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Keywords:

  • cranial base;
  • evolution;
  • hominin

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

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. We use a parsimony analysis of published cranial and dental data to predict the cranial base morphology expected in the hypothetical last common ancestor 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.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

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 & 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 the densest part of the cranial skeleton. Paradoxically, it is better represented in the early part of the hominin fossil record than in the later parts of the Homo clade. 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 to 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 many to assume 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 it has been relatively neglected by palaeoanthropologists. This has changed since imaging methods have enabled researchers to access non-destructively information about the structure of the 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 to differences in habitual posture and locomotor mode. These findings, and the use of computed tomography (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 review.

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 morphometric (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 & Ravosa, 1993; Ross & Henneberg, 1995; Lieberman & McCarthy, 1999; Strait, 1999; McCarthy, 2001; Jeffery & Spoor, 2002, 2004; Bookstein et al. 2003; Jeffery, 2005). Traditional studies of the cranial base as a whole have concentrated on the gross morphology that can be seen not from the endocranial surface, but also 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 to compare the orientation of the petrous bones and the tympanic components of the temporal bones (Dean & Wood, 1981, 1982, 1984; Lockwood et al. 2002; Bastir et al. 2004) (Fig. 1).

image

Figure 1. Cranial base morphology in Homo sapiens (left) and Pan troglodytes (right). 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 adapted from Bookstein et al. (2003), of Pan troglodytes adapted from http://www.digimorph.org (2008).

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We use a parsimony analysis of published cranial and dental data (Strait & 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

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

Fourteen fossil hominin taxa were included in the cladistic analyses (Table 1) including 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. We excluded three taxa (i.e. Ardipithecus kadabba, Orrorin tugenensis, Australopithecus bahrelghazali) from detailed consideration because there are either limited or no cranial base data available. 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. Cercopithecoid phylogeny is beyond the scope of this paper. Details of all these samples are previously published (Strait & Grine, 2004).

Table 1.  Specimens included in the hypodigms of early hominin species
Sahelanthropus tchadensis:Paranthropus aethiopicus:
TM 266-01-060-1KNM-WT 16005, 17000
 L 55 s-33, 338y-6, 860-2
Ardipithecus ramidus:Omo 18-1967-18, 44-1970-2466,
ARA-VP 6/1, 1/128, 1/50057-4-1968-41
KNM-TH 13150 
KNM-LT 329Paranthropus robustus:
 DNH 7
Australopithecus anamensis:SK 6, 12, 13/14, 23, 34, 46, 47, 48, 49,
KNM-KP 29181, 29283, 2928652, 55, 65, 79, 83, 848, 1586
 SKW 5, 8, 11, 29, 2581,
Australopithecus afarensis:SKX 265, 4446, 5013
A.L. 33-125, 58-22, 128-23, 145-35,TM 1517
162-28, 188-1, 198-1, 199-1, 
200-1, 207-13, 266-1, 277-1,Paranthropus boisei:
288-1, 311-1, 333-1, 333-2,OH 5
333-45, 333-105, 333w-1,KGA 10-506, 10-525
333w-12, 333w-60,KNM-CH 1
400-1a, 417-1, 444-2KNM-ER 403, 404, 405, 406, 407, 725,
Garusi 1727, 728, 729, 732, 733, 801,
KNM-ER 2602805, 810, 818, 1468, 1469,
LH 41483, 1803, 1806, 3229, 3230,
MAK-VP 1/123729, 3954, 5429, 5877, 13750,
 15930, 23000
Australopithecus garhi:KNM-WT 16841, 17400
BOU-VP 12/130L 7a-125, 74a-21
 Natron
Australopithecus africanus:Omo 323-76-896
MLD 1, 2, 6, 9, 12, 22, 29, 34, 
37/38, 40, 45Homo habilis:
Sts 5, 7, 17, 20, 26, 36, 52a and b,A.L. 666-1
67, 71L 894-1
Stw 13, 73, 252, 384, 404, 498,OH 7, 13, 24, 62
505, 513KNM-ER 1478, 1501, 1502, 1805, 1813, 3735
Taung 1SK 15, 27, 45, 847
TM 1511, 1512Sts 19
 Stw 53
Kenyanthropus platyops: 
KNM-WT 38350, 40000Homo rudolfensis:
KNM-ER 819, 1470, 1482, 1483, 1590, 1801, 1802, 3732, 3891KNM-ER 819, 1470, 1482, 1483, 1590, 1801, 1802, 3732, 3891
UR 501UR 501
Homo ergaster: 
KNM-ER 730, 820, 992, 1507, 3733, 3883 
KNM-WT 15000 

A relatively recent comprehensive cladistic analysis of fossil hominins (Strait & Grine, 2004) used metric and non-metric characters taken from the literature (Delson & Andrews, 1975; Wood, 1975; Schwartz, 1984; Andrews & Martin, 1987; Chamberlain & Wood, 1987; Groves & Eaglen, 1988; Braga, 1995; Shoshani et al. 1996; Strait et al. 1997; Collard & Wood, 2000). The character matrix is made up of 198 characters, of which 89 are metric. Like Strait & 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 & 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 as 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 & 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 & 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; 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 & 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 & Miller, 1958; Cheverud, 1982, 1995, 1996; Zelditch, 1987, 1988; Chernoff & Magwene, 1999; Ackermann & Cheverud, 2000; Strait, 2001). Strait & Grine (2004) identified hypothesized character complexes and reduced the weight of characters within these complexes in order more closely to 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 & Cracraft, 1980; Wheeler, 1986), so 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 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). Finally, we randomized the order in which taxa were entered into the analysis.

A majority rules consensus cladogram of the most parsimonious trees is reported, and the percentage of trees supporting a given branch in the consensus cladogram is 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 uses only characters that are based on cranial base morphology.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

A 10 000 replicate bootstrap analysis resulted in 112 most parsimonious trees out of 10 113 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 Fig. 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 Fig. 3 (TL = 1059, CI = 0.68, 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.

image

Figure 2. Majority-rule 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 10 113 possible trees. (TL = 1007, CI = 0.71, 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.

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image

Figure 3. Majority-rule consensus of marginally less parsimonious trees. A majority rules consensus diagram of marginally less parsimonious trees is reported. The analysis reported in Fig. 2 resulted in10 086 trees within 1% of the most parsimonious tree (TL < 1107). (The consensus topology has a TL = 1059, CI = 0.68 and RI = 0.47.)

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In the consensus tree generated from the most parsimonious trees (Fig. 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 taxon to that clade, 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, as 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 taxa 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).

Tree topology is sensitive to the order in which taxa are entered into the analysis. A random order of the taxa was submitted to a 10 000 replicate bootstrapped analysis under similar conditions as the analysis described above. In Fig. 4A, a strict consensus tree of the seven most parsimonious trees is reported. The strict consensus tree results in a TL = 1007, a CI = 0.70 and a RI = 0.51. This analysis also supports the monophyletic relationship among Paranthropus and Homo, but not among the australopith archaic hominins. In both analyses Au. africanus is the sister taxon of Homo. In both analyses an unresolved polytomy is observed among Au. garhi and Paranthropus, and a (Kenyanthropus, Au. africanus, Homo) clade. The topology remains unchanged with respect to Au. afarensis and Au. anamensis. Note, however, that when S. tchadensis is not entered into the analysis first, its position as the sister taxon of all later hominins changes: Ar. ramidus is the PanHomo LCA, Au. anamensis is the proposed stem hominin and S. tchadensis is the sister taxon to the outgroups.

image

Figure 4. Portions of the topology are sensitive to the order in which taxa are analysed. A random order of the taxa was submitted to a 10 000 replicate bootstrapped analysis under similar parameters as the analysis described above. In (A), a strict consensus tree of the seven most parsimonious trees is reported (TL = 1007, CI = 0.70, RI = 0.51). A majority rules consensus of marginally less parsimonious trees is reported in B. A total of 17179 trees were considered (TL = 1055, CI = 0.65, RI = 0.44).

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A majority-rule consensus of marginally less parsimonious trees is reported in Fig. 4B. A total of 17 179 trees were considered. The consensus TL = 1055, the CI = 0.65 and the RI = 0.44. This analysis supports Paranthropus and Homo as monophyletic sister taxa; otherwise the relationships of S. tchadensis, Ar. ramidus and Au. anamensis are similar to those outlined above.

Character analysis

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 PanHomo 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 a more coronally orientated petrous. 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 Paranthropus and Homo.

The primitive condition among hominoids is for the apex of the petrous bone to be ossified anterior to the sphenoccipital synchondrosis. The PanHomo 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. habilis show the primitive condition of ossification anterior to the sphenoccipital synchondrosis. Data are not available with respect to this character for H. ergaster, P. robustus, H. rudolfensis, S. tchadensis, Au. afarensis, Ar. ramidus, Au. anamensis, Au. garhi and K. platyops.

Mastoid and temporomandibular joint.  A large and anteriorly placed postglenoid process is the primitive condition for hominoids, and the primitive condition is expected to be shared by the PanHomo LCA, the stem panin and the stem hominin. A trend towards a reduction in the size of the postglenoid process and increasing frequency of fusion between the tympanic bone and petrous bone is shared by the Paranthropus–Kenyanthropus–Australopithecus–Homo clade. However, according to the proposed topology this trend is followed independently within Paranthropus and Homo. S. tchadensis and Au. afarensis share the primitive condition and data are not available for Ar. ramidus, Au. anamensis, Au. garhi and K. platyops.

A small or absent vaginal process is primitive among hominoids and this state is also likely to be shared by the PanHomo LCA, the stem panin and the stem hominin. A trend towards a more substantial vaginal process is seen independently in early Homo and Paranthropus. The first appearance of a substantial bony vaginal process occurs as a synapomorphy in the (H. ergaster, H. sapiens) subclade. None of the proposed stem hominins, plus Au. afarensis, shares the derived state of a larger vaginal process. Note, however, that data with respect to the size of the vaginal process are not available for S. tchadensis, Ar. ramidus, Au. anamensis, Au. garhi or K. platyops.

A broad and shallow attachment of the posterior digastric muscle is the primitive condition for hominoids, the PanHomo LCA, the stem panin and the stem hominin. Missing data and homoplasy obscure the earliest appearance of the modern human character state. Au. afarensis has the primitive state, whereas Ar. ramidus has a deeper origin; no data are available for S. tchadensis, Au. anamensis, Au. garhi or K. platyops. A deeper origin of the posterior digastric is seen in P. robustus (but not in the other taxa within the Paranthropus clade), and a deeper origin of the posterior digastric is seen in the Homo clade. Apparently three independent acquisition or reversal events occurred within the hominin clade.

A wide supraglenoid gutter width is the primitive condition among hominoids, and it is likely to have been shared by the PanHomo LCA, and the stem panin. The stem hominin condition is ambiguous with respect to supraglenoid gutter width. There is limited information on the character for the earliest members of the hominin clade; data are not available for S. tchadensis, Ar. ramidus, Au. anamensis, Au. garhi or K. platyops. Au. afarensis has a modern human-like narrow supraglenoid gutter and the Paranthropus clade has the primitive condition of a wide supraglenoid gutter.

The primitive condition for hominoids is to lack lateral inflation of the mastoid process relative to the supramastoid crest. The PanHomo LCA, the stem panin and the stem hominin are expected to have expressed the primitive condition. Au. afarensis has the primitive condition; data are not available for S. tchadensis, Ar. ramidus, Au. anamensis, Au. garhi or K. platyops. Lateral inflation of the mastoid relative to the supramastoid crest is shared by each taxon within Paranthropus. Homo sapiens, H. ergaster and H. rudolfensis share the primitive condition. The variable presence of this character state in H. habilis raises the possibility that this trait might have arisen before Paranthropus diverged, in which case the trait was lost within Homo. However, there is strong support for Au. africanus as the sister taxon to Homo and this taxon expresses the primitive character state. Consequently, an independent acquisition of the trait in H. habilis is a more parsimonious hypothesis. The lateral inflation of the mastoid process relative to the supramastoid crest may be a synapomorphy of Paranthropus.

Tympanic.  A tubular tympanic bone (or a weak tympanic crest) is the primitive state among hominoids, and it is inferred to be the condition in the PanHomo LCA, and in the stem panin. The primitive condition is seen in S. tchadensis, Ar. ramidus, Au. anamensis, Au. afarensis and K. platyops; data are not available for Au. garhi. Consequently, the stem hominin was likely to have expressed the primitive condition. The modern human-like condition of a tympanic crest with a vertical tympanic plate is seen in P. aethiopicus, P. robustus, Au. africanus, H. habilis and H. ergaster. Note that P. boisei has an autapomorphic tympanic crest with an inclined plane. If the proposed topology is correct, at least one homoplastic event occurred within the hominin lineage. In one scenario, Kenyanthropus evolved the primitive condition of a tubular tympanic; alternatively Paranthropus and (Au. africanus, Homo) may have independently developed a tympanic crest with a vertical plate.

The mediolateral placement of the external auditory meatus (EAM) expresses a complex phylogenetic distribution. A medial placement of the EAM is observed in Au. anamensis, Au. afarensis, K. platyops, P. aethiopicus, Au. africanus, H. rudolfensis, H. ergaster and H. sapiens. A lateral position of the EAM is shared by Ar. ramidus, P. robustus and P. boisei. H. habilis has a variable placement of the EAM. No data are available for S. tchadensis or A. garhi. A medial external auditory meatus is present in Pan, Pongo and Hylobates, while a lateral position is present in Gorilla and Papio. Consequently, the outgroup taxa provide modest support for the hypothesis that the primitive condition among hominoids is a medial position of the EAM. The stem panin was likely to have retained a medial EAM. The character state of the stem hominin is completely ambiguous due to extensive homoplasy. At least three independent acquisitions of a more lateral external auditory meatus have occurred within Homo, Paranthropus and Ardipithecus according to the proposed topology.

A small external auditory meatus is the primitive state among hominoids, and is shared by the PanHomo LCA, the stem panin and the stem hominin. A large EAM is shared by many hominins, including Au. afarensis, Au. africanus, Paranthropus and Homo. Note that Ar. ramidus, Au. anamensis and K. platyops have the primitive state; data are not available for S. tchadensis or Au. garhi.

A prominent eustachian process of the tympanic is the primitive condition among hominoids; the PanHomo LCA and the stem panin retain the primitive condition. A prominent eustachian process is shared by S. tchadensis, Au. africanus and P. robustus. The eustachian process is absent or slight in Au. afarensis, P. aethiopicus, P. boisei, H. habilis, H. ergaster and H. sapiens. Data are not available for Ar. ramidus, Au. anamensis, Au. garhi, K. platyops or H. rudolfensis. These data predict that the stem hominin shared the primitive condition of a prominent eustachian process. Note, however, that this topology is supported by just one character state change, and that the morphology of a number of taxa remain unknown, and that this trait is characterized by a high level of homoplasy. There is homoplasy in Paranthropus; neither P. aethiopicus nor P. boisei have a marked eustachian process, but P. robustus does. The first appearance of the modern human pattern cannot be determined without additional fossil material, and different types of character optimizations result in equally parsimonious trees. A fast character optimization suggests that the reduced eustachian morphology seen in modern humans can be traced as far back as Au. afarensis. However, a slow character optimization suggests that a reduced eustachian morphology occurred later, within the genus Homo.

Squamous temporal bone.  The presence of an asterionic notch in the squamosal portion of the temporal bone is a hominoid synapomorphy. The PanHomo LCA, the stem panin and the stem hominin are each predicted to have had the primitive condition of an asterionic notch, as do Au. afarensis and P. aethiopicus. Hominin taxa lacking an asterionic notch include P. robustus, P. boisei, Au. africanus and Homo. Data are not available for S. tchadensis, Ar. ramidus, Au. anamensis, K. platyops or Au. garhi. Several evolutionary scenarios are consistent with these data. The presence of an asterionic notch may represent an autapomorphic character within P. aethiopicus, in which case the absence of the notch among the more derived Paranthropus taxa could represent a primitive retention. Alternatively, the absence of an asterionic notch may have evolved in parallel in the (P. robustus, P. boisei) and (Au. africanus, Homo) clades.

The absence of overlap between the parietal bone and the occipital bone at asterion is the primitive condition among hominoids. The PanHomo LCA, the stem panin and the stem hominin are likely to have shared the primitive condition. Australopithecus afarensis, Au. africanus, P. robustus and Homo share the primitive condition. Both P. aethiopicus and P. boisei have an overlap at asterion. Data are not available for S. tchadensis, Ar. ramidus, Au. anamensis, Au. garhi or K. platyops. Given the strength of the general support for a (P. aethiopicus, P. boisei, P. robustus) clade at least two character state changes are required to account for the observations about the parieto-occipital relationships at asterion.

The primitive condition among hominoids is to lack extensive parietal overlap at the parietosquamosal suture. We predict that the PanHomo LCA, the stem panin and the stem hominin had the primitive condition. Australopithecus afarensis, Au. africanus and Homo share the primitive condition, whereas P. aethiopicus, P. boisei and P. robustus express the derived condition of extensive overlap. Data are not available for S. tchadensis, Ar. ramidus, Au. anamensis, Au. garhi or K. platyops. Modern humans express the primitive condition and extensive overlap appears to be a Paranthropus synapomorphy.

The primitive condition among hominoids is to have extensive pneumatization of the temporal squama. The PanHomo LCA, the stem panin and the stem hominin are predicted have shared the primitive condition. Sahelanthropus tchadensis, Ar. ramidus, Au. anamensis, Au. afarensis, P. aethiopicus and Au. africanus each have the primitive condition. A reduction in the degree of pneumatization of the temporal squama is observed among P. robustus and Homo; P. boisei shows a variable degree of pneumatization. Data are not available for Au. garhi or K. platyops. Apparently there were two independent morphoclines of reduction in the degree of pneumatization of the temporal squama, one in Paranthropus, the other within Homo.

External cranial base flexion

External cranial base flexion is reduced among hominoids and we predict that this condition was shared by the PanHomo LCA and the stem panin. The morphology of the stem hominin with respect to external cranial base flexion remains ambiguous. A flat external cranial base angle is present in P. aethiopicus. A moderate degree of flexion is present in Au. africanus. A flexed external cranial base angle is seen in P. robustus, P. boisei, H. habilis, H. ergaster and H. sapiens. Data are not available for S. tchadensis, Au. afarensis, Ar. ramidus, Au. anamensis, Au. garhi, K. platyops or H. rudolfensis. Based on the proposed reference tree topology, a flexed external cranial base angle appears to have developed independently within Paranthropus and the (Australopithecus, Homo) clade.

Occipital bone

The foramen magnum is posteriorly situated among hominoids, and that is inferred to be the condition for the PanHomo LCA and the stem panin. The anterior margin of the foramen magnum is located at the bi-tympanic line in S. tchadensis, Ar. ramidus, P. aethiopicus, Au. africanus, H. ergaster, H. erectus and H. sapiens. In H. habilis the anterior margin of the foramen magnum is located at the bi-tympanic line, or anterior to it; in Au. afarensis, P. robustus and P. boisei it is placed well in advance of the bi-tympanic line. Data are not available for Au. anamensis, Au. garhi, K. platyops or H. rudolfensis. The modern human condition, where the anterior margin of the foramen magnum is at the bi-tympanic line can be traced back possibly to S. tchadensis and Ar. ramidus. The placement of the foramen magnum in a hypothetical stem hominin remains ambiguous; a posterior placement, or placement at the bi-tympanic line are equally parsimonious hypotheses.

A posteriorly inclined foramen magnum is a state that is shared among hominoids, the PanHomo LCA and the stem panin, and Au. africanus shares this primitive condition. A horizontal orientation of the foramen magnum is seen in P. robustus, P. boisei, H. habilis and H. sapiens. In H. ergaster the foramen magnum is inclined anteriorly; no data are available for S. tchadensis, Ar. ramidus, Au. anamensis, Au. garhi, K. platyops, Au. afarensis, P. aethiopicus or H. rudolfensis.

Two evolutionary scenarios are consistent with these data. In the first scenario, modern humans trace their horizontal foramen magnum back at least as far as the common ancestor of Paranthropus and Homo, in which case the primitive condition observed in Au. africanus would be an autapomorphic reversal. In a second scenario, the horizontal foramen magnum orientation evolved independently within Paranthropus and Homo. Additional fossil evidence is required in order to predict the morphology of the stem hominin. On the current evidence, a posteriorly inclined and a horizontal foramen magnum are equally parsimonious.

An ovoid rather than a heart-shaped foramen magnum is the predicted character state for the PanHomo LCA, the stem panin and the stem hominin. Hominin taxa lacking a heart-shaped foramen magnum include S. tchadensis, K. platyops, Au. afarensis, Au. africanus, P. robustus, H. habilis and H. sapiens. Paranthropus aethiopicus and P. boisei both have a heart-shaped foramen magnum, and it is also seen in some H. ergaster cranial bases. Data are not available for Ar. ramidus, Au. anamensis, Au. garhi or H. rudolfensis. Modern humans probably inherited their ovoid foramen magnum from the LCA shared with hominoids. Note, however, that the sister taxon to the later Homo clade, H. ergaster, has a variable expression of the foramen magnum outline. Within the hominin clade a heart-shaped foramen magnum is only seen in the Paranthropus subclade, and even then there was an apparent reversal of this feature in P. robustus.

The primitive condition among hominoids is to have a steep and posteriorly inclined nuchal plane and we infer this to be the morphology shared by the PanHomo LCA and the stem panin. A steep and posteriorly inclined nuchal plane is also seen in Au. africanus. A more horizontal nuchal plane is seen in P. robustus, P. boisei, H. habilis and H. sapiens; the only early hominin taxon to have a mildly anteriorly inclined nuchal plane is H. ergaster. Data about the inclination of the nuchal plane are not available for S. tchadensis, Ar. ramidus, Au. anamensis, Au. garhi, K. platyops, Au. afarensis, P. aethiopicus or H. rudolfensis. Two equally parsimonious scenarios are consistent with these data. In one scenario, Au. africanus inherits the primitive condition of a steep and posteriorly inclined nuchal plane from hominoids, with a horizontal orientation evolving independently within both the Paranthropus and the Homo clades. In this scenario the stem hominin would have a steep and posteriorly inclined nuchal plane. In the alternative scenario, a horizontally orientated nuchal plane arose in the hypothetical common ancestor of Paranthropus and Homo, and then Au. africanus subsequently reverted its character state back to a steep and posteriorly inclined nuchal plane. In this scenario, a horizontal or a steep and posteriorly inclined nuchal plane are equally parsimonious character states for the stem hominin.

A large longus capitus insertion is the primitive state for hominoids, is inferred to be the condition in the PanHomo LCA and the stem panin, and it is also seen in Au. africanus. A smaller longus capitus insertion is seen in P. aethiopicus, P. robustus, P. boisei, H. habilis, H. ergaster and H. sapiens; data are not available for S. tchadensis, Ar. ramidus, Au. anamensis, Au. afarensis, Au. garhi or K. platyops. The character state distribution of the size of the longus capitus insertion matches that of nuchal plane orientation. Two scenarios are consistent with these data. In one scenario, modern humans inherit a small longus capitus insertion from the LCA of Paranthropus and Homo, and the large insertion area seen in Au. africanus is an autapomorphic feature. According to this scenario, the stem hominin could have had either a primitive (or large) longus capitus insertion, or the derived (or small) condition. In an alternative scenario, a small insertion of longus capitus might have arisen in parallel in the Paranthropus and early Homo clades. In this scenario a large (or primitive) insertion of longus capitus is predicted for the stem hominin.

An occipital marginal (OM) sinus is relatively rare in hominoids, and we predict that the PanHomo LCA and the stem panin shared this inferred primitive condition. Hominin taxa with a low OM sinus frequency include K. platyops, P. aethiopicus, H. habilis and H. rudolfensis. An intermediate frequency is seen in Au. africanus and H. sapiens. The OM sinus occurs in a higher frequency in Au. afarensis, P. robustus and P. boisei; data are not available for S. tchadensis, Ar. ramidus, Au. anamensis, Au. garhi or H. ergaster. These observations suggest that the evolution of the endocranial venous sinus system in the posterior cranial fossa is complex. In the most parsimonious scenario an increase in the frequency of an OM sinus evolved on four separate occasions and we predict that the stem hominin had a low frequency of an OM sinus pattern.

Male hominoids have a well-developed compound temporonuchal crest, and we predict this would have been the case in the PanHomo LCA, the stem panin and in the stem hominin. This feature is seen in the presumed male specimens of S. tchadensis, Au. afarensis, P. aethiopicus and K. platyops, and in some, but not all, of the probable male fossils attributed to P. boisei and H. habilis. This morphology is not seen in Au. africanus, H. rudolfensis, H. ergaster and H. sapiens. Data are not available for Ar. ramidus, A. anamensis, A. garhi or P. robustus. There is a morphocline towards reducing the size and the frequency of the compound temporonuchal crest in the more recent part of the hominin clade.

Diagnostic characters

The results of comparing the observed and inferred distributions of the characters reviewed above in extant hominoids and in the LCA of the PanHomo clade are reported in Table 2. This analysis results in the hypothesis that the PanHomo LCA had up to eight cranial base synapomorphies. Several character states are ambiguous at the PanHomo LCA node. We suggest that the Pan–Homo ancestor differed from hominoids in the following ways: a shorter posterior skull length, a narrower supraglenoid gutter, greater bi-jugular foramen and bi-carotid widths, and an increase in the distance between the apices of the right and left temporal bones. Additional character states whose expression might change at the PanHomo LCA node include the position of the articular eminence above the occlusal plane (it may stay high, or be reduced), the distance between the mandibular fossae (it may be reduced), and the distance between the mastoid processes (it may increase). In Table 3 we compare the morphology of the cranial base seen in norma basilaris of the LCA of the PanHomo clade with the equivalent morphology of modern humans and chimpanzees. Traits reported in bold indicate a character state difference between that taxon and the LCA of the PanHomo clade. Finally, we identify character state changes that might distinguish the LCA of the PanHomo clade from a stem hominin (Table 4). In this analysis there is one unambiguous character state change and as many as 15 possible character state changes at this node.

Table 2.  Eight synapomorphies differentiate the PanHomo LCA from extant hominoids
Extant hominoidsPanHomo LCA
Posterior skull length – longer.Posterior skull length – shorter.
Supraglenoid gutter – wider.Supraglenoid gutter – narrower.
Articular eminence above occlusal plane.Articular eminence close to the occlusal plane.
Inter-mandibular fossa distance – larger.Inter-mandibular fossa distance – smaller.
Inter-mastoid process distance – smaller.Inter-mastoid process distance – larger.
Bi-carotid canal distance – larger.Bi-carotid canal distance – smaller.
Bi-jugular foramen distance – smaller.Bi-jugular foramen distance – larger.
Distance between the apices of the petrous temporal bones – smaller.Distance between the apices of the petrous temporal bones – larger.
Table 3.  Predicted cranial base and cranial base-related morphology of the PanHomo LCA compared with the predominant character states seen in H. sapiens and P. troglodytes
PanHomo LCAHomo sapiensPan troglodytes
Variable frequency of contact between the ethmoid bone and the lacrimal bone.High frequency of contact between the ethmoid bone and the lacrimal bone.Variable frequency of contact between the ethmoid bone and the lacrimal bone.
Contact between the ethmoid bone and the sphenoid bone present in the majority (i.e. 50–75% of cases).Contact between the ethmoid bone and the sphenoid bone present in c. 100% of cases.Contact between the ethmoid bone and the sphenoid bone present in most (i.e. > 75% of cases).
Ovoid foramen magnum.Ovoid foramen magnum.Ovoid foramen magnum.
Anterior boundary of the foramen magnum well behind the bi-tympanic line.Anterior boundary of the foramen magnum at the bi-tympanic line.Anterior boundary of the foramen magnum well behind the bi-tympanic line.
Flat external cranial base.Flexed external cranial base. Flat external cranial base.
Posteriorly inclined foramen magnum.Horizontal foramen magnum.Posteriorly inclined foramen magnum.
Anteroposterior length of the foramen magnum – ambiguous.Anteroposterior length of the foramen magnum – long.Anteroposterior length of the foramen magnum – short.
Foramen magnum width – ambiguous.Foramen magnum width – wide.Foramen magnum width – narrow.
Steeply inclined and posterior facing nuchal plane.Horizontal nuchal plane.Steeply inclined and posterior facing nuchal plane.
Broad and shallow posterior digastric attachment.Narrow and deep posterior digastric attachment.Broad and shallow posterior digastric attachment.
Attachment of longus capitus – large.Attachment of longus capitus – small.Attachment of longus capitus – large.
Asterionic notch present.Asterionic notch absent.Asterionic notch present.
No overlap between the parietal bone and the occipital bone at asterion.No overlap between the parietal bone and the occipital bone at asterion.No overlap between the parietal bone and the occipital bone at asterion.
Occipital marginal sinus – low incidence.Occipital marginal sinus – variable incidence.Occipital marginal sinus – low incidence.
Cerebellar morphology – lateral flare and posterior protrusion.Cerebellar morphology – tucked.Cerebellar morphology – lateral flare and posterior protrusion.
Opisthion to inion distance – large.Opisthion to inion distance – small.Opisthion to inion distance – large.
Posterior skull length – short.Posterior skull length – long.Posterior skull length – short.
Opisthion to infratemporal subtense – large.Opisthion to infratemporal subtense – small.Opisthion to infratemporal subtense – large.
Cranial base – long.Cranial base – short.Cranial base – long.
Occipital sagittal chord – long.Occipital sagittal chord – long.Occipital sagittal chord – short.
Occipital sagittal arc – large.Occipital sagittal arc – large.Occipital sagittal arc – small.
Bi-foramen ovale distance – small.Bi-foramen ovale distance – large.Bi-foramen ovale distance – small.
Bi-infratemporal crest distance – small.Bi-infratemporal crest distance – small.Bi-infratemporal crest distance – small.
Middle ear – deep.Middle ear – deep.Middle ear – deep.
Axis of ear bones – > 90°.Axis of ear bones – > 90°.Axis of ear bones – > 90°.
Area of inner ear – large.Area of inner ear – large.Area of inner ear – large.
Vaginal process – small or absent.Vaginal process – present and substantial.Vaginal process – small or absent.
Supraglenoid gutter – narrow.Supraglenoid gutter – narrow.Supraglenoid gutter – narrow.
Horizontal distance between the TMJ and the M2/ M3 boundary – long.Horizontal distance between the TMJ and the M2/ M3 boundary – short.Horizontal distance between the TMJ and the M2/ M3 boundary – long.
Large anteriorly placed postglenoid process.Small postglenoid process fused to the tympanic.Large anteriorly placed postglenoid process.
Articular eminence above the occlusal plane.Articular eminence close to the occlusal plane.Articular eminence close to the occlusal plane.
Inter-mandibular fossa distance – large.Inter-mandibular fossa distance – small.Inter-mandibular fossa distance – large.
Infratemporal fossa – short.Infratemporal fossa – very short.Infratemporal fossa – short.
Temporal fossa – varies in width.Temporal fossa – narrow.Temporal fossa – varies in width.
Lateral inflation of the mastoid process relative to the supramastoid crest – none.Lateral inflation of the mastoid process relative to the supramastoid crest – none.Lateral inflation of the mastoid process relative to the supramastoid crest – none.
Mastoid process length – medium.Mastoid process length – long.Mastoid process length – medium.
Inter-mastoid distance – medium.Inter-mastoid distance – small.Inter-mastoid distance – large.
Petrous orientation – sagittal.Petrous orientation – coronal.Petrous orientation – sagittal.
Petrous apex – ossified with projection.Petrous apex – not ossified with projection.Petrous apex – ossified with projection.
Bi-carotid canal distance – small.Bi-carotid canal distance – large.Bi-carotid canal distance – small.
Distance between the apices of the right and left petrous temporal bones – small.Distance between the apices of the right and left petrous temporal bones – large.Distance between the apices of the right and left petrous temporal bones – small.
Petrous portion of the tympanic – long.Petrous portion of the tympanic – short.Petrous portion of the tympanic – long.
Little or no overlap of the squamosal portion of the temporal bone.Little or no overlap of the squamosal portion of the temporal bone.Little or no overlap of the squamosal portion of the temporal bone.
Pneumatization of the temporal squama – extensive.Pneumatization of the temporal squama – less extensive.Pneumatization of the temporal squama – extensive.
Compound temporonuchal crest in males – present and marked.Compound temporonuchal crest in males – absent.Compound temporonuchal crest in males – present and marked.
Tympanic – tubular with a weak crest.Tympanic – plate-like and inclined.Tympanic – tubular with a weak crest.
External auditory meatus – medial.External auditory meatus – medial.External auditory meatus – medial.
Eustachian process – marked.Eustachian process – marked.Eustachian process – marked.
External auditory meatus small.External auditory meatus large.External auditory meatus small.
Bi-porionic breadth – ambiguous.Bi-porionic breadth – small.Bi-porionic breadth – small.
Bi-jugular foramen width – large.Bi-jugular foramen width – small.Bi-jugular foramen width – large.
Bi-tympanic breadth – large.Bi-tympanic breadth – small.Bi-tympanic breadth – large.
Tympanic length – large.Tympanic length – small.Tympanic length – large.
Very low auricular height.High auricular height.Low auricular height.
Bold text indicates a character state change from the state expressed by the PanHomo LCA.
Table 4.  Comparison between the PanHomo LCA and a hypothetical stem hominin
PanHomo LCAStem Hominin
  1. Bold text is an unambiguous character state change in the stem hominin.

Anterior boundary of the foramen magnum well behind the bi-tympanic line.Anterior boundary of the foramen magnum at the bi-tympanic line.
Flat external cranial base.Flat external cranial base or a more flexed cranial base.
Steeply inclined and posterior facing nuchal plane.Steeply inclined and posterior facing nuchal plane or horizontal nuchal plane.
Attachment of longus capitus – large.Attachment of longus capitus – large or small.
Posterior skull length – short.Posterior skull length – long.
Opisthion to infratemporal subtense – large.Opisthion to infratemporal subtense – large or small.
Cranial base – long.Cranial base – long or short.
Bi-foramen ovale distance – small.Bi-foramen ovale distance – small or medium.
Mastoid process length – medium.Mastoid process length – medium or long.
Inter-mastoid distance – medium or large.Inter-mastoid distance – medium.
Petrous orientation – sagittal.Petrous orientation – sagittal or intermediate.
Bi-carotid canal distance – smaller.Bi-carotid canal distance – smaller or larger.
Tympanic – tubular with a weak crest.Tympanic – tubular with a weak crest or plate-like and inclined.
Bi-porionic breadth – ambiguous.Bi-porionic breadth – small.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

Interest in the morphology of the cranial base as seen in norma basilaris can be divided into studies that are concerned with identifying relatively discrete, small-scale, taxonomically distinctive morphological features, and studies that are more concerned with larger scale changes in the size and shape of the major components of the cranial base. The former studies are exemplified by the observations of Weidenreich (1943) and by Rightmire (1990), suggesting that the shape and the relationships of the tympanic are distinctive in H. erectus and detailed differences in temporomandibular joint morphology were among the reasons why Spoor et al. (2007) assigned KNM-ER 42700 to H. erectus and not to H. habilis (see below). Larger scale studies have focused more on the relative and absolute contributions of the sphenoid and the temporal bones to the width of the cranial base and on the orientation of the tympanic and petrous components of the temporal bone. This analysis has focused on these larger scale relationships.

There is now substantial agreement among molecular biologists that the PanHomo divergence occurred between 4 and 8 Ma (for a review of this evidence see Bradley in this issue). Considerable effort is now being expended to identify and survey fossiliferous deposits within this time span in order to expand the early hominin hypodigm. Because of the paucity of the fossil evidence for many of the pre-1.5 Ma taxa recognized in speciose interpretations of the early hominin record the phylogenetic relationships represented in Fig. 2 must be considered provisional. However, this is probably the best we can do with the existing evidence, so we have used the results of this parsimony analysis to predict the expected cranial base morphology of the LCA of the PanHomo clade and in Table 3 we compare these predictions with the observed expressions of the same characters in modern humans and in common chimpanzees. Eight potential cranial base synapomorphies (i.e. differences between the states observed in extant hominoids and our predictions) are noted for the PanHomo LCA (Table 2) and at least two synapomorphies, an increase in posterior skull length and a more forwardly situated foramen magnum, together with ambiguity in several other characters is expected to distinguish the PanHomo LCA and the stem hominin (Table 4).

Evolutionary trends in the Paranthropus subclade

Whereas the total evidence analysis suggests a close relationship between P. aethiopicus and (P. robustus, P. boisei), the cranial base characters do not provide support for a close relationship among these taxa. This lack of support for a Paranthropus clade is also manifest in the results of the character state analyses presented above, as these consistently found evidence of homoplasy in Paranthropus cranial base morphology. However, a number of cranial base characters are synapomorphic for the Paranthropus clade and do not show evidence of homoplasy (based on the cranial and dental data set). For example, lateral inflation of the mastoid process relative to the supramastoid crest is shared among Paranthropus (Fig. 5). Similarly, all the Paranthropus taxa share a laterally placed external auditory meatus, overlap between the parietal bone and occipital bone at asterion, and overlap of the parietal where it articulates with the squamosal portion of the temporal squama. At least 14 cranial base traits suggest parallel evolution within Paranthropus and Homo.

image

Figure 5. Hominin cranial base morphological grades. This diagram shows early hominin taxa plotted against a vertical axis of time. The columns for each taxon are the best current estimate of the time of its first and last appearance, so the height of the columns represents their temporal span. The horizontal axis is an approximate reflection of the phenotype, with hominins with large brains, small chewing teeth and obligate bipedalism to the left and hominins with relatively small brains and/or large chewing teeth to the right. The taxa are colour coded according to whether they do, or do not, have the cranial base attributes set out in the key to the figure.

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Evolutionary trends in the Homo subclade

A number of characters unite the genus Homo (Fig. 5) including a more coronal orientation of the petrous bone, a deep and narrow posterior digastric notch, reduced pnuematization of the temporal squama, a more flexed cranial base, anterior inclination of the foramen magnum, and a reduced insertion of longus capitus. Among the characters supporting a (H. habilis (H. ergaster, H. sapiens)) grouping is a reduction in the distance between the TMJ and the occlusal plane. The presence of a prominent vaginal process links H. ergaster and H. sapiens, and modern humans are autapomorphic in that the apex of the petrous bone is not ossified anterior to the sphenoccipital synchondrosis.

Cranial base characters figured prominently in recent analysis of KNM-ER 42700, the calvaria of a young adult that was recovered in 2000 from Ileret, east of Lake Turkana, in Kenya. In H. habilis the TMJ retains the primitive condition of being mediolaterally relatively broad, and both the tympanic and the petrous are relatively sagittally orientated (as measured by the tympanomedian angle and the petromedian angles, respectively), so that they are similarly aligned. In contrast, in H. erectus the TMJ is narrow mediolaterally, the tympanic is relatively coronally orientated and the petrous is more sagittally orientated, so that contra the condition in H. habilis, the tympanic and the petrous components of the temporal bone meet at an angle (Spoor et al. 2007).

The cranial base is relatively well represented in the intriguing collection of hominins recovered from Dmanisi, and this review of the polarity of characters based on the macromorphology of the cranial base as seen in norma basilaris will help in the assessment of this material. A companion study of patterns of intraspecific variation in the cranial base of the extant higher primates (our unpublished data) will also aid in the ongoing discussions about the taxonomy of the Dmanisi cranial fossils.

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

Cranial base morphology has played a prominent role in hominin systematics since the earliest investigators began looking at the hominin record. When Raymond Dart assigned Taung to Au. africanus in 1925 the placement and orientation of the foramen magnum featured in his decision to assign the Taung skull to a novel species and genus. Cranial base morphology continues to contribute prominently to recent hominin diagnoses for Ar. ramidus and S. tchadensis. The importance of cranial base morphology can be attributed to its taphonomic as well as its morphological characteristics. The petrous bones are dense, well protected and they are evidently not preferred by carnivores. The cranial base is well represented in the hominin record and fossils recovered over the past 20 years have further expanded the geographical and temporal range of hominin fossil evidence of the cranial base (White et al. 1994; Gabunia et al. 2000; Brunet et al. 2002; Vekua et al. 2002; Brown et al. 2004; Spoor et al. 2007).

We used a parsimony analysis of published cranial and dental data (Strait & Grine, 2004) to predict the cranial base morphology expected in the hypothetical LCA of the PanHomo clade. We also predicted the primitive condition of the cranial base for the hominin clade, and documented 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.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

We are grateful to the Anatomical Society of Great Britain and Ireland for their invitation to take part in this symposium, and to Nick Lonergan and Rui Diogo for their constructive and insightful comments on previous versions of the manuscript. L.N. is supported by a George Washington University Academic Excellence Graduate Fellowship. The participation of B.W. was supported by George Washington University's Academic Excellence initiative, the George Washington VPAA, the George Washington University Professorship in Human Origins, and by the ASGB&I.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References