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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 98:575-593 (1995)

Basicranial Flexion, Relative Brain Size, and Facial Kyphosis in Homo Sapiens and Some Fossil Hominids

CALLUM ROSS AND MACIEJ HENNEBERG Biological Anthropology Research Programme, Department of Anatomy and Human Biology, University of the Witwatersrand, Parktown, Johannesburg, Republic of South Africa

KEY WORDS tomography, Brain evolution

Fossil hominids, Basicranial flexion, Computed

ABSTRACT Comparative work among nonhominid primates has demonstrated that the basicranium becomes more flexed with increasing brain size relative to basicranial length and as the upper and lower face become more ventrally deflected (Ross and Ravosa [19931 Am. J. Phys. Anthropol. 91:305- 324). In order to determine whether modern humans and fossil hominids follow these trends, the cranial base angle (measure of basicranial flexion), angle of facial kyphosis, and angle of orbital axis orientation were measured from computed tomography (CT) scans of fossil hominids (Sts 5, MLD 37/38, OH9, Kabwe) and lateral radiographs of 99 extant humans. Brain size relative to basicranial length was calculated from measures of neurocranial volume and basicranial length taken from original skulls, radiographs, CT scans, and the literature. Results of bivariate correlation analyses revealed that among modern humans basicranial flexion and brain sizehasicranial length are not significantly correlated, nor are the angles of orbital axis orientation and facial kyphosis. However, basicranial flexion and orbit orientation are significantly positively correlated among the humans sampled, as are basicranial flexion and the angle of facial kyphosis. Relative to the comparative sample from Ross and Ravosa (1993), all hominids have more flexed basicrania than other primates: Archaic Homo sapiens, Homo erectus, and Australopithecus africanus do not differ significantly from Modern Homo sapiens in their degree of basicranial flexion, although they differ widely in their relative brain size. Comparison of the hominid values with those predicted by the nonhominid reduced major-axis equations reveal that, for their brain sizehasicranial length, Archaic and Modern Homo sapiens have less flexed basicrania than predicted. H. erectus and A. africanus have the degree of basicranial flexion predicted by the nonhominid reduced major-axis equation. Modern humans have more ventrally deflected orbits than all other primates and, for their degree of basicranial flexion, have more ventrally deflected orbits than predicted by the regression equations for hominoids. All hominoids have more ventrally deflected orbital axes relative to their palate orientation than other primates. It is argued that hominids do not strictly obey the trend for basicranial flexion to increase with increasing relative brain size because of con-

Received October 28, 1994; accepted August 15, 1995. Address reprint requests to Dr. Callum Ross, Dept. Anatomical

Sciences, Health Sciences Center, SUNY Stony Brook, Stony Brook. NY 11794-8081.

0 1995 WILEY-LISS. INC.

576 C. ROSS AND M. HENNEBERG

straints on the amount of flexion that do not allow it to decrease much below 90". Therefore, if basicranial flexion is a mechanism for accommodating an expanding brain among non-hominid primates, other mechanisms must be at work among hominids. o 1995 Wiley-Liss, Inc.

Homo sapiens have long been known to have highly flexed basicrania. Extreme basicranial flexion in humans has been claimed to provide better balance of the head on the vertebralcolumn(Wood Jones, 1917; Weiden- reich, 1924, 1941; DuBrul, 19501, to create a pharynx shape suitable for the generation of vowel sounds used in human speech (Lait- man, 1985), to reduce stresses in the anterior part of the cranial base due to loading of the jaw joint or occipital condyles (Demes, 1985), or to be the result of increases in relative brain size and decreases in the relative size of the masticatory apparatus (Biegert, 1963; see review in Ross and Ravosa, 1993). Basi- cranial flexion has also been invoked as evidence in support of various hypotheses regarding the taxonomic relationships among hominoids (Dean and Wood, 1981, 1982, 1984; Shea, 1985). Elucidation of the functional and structural correlates of basicranial flexion is therefore of importance for the understanding of hominoid cranio-facial evolution.

Previous comparative work on basicranial flexion in non-hominid primates found no correlation between the degree of basicranial flexion and habitual orthograde posture but confirmed Gould's (1977) suggestion that increases in brain size relative to basicranial length are correlated with increased basicranial flexion (as indicated by decreases in the cranial base angle [CBA]) (Ross and Ravosa, 1993). In addition, positive correlations were found between measures of basicranial flexion and the orientation ofthe orbital axes and palate, indicating that as the basicranium becomes more flexed, the face becomes more ventrally deflected (i.e., more kyphotic or klinorhynch).

The present study was carried out to determine whether the degrees of basicranial flexion and palatal and orbital kyphosis in hominids follow the trends documented by Ross and Ravosa (1993) for non-hominid primates. Specifically, we sought to test the fol-

lowing null hypotheses: a ) that hominids have the degree of basicranial flexion expected for primates with their brain size relative to basicranial length, and b) that hominids have the orbit and palate orientation expected for primates with their degree of flexion. If hominids follow the trends of asso- ciation exhibited by nonhominid primates, then the same causes of flexion may be postulated; if all or some of the hominids deviate from these trends, then different mechanisms may be invoked.

MATERIALS AND METHODS Sample

The sample of Homo sapiens used in this study consisted of 99 adult skulls from the Raymond A. Dart Collection of Human Skel- etons in the Department of Anatomy and Human Biology at the University of the Wit- watersrand. The specimens were selected to represent both sexes, a wide size-range and diverse ethnic origins. An attempt was made to measure five males and five females from all the major ethnic groups sampled in the Dart Collection, with the remaining skulls chosen to sample specimens of unusual origins (see Table 1). Lateral radiographs of the skulls were made in the Department of Radiology, Johannesburg General Hospital.

The measurement techniques employed here (see below) require fairly complete and undistorted basicrania, a rare situation in fossil hominids. Moreover, of those hominids with complete basicrania, only a small proportion have been scanned by computed tomography, the East African hominids being the most important exceptions. As a result, the sample of fossil hominids available for this study was restricted to Sts 5, MLD 37/ 38 (Australopithecus africanus), OH9 (Homo erectus), and the Kabwe Skull (archaic Homo sapiens).

Measurements of Sts 5 and MLD 37/38 were taken from computed tomography (CT)

HOMINID BASICRANIAL FLEXION 577

TABLE 1. Composition of human sample (ethnicity and sex as recorded in Dart Collection records)‘

Unknown Ethnicity sex

Zulu Xhosa Venda Indian Tswana Malawi Dama Fingoe Amazon Sotho Ndebele European Chinese 8 Griqua 10 Bushman Kalanga Totals 18

Males

5 5 3 4 5 5 1

1

5 5

6 1

46

Females

6 5

4 5

1

1 5 5

3

35

Total

11 10 3 8

10 5 1 1 1 1

10 10 8

10 9 1

99

‘Zulu = Zulu-speaking; Xhosa = Xhosa-speaking; Venda = Venda- speaking; Indian = individuals classified by South African apartheid laws as Indian (probably related to peoples originating in Indian suh- continent); Tswana = Tswana-speakingpeoples; Malawi = Malawians; Dama = from Damaraland in northern Namibia; Fingoe = South African tribe; Amazon = Amazonian Indian; Sotho = Sotho-speaking peoples; Ndebele = Ndebele-speaking peoples; European = individuals classified as “White” by South African apartheid laws; Chi- nese = immigrants from China to South Africa; Griqua = individuals from old frontier community mostly of Khoi-San descent with European admixture; Kalanga = tribe from Botswana, Zimbabwe, and northern South Africa.

scans (1.5 mm slice thickness) of these fossils stored on tapes at the Transvaal Museum in Pretoria. They were displayed on the console of the CT scanner in the Department of Radiology, Johannesburg General Hospital, where measurements were taken. Hard copies of the scans were also made to enable comparison with the original fossils. CT scans (1.5 mm slice thickness) of the Kabwe (Broken Hill) Skull and OH9, in the care of the Foundation for Hominid Palaeoradiol- ogy, were measured on a console a t Philips Headquarters, Best, and University Hospi- tal, Utrecht, Holland. Table 2 lists the scan slices used for measurement of fossil hominids.

Measures The measures taken in this study are

those described by Ross and Ravosa (1993). Although a plethora of measures has been used in the study of basicranial flexion, we believe these new measures to be more appropriate for determining the relative orientations of the anterior and posterior portions of the basicranium than more “traditional”

TABLE 2. Computed tomography scans used to take measurements

Specimen Tape location Scan numbers

Sts 5 Transvaal Museum 41 (mid-sagittal) 32 & 51 (orbit)

MLD 37/38 Transvaal Museum 5 (mid-sagittal)

Kabwe skull Philips HQ All scansL

OH9 Philips HQ 201 (mid-sagittal)

Pretoria, SA

Pretoria, SA

Best, Netherlands

Best, Netherlands

‘Measurements of mid-sagittal variables were taken from the mid- sagittal section of a three-dimensional reconstruction of the Kabwe Skull.

(i.e., older) measures. As noted previously, the advantage of Ross and Ravosa’s measure of the cranial base angle is that the points used to define the orientation of the occipital clivus both lie on the clivus, and those used to define the orientation of the planum sphenoideum both lie on the planum sphenoideum. Thus, our cranial base angle measures only the relative orientation of the endocranial surfaces of these two bones, without the confounding effects introduced by using points such a s nasion or foramen cecum which lie off these bones.

Angles. The cranial base angle (CBA) is an attempt to quantify the relative orientations of the endocranial surfaces of the clivus ossis occipitalis (CO) and the planum sphenoideum (PS) (Fig. 1). The orientation of the endocranial surface of CO is represented by a line from basion (B in Fig. 1) to the point on clivus where dorsum sellae begins to curve away superiorly (D). This latter point is an attempt to estimate the position of the endocranial edge of the spheno-occipital syn- chrondrosis, which defines the anterior edge of the occipital clivus. The orientation of PS is represented by a line from the apex of the declivity above the sulcus chiasmatis (P) to the apex of the sloping posterior surface of the pit in which the cribriform plate is set (A). The angle between CO and PS is the CBA.

The angle of facial kyphosis (AFK) measures the Orientation of the palate to CO (Fig. 1). The plane of the palate is represented by a line between anterior (AN) and posterior (PN) nasal spines. This line is extended pos- teriorly to reach the plane of clivus: the angle between these lines is the AFK. Identifica-


Fig. 1. Diagram illustrating points and planes used to measure cranial base angle (LBA) and angle of facial kyphosis (AFK). B, basion; D, point where dorsum sellae curves away; P, top of declivity above sulcus chiasmatis; A, top of slope above cribriform plate; PN, posterior nasal spine; AN, anterior nasal spine.

tion of the posterior nasal spine on the radiographs was facilitated by comparisons with the skulls themselves. These comparisons with the original skulls were performed because in the replications performed to evaluate measurement error, these comparisons were not made and the measurement error of the AF'K was higher than that for the other measures (see below, Table 2).

The angle of orbital axis orientation (AOA) was measured according to the method defined by Ravosa (1988). The orbital axis is drawn in the following way (see Fig. 2). On lateral radiographs, the distance (X) from the center of the optic canal to the anterior point of overlap of the orbital roof and the contour of the anterior cranial fossa (A) was measured. The point on the inferior orbital border a t distance (X) was then marked (B) and a line drawn between point B and point A. The line AB defines the plane of the orbital aperture. The line bisecting this plane and passing through the center of the optic canal is the orbital axis (OA). The angle between OA and CO was measured as AOA (Fig. 2).

Linear measures. Linear measures of endocranial basicranial length (BL) were taken

Fig. 2. Diagram illustrating points and planes used to measure angle of orbital axis orientation (AOA). AOA is the angle between orbital axis (OA) and clivus (CO). A is point of overlap of the orbital roof and the contour of the anterior cranial fossa. X is distance from optic canal to A. B is point on inferior orbital margin at distance X from optic canal.


Fig. 3. Photo of midsagittal CT scan of Sts 5 illustrating positions of planum sphenoideum (PS) and clivus (CO). The angle between these planes is the CBA.

from the radiographs of the humans (Fig. l ) , from the mid-sagittal scans of MLD 37/38 and the Kabwe Skull, from the original of Sts 5 (Fig. 31, from a cast of OH9 in the Palaeoanthropology Research Unit at the University of the Witwatersrand, and from Maier and Nkini’s (1984) reconstruction of OH9. (In the latter instance the scale of the drawing [Maier and Nkini, 1984: Fig. 11 was assessed by measuring the width of the foramen magnum on the cast and on the drawing.) BL is estimated by measuring three segments: a) basion to pituitary point (Zuck- erman, 19551, b) pituitary point to the posterior point on PS (as defined above), and c) posterior to anterior point of PS. For each individual, a, b, and c were summed. The linear measures taken from each radiograph were then corrected by multiplying them by the ratio

prosthion-basion length measured on the radiopraDh

prosthion-basion length measured on the skull.

Measurements from scans of Sts 5, MLD 37/38,0H9, and the Kabwe Skull. Angu-

lar and linear measurements of the basicrania of Sts 5, MLD 37/38, OH9, and the Kabwe Skull were taken from CT images.

Sts 5. Hard copies of Images 27-55 were made and the mid-sagittal scan identified as Image 41 (Fig. 3) by comparison with the original specimen. To control for slight distortion in the specimen, the mid-sagittal measurements (CBA and AFK) were also made on Images 39,40, and 42 for comparison with those taken on Image 41 and found to differ by, a t most, one degree.

The CBA was measured as shown in Fig- ure 3. Figure 4A illustrates the positions of CO and PS on the original specimen: Figure 4B illustrates the position of the anterior edge of PS in more detail (APS).

The AOA used here was defined by Ravosa (1988) for measurement from lateral radiographs and uses points in parasagittal planes projected onto the radiograph. Be- cause measurements ofAOA in sts 5 were by necessity taken from parasagittal CT slices passing through the optic foramina, some modification of this technique was necessary. This was done by measuring the orientation of CO to the horizontal on Image 41 and the orientation of the orbital axes to the hori-


A

B

Fig. 4. Stereophotos of interior of Sts 5 calvaria illustrating A) position of CO and PS, and B) position I

of anterior point on planum sphenoideum (APS) .

zontal on Images 32 (left) and 51 (right), enabling the angles between the orbital axes and CO to be calculated trigonometrically. The optic foramen on the right side of Sts 5 is not preserved, so it was necessary to estimate its position using the optic foramen on the left side. This was done by positioning the cursor in the center of the optic foramen in Image 32 then bringing up Image 51 without moving the cursor. Although this technique does not take into account some minor distortion in the fossil, the results are deemed acceptable because the measurements obtained from the two sides of the skull are comparable (Table 5). The mean of these two values (144.8') was used.

MLD 37138. In this specimen, the anterior portion of planum sphenoideum and most of the face are missing. Consequently, it was not possible to measure AOA, AFK, or BL. Nor was it possible to measure the CBA using the anteriormost point on PS as defined above, as the pit in which the cribriform plate sits is missing. However, because a significant portion of PS is preserved, it was possible to estimate the CBA, yielding

a value not dissimilar to that obtained for Sts 5 (Table 5).

An estimate of BL in MLD 37/38 was also attempted. The distances from basion to pituitary point, and from pituitary point to the posterior edge of PS were measured from the CT scan. The values for these measurements are similar to those for Sts 5, so the length of PS in Sts 5 was used as the estimate of PS length used to calculate BL in MLD 37/ 38 (Table 5).

OH9. The only skull of Homo erectus for which scans were available is OH9, the midline basicranium of which is badly damaged, missing much of the dorsal surface of clivus and dorsum sellae. Maier and Nkini (1984) have attempted a reconstruction of the basicranium (redrawn in Fig. 5). The position of basion can be reconstructed from the out- line of the foramen magnum as only several millimeters of bone appear to be missing. The position of tuberculum sellae on the left side has allowed Maier and Nkini to approxi- mate the position of dorsum sellae; however, it is possible that dorsum sellae may have been lower than they suggest. The preserved


Fig. 5. Slightly parasagittal scan of OH9 with reconstruction of clivus by Maier and Nkini (1984). Redrawn from Maier and Nkini (1984). CO’A’ and CO’B’ represent our estimates of the likely upper and lower limits possible for the position of CO. MN is the position of CO as measured from Maier and Nkini’s reconstruction.

portions of tuberculum sellae and lamina cri- brosa can be used to reconstruct planum sphenoideum, although with limited confidence. One estimate of CBA (MN in Fig. 5) was taken from Maier and Nkini’s (1984) reconstruction, one was taken from a slightly modified version of this reconstruction, with the dorsum sellae slightly lower (CO’B’ in Fig. 5) and one was taken by rough estimate from the mid-sagittal scan stored at Philips Headquarters in Best (CO’A’ in Fig. 5). This latter reconstruction assumes that basion was further forward than Maier and Nkini reconstruct it. The three differing results, listed in Table 5, are presented with some circumspection. We have not examined the original skull itself, and would not be surprised if our estimates were significantly in error. Consequently, OH9 is given only cursory treatment in this study. Kabwe skull. Measurements of the

Kabwe Skull were taken from the mid-sagittal section (CBA, AFK, BL) or parasagittal section (AOA) of a three-dimensional reconstruction of CT slices stored at Philips. All points on the midline basicranium used for measurements in this study are preserved. The method used for measuring AOA in Sts 5 was applied to the left side of the Kabwe Skull with the assistance of Prof. Zonneveld.

Volumes. Estimates of overall brain size in the human sample were made by measuring the endocranial volume of the skulls with mustard seed. After plugging up foramina and holes due to breakage, the neurocranium was filled with mustard seed through the foramen magnum while gently shaking the skull from side to side. The skull was then tapped with a finger and more seed added. This procedure was repeated until no more mustard seed would fit into the braincase. The mustard seed was then poured into a graduated cylinder which was shaken until the column of seed had a flat surface at the top. The volume was then read off the cylinder.

The volumes of the neurocrania of OH9, Sts 5, and MLD 37/38 were taken from Hol- loway (1973a,b). Neurocranial volume in the Kabwe Skull was obtained from Pycraft (1928) rather than Holloway (1981) because Zonneveld’s (pers. comm.) estimate of neurocranial volume in Broken Hill using 3-dimensional reconstruction of the CT slices of the skull is identical to that obtained by Pycraft .

Statistical analyses Reliability of the radiographic methods

and the techniques for taking measurements


TABLE 3. Measurement error: Results o f the analysis of variance in measurements of CBA, AOA, and AFK

repeated fiue times on fiue human skulls (n = 25, k = 51

Total Error Mean variance variance Erroritotal

CBA 109.04 69.14 4.02 0.0582 AOA 118.78 37.98 2.23 0.0587 AFK 123.88 35.91 4.98 0.1386

from the radiographs were evaluated by taking five lateral radiographs of each of five skulls and measuring the CBA, AFK, and AOA once on each radiograph. The radiographs were all taken on one day; one radiograph for each specimen was measured each day for five days. One-way analysis of variance (ANOVA) (P < 0.05) indicated that the variance of the replicate measurements taken on single individuals is significantly less than the inter-individual variance. Er- ror variance calculated from the one-way ANOVA is shown in Table 3.

Descriptive statistics for all measurements taken from Homo sapiens are presented in Table 4. Sample sizes for H. sapiens are less than 99 because it was not possible to take measurements from all skulls that were radiographed. Index of Relative En- cephalization 1 (IREl)--cube root of neurocranial volume divided by basicranial length (Ross and Ravosa, 1993)-was calculated for each individual measured. Descriptive statistics for IREl are also given in Table 4. The values obtained for the fossil hominids are presented in Table 5, as are the means for the same measurements taken from the great apes.

In order to evaluate the null hypotheses, Student’s t-tests were used to determine whether the mean values for Homo sapiens and the individual variates for the fossil

hominids are significantly different from the values predicted from the nonhominid primate reduced major-axis (RMA) regression equations. Formulae for the standard errors used to calculate 95% confidence limits of the predicted individual values and the sample means were taken from Sokal and Rohlf (1981). RMA equations were used for predic- tion because we have no reason to believe that our x-values are measured without error. More importantly, many of our predic- tions pertain to x-values lying well outside the range of values used to generate the equation, a situation in which RMA per- forms better than least-squares (Draper and Smith, 1981; Ricker, 1984).

RMA slopes were compared using a computer program written by Tim Cole; y-intercepts were compared using the “quick test” of Tsutakawa and Hewett (1977). Table 6 lists the RMA equations for nonhominids, nonhominoids and hominoid primates, along with the standard errors of b and the y-intercept. Bivariate plots of the comparisons listed above were created and the RMA lines for non-hominid primates were added (Figs. 6-9). These lines enable the position of hominids relative to the general nonhominid primate trends to be evaluated. In Figures 7-9, the RMA for hominoids was also added to illustrate the divergent trends seen in hominoids. Note that the RMAs for nonhominid and nonhominoid primates are very similar (Table 6).

Loess (Lowess) curve-fitting was applied to the data on CBA and IREl in order to describe the nature of the change in CBA with increasing relative brain size. Being a nonparametric regression technique, Loess has the advantage of making fewer assump- tions about the form of the relationship be-

TAELE 4. Descriptiue statistics for measurements taken on sample of Homo sapiensl

Homo sapiens n Mean S.D. Min. Max.

Neurocranial volume (cc) 92 1,351 148.1 1,050 1,835 CBA (deg.) 93 111.8 7.43 92 135 AFK (deg.) 93 122.2 6.52 103 140 AOA (deg.) 93 114.6 6.40 102 133 Na-S-Ba (deg.) 83 134.7 6.09 116 149 IRE 1 89 1.65 0.172 1.36 2.25 BL (mm) 94 67.6 6.45 45.23 79.3

‘CBA = cranial base angle; AFK = angle of facial kyphosis; AOA = angle of orbital axis orientation; Na-S-Ba = nasion-sella-basion angle; IREl = index of relative encephalization 1; BL = basicranial length.


TABLE 5. Mean values for measurements on great apes (from Ross, 19931 and values for measurements on fossil specimens

Neuro volume CBA AFK AOA BL

n (cc1 (deg.) (deg.1 (deg.1 (mm) IRE 1

Pongo pygmaeus Pan troglodytes Gorilla gorilZa Sts 5

MLD 37/38 OH9'

(M&N) (CO'B') (CO'A)

Kabwe Skull

396.8 398.3 489.6 485.0

435.0

1,067.0 1,067.0 1,067.0 1.280.0

135.0 152.0 148.0 114.0

110.5

99.0 92.7

104.0 128.0

162.0 167.0 71.62 156.0 161.0 70.85 150.0 163.0 77.22

60.67 141.0 L = 144.2 R = 145.5

58.80

88.92' 88.92 88.92

125.0 127.0 81.60

1.03 1.04 1.02 1.30

1.29

1.15 1.15 1.15 1.33

'M&N, estimate of CO onentation taken from Maier and Nkini's (1984) reconstruction; CO'A, estimate of upper limit possible for orientation of CO; CO'B', estimate of lower hmit possible for onentation of CO. Estimate of basicranial length based on Maier and Nkini (1984) reconstruction,

TABLE 6. Statistics for RMA regression eauations for nonhominid. nonhominoid. and hominoid urimates

CBA x IREl Nonhominid

primates Nonhominoid

primates Hominoids

AOA x CBA Nonhominid


primates Hominoids

AFK X CBA Nonhominid

primates No n h o m i n o i d

primates Hominoids

AOA x AFK Nonhominid


primates Hominoids

n

64

58

11

66

60

8

66

60

9

65

59

8

b sb

- 132.30

- 138.92

-120.83

12.776

15.239

29.596

1.085 0.1049 1.204 0.8522 0.923 0.2035

1.045 0.1138 1.140 0.1130 0.786 0.1861

1.026 0.0667 1.050 0.0673 1.419 0.1246

277.62

304.81

216.51

10.868

12.758

33.074

-14,037 17.3364

-35.268 18.5827 25.198 29.2802

-23.201

-40.924 18.7914

18.7962 36.670 26.4301

12.145 9.9655 9.402 9.9932

-56.631 18.8118

r

-0.649

-0.571

-0.677

0.633

0.708

0.841

0.490

0.650

0.779

0.856

0.875

0.976

P

***

*:i*

***

**:!:

***

***

***

***

***

+**

tween CBA and IREl than standard para- metric techniques (Efron and Tibshirani, 1991).

Despite the fact that our measurement techniques are not ideal for intraspecific studies (Ross and Ravosa, 19931, the large size of our human sample and the low errors

associated with our measurements of the human sample (Table 2) suggest that it would be of interest to determine whether the nonhominid primate trends discussed in the In- troduction hold within humans as a group. Consequently, RMA equations and Pearson correlations were computed for the following

584

-

-

-

C. ROSS AND M. HENNEBERG

- H F y . 1 0 Non-hominoid primates

1 . - - 1 - . . 1 . 9 - 1 - . - 1 . . . , . .

W Hylohatids 0 GreatApes

+ Homo sapiens

Australopithecus africunus

0 Kabwe Nonhominid RMA

Hylohatids

0 Great Apes + Homo sapiens

4 Australopithecus africanus

Kahwe

190

180

170

160

150

140

130

120

110

I00

90

I "\

X ## X \u .6 .8 1 1.2 1.4 1.6 1.8 2 2.2

Index of relative encephalization 1

Fig. 6. Bivariate plot of the cranial base angle (CBA) and index of relative encephalization 1 (IRE11 in primates. The latter variable is calculated as the cube root of neurocranial volumehasicranial length. The reduced major-axis (RMA) regression line for nonhominid primates is shown. The polygon surrounding the human mean defines the range of values obtained over the human sample

190

8

8 - 's 2 9 130

'3 170 Q Y

150 Y

o a

22 110

+I- 0

90

70

Fig. 7. Bivariate plot of the angle of orbital axis orientation (AOA) and the cranial base angle (CBA) in primates. The reduced major-axes (RMA) for nonhominid and hominoid primates are added. The polygon surrounding the human mean defines the range of values obtained over the human sample.

bivariate comDarisons within the Homo sa- RESULTS ~~

Homo sapiens piens sample:*CBA vs. IRE1; CBA vs. AOA; CBA VS. AF'K; AOA vs. AF'K. Pearson correlation coefficients were acceDted as signifi- Descriptive statistics for the measures - cant at P < 0.05. taken o n the human sample are presented


180

170

6)

120 $ 110

100

Hylobatids

I 0 Great Apes I I + Homo sapiens I

I I 0 Ausfrulopithecus africanus

0 Kabwe

Cranial base angle (degrees)

Fig. 8. Bivariate plot of the angle of facial kyphosis (AFK) and the cranial base angle (CBA) in primates. The reduced major-axes (RMA) for nonhominid and hominoid primates are added. The polygon surrounding the human mean defines the range of values obtained over the human sample.

cu 0

90 I I . " I . ' ' I . . . I . . ~ l ' ' ' I ' ' ' I ~ ~ ' t

100 110 120 130 140 150 160 170 180

Angle of facial kyphosis (degrees)

Fig. 9. Bivariate plot of the angle of orbital axis orientation (AOA) and the angle of facial kyphosis (AFK) in primates. The reduced major-axes (RMA) for nonhominid and hominoid primates are added. The polygon surrounding the human mean defines the range of values obtained over the human sample.

in Table 4. Mean neurocranial volume and 1990; Tobias, 19941); other sample statistics the standard deviation of the sample are obtained in the present study may therefore very similar to those estimated for the hu- also be good estimates of the degree of human species by larger samples (approx. man variation. Similarly, the values for neu- 1,350 cc and 157 cc, respectively [Henneberg, rocranial volume obtained for the great apes


(Table 5) are comparable with those given by Tobias (1994).

Figure 6 is a plot of CBA (cranial base angle) against IREl (Index of Relative En- cephalization 1) with the RMA regression line for nonhominid primates added. Homo sapiens has a much larger brain size (estimated by neurocranial volume) relative to basicranial length than do other primates (Fig. 6): the mean IREl for humans lies outside the range for nonhominid primates. Also, as has long been known, modern humans have more flexed basicrania than other primates (Fig. 6). The range of values for the CBA in modern humans extends from 135", equal to the mean for the Pongo sample, to 92" (three standard deviations from the mean). The correlation between CBA and IREl that was observed across nonhominid primates (Ross and Ravosa, 1993) was not seen within the human sample. One-tailed t-tests reveal that the mean value for the cranial base angle in humans is significantly higher than the mean predicted by the nonhominid RMA.

The entire range of values for the angle of orbital axis orientation (AOA) