-525 KNM-CH 1 KNM-ER 403, 404, 405, 406, 407, 725, 727, 728, 729, 732, 733, 801, 805, 810, 818, 1468, 1469, 1483, 1803, 1806, 3229, 3230, 3729, 3954, 5429, 5877, 13750, 15930, 23000 KNM-WT 16841, 17400 L 7a-125, 74a-21 Natron Omo 323-76-896

Homo habilis: A.L. 666-1 L 894-1 OH 7, 13, 24, 62 KNM-ER 1478, 1501, 1502, 1805, 1813, 3735 SK 15, 27, 45, 847 Sts 19 Stw 53

Homo rudolfensis: KNM-ER 819, 1470, 1482, 1483, 1590, 1801, 1802, 3732, 3891 UR 501

Homo ergaster: KNM-ER 730, 820, 992, 1507, 3733, 3883 KNM-WT 15000

12 Cercopithecoid phylogeny is beyond the scope of this paper. Details of all these samples are previously published (Strait and Grine, 2004). A relatively recent comprehensive cladistic analysis of fossil hominins (Strait and Grine, 2004) used metric and non-metric characters taken from the following sources (Delson and Andrews, 1975, Wood, 1975, Schwartz, 1984, Andrews and Martin, 1987, Chamberlain and Wood, 1987, Groves and Eaglen, 1988, Braga, 1995, Strait et al., 1997, Shoshani et al., 1996, Collard and Wood, 2000). The character matrix is made up of 198 characters, of which 89 are metric. Like Strait and Grine (2004) we excluded 40 characters because missing data meant that shape indices could not be calculated for many of the fossil hominin specimens, and like Strait and Grine (2004) we excluded redundant characters (i.e., characters that are components of more inclusive characters, or measurements that are included within another more inclusive measurement) from the published literature for such characters violate the assumption of character independence and can obscure true relationships (Farris, 1983, Kluge, 1989). Qualitative character states were assigned as absent, variable, or present (Strait and Grine, 2004). Traditional quantitative character states are determined by a range-based method where taxa are assigned different states when ranges are discontinuous or exhibited minimal overlap (Almeida and Bisby, 1984). Craniometric character states are determined using homogeneous subset coding (HSC). In HSC taxa may share the same state when they meet two criteria: first, two taxa share a state when they are not significantly different from one another;

13 second, two taxa share a state when they differ significantly from a common set of taxa (Simon, 1983, Rae, 1997). This paper departs from Strait and Grine (2004) with respect to character weighting. Character independence is a fundamental assumption of cladistic analysis (Farris, 1983, Kluge, 1989), however characters that share some aspect of function or development are likely to covary in a non-independent fashion ( Olson and Miller, 1958, Cheverud, 1982, Zelditch, 1987, Zelditch, 1988, Cheverud, 1995, Cheverud, 1996, Chernoff and Magwene, 1999, Ackermann and Cheverud, 2000, Strait, 2001). Strait and Grine (2004) identified hypothesized character complexes and reduced the weight of characters within these complexes in order to more closely approximate character independence. The authors assigned all of the characters in each hypothesized complex equal to the weight of one independent character and assigned equal weights to each character within that complex. Testing the validity of the hypothesized character complexes is beyond the scope of the present paper. In the absence of an empirically tested hypothesis of non-independence, equal weighting of all characters is a more conservative approach (Eldredge and Cracraft, 1980, Wheeler, 1986). In this study we give equal weight to all of the characters. Cladistic analysis was performed using the maximum parsimony and bootstrap search option of Winclada (Nixon, 1999) and NONA 2.0 (Goloboff, 2007). In a bootstrap analysis a data set is resampled with replacement and each resulting new data set is subjected to parsimony analysis (Felsenstein, 1985, 2004). Characters were treated as unordered, the distant outgroups were not

14 constrained to be monophyletic and the trees were unrooted. In all analyses, 10,000 replicates are performed. We report the most parsimonious trees, and in a separate analysis we report an analysis that includes trees that are marginally less parsimonious (i.e., trees that are within 1% of the shortest tree length). A majority rules consensus cladogram of the most parsimonious trees is reported, the percentage of trees supporting a given branch in the consensus cladogram are reported at each node. This consensus tree cladogram is the phylogenetic hypothesis used in the subsequent character analysis. Note that the tree topology is determined by characters from the whole cranium, but the character analysis is confined to characters that are based on cranial base morphology.

Results A 10,000 replicate bootstrap analysis resulted in 112 most parsimonious trees out of 10113 possible trees. [Tree Length (TL) =1007; Consistency Index (CI) = 0.71; Retention Index (RI) = 0.55]. A majority-rule consensus of the most parsimonious trees is shown in Figure 2; the percentage of trees supporting a particular node is reported on each branch. A majority rules consensus diagram of marginally less parsimonious trees is reported in Figure 3 [Tree Length (TL) = 1059, Consistency Index CI = 0.68 and Retention Index RI = 0.47]. The tree topology resulting from the most parsimonious trees is discussed in further detail below and resembles the topology resulting from marginally less parsimonious trees.

15 Figure 2: Majorityrule consensus based on the most parsimonious trees from a 10,000 bootstrapped replicate analysis

A 10,000 replicate bootstrap analysis resulted in 112 most parsimonious trees out of 10113 possible trees. [Tree Length (TL) =1007; Consistency Index (CI) = 0.71; Retention Index (RI) = 0.55]. A majority-rule consensus of the most parsimonious trees is shown with the percentage of trees supporting a particular node reported on each branch. In the consensus tree generated from the most parsimonious trees (Figure 2), there is support for the hypothesis that hominins form a clade to the exclusion of other hominoids (82%), and Pan troglodytes shares a sister taxa relationship with hominins in the majority of our most parsimonious tree topologies (77%). The results of this analysis are consistent with many of the most widely used taxonomic schemes for the Pan-Homo clade. For example, all of our tree topologies placed H. sapiens and H. ergaster as sister taxa, H. habilis as the sister

16 taxon to that subclade, and there is strong support for the genus Homo being a monophyletic group, or subclade. There is even stronger support for the genus Paranthropus forming a monophyletic group, for 100% of the most parsimonious trees generated by the present analysis support this interpretation. Within the hominin clade, a substantial majority of the most parsimonious trees (91%) support a (Au. garhi, Paranthropus) and a (Homo, Au. africanus, K. platyops) grouping. Likewise, a substantial majority of our tree topologies (91%) also support Au. afarensis as the sister taxon to a ((Au. garhi, Paranthropus) (Homo, Au. africanus, K. platyops)) grouping. A majority of our tree topologies (82%) also suggest that S. tchadensis is the sister taxon of a clade comprising all other hominin taxa. In addition 79% of our tree topologies suggest that K. platyops is the sister clade of a (Homo, Au. africanus) grouping and 76% of the tree topologies support Au. africanus as the sister taxon to the Homo clade. In contrast the phylogenetic relationships of Au. anamensis and Au. afarensis are more variable in our most parsimonious tree topologies. The results of this analysis are consistent with widely accepted hypotheses of the phylogenetic relationships within the hominin clade (reviewed in Kimbel et al. 2004). A majority rules consensus diagram of marginally less parsimonious trees is reported in Figure 3. The analysis reported in Figure two resulted in 10086 trees within 1% of the most parsimonious tree (TL < 1107). The consensus topology of marginally less parsimonious trees has a TL= 1059, CI = 0.68, and RI = 0.47.

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Figure 3: Majorityrule consensus of marginally less parsimonious trees

A majority rules consensus diagram of marginally less parsimonious trees is reported above. The analysis reported in Figure 1 resulted in10086 trees within 1% of the most parsimonious tree (TL < 1107). [The consensus topology has a TL= 1059, CI = 0.68, and RI = 0.47.]

Character Analysis

18 In addition to characters involved in the cranial base as seen in norma basilaris we also include discussions of characters that relate to the non-cranial base parts of the parietal bones and the occipital (i.e., the squamous parts of the two parietal bones and the squamous part, or upper scale, of the occipital).

Temporal Bone

Petrous

The primitive hominoid condition is to have a sagittally-orientated long axis of the petrous bone, and this is the probable condition of the Pan-Homo LCA and the stem panin. The long axis of the petrous bone of S. tchadensis is relatively sagittal, the orientation of the long axis of the petrous of Au. afarensis and Au. africanus is intermediate, and P. aethiopicus, P. robustus, P. boisei, H. habilis, H. rudolfensis, H. ergaster, and H. sapiens all possess more coronally- orientated petrous bones. Data are not available for Ar. ramidus, Au anamensis, Au. garhi, or K. platyops. There is an obvious morphocline towards a coronally- orientated petrous long axis in the genus Homo and in Paranthropus, but it seems that coronally-orientated petrous bones arose independently within the Paranthropus and Homo subclades. The primitive condition among hominoids is for the apex of the petrous bone to be ossified anterior to the sphenoccipital synchondrosis. The Pan-Homo LCA, the stem panin, and the stem hominin most likely had the primitive condition of an ossified petrous apex. The first appearance of an un-ossified petrous apex occurs in H. sapiens. P. aethiopicus, P. boisei, Au. africanus, and H.

Full document contains 171 pages
Abstract: Cranial base morphology features in some hominin species diagnoses. One of the unsolved puzzles of the hominin cranial base is the evolutionary history of the apparent convergence seen in the cranial base morphology of two hominin subclades, Homo and Paranthropus. Both subclades apparently share the same suite of cranial base characters, namely, a reduction in anteroposterior length of the cranial base, more coronally-orientated long axes of the petrous component of the temporal bones of the cranial base, and a more centrally-located foramen magnum. Did the two subclades inherit this morphology from a recent common ancestor, or is the morphology homoplasic in the two subclades? This thesis had two main aims. The first was to provide a better comparative context for the study of hominin cranial base evolution; this forms Chapter 2 of the thesis. The second was to use animal models to investigate (A) the extent to which the cranial base is affected by morphological integration, and (B) whether three genes that have been implicated in one way, or another, in the development of the cranial base, affect its development in ways that are analogous to the differences between the cranial base of modern humans and our close living relatives, chimpanzees, bonobos and gorillas. Disruption of Disp1, Pax7, or FGFr3 each resulted in an increase on the length of the basioccipital bone in newborn mice. The basioccipital responded in a highly integrated fashion to various perturbations of normal growth. Morphological integration may facilitate apparent homoplasy in the hominin cranial base.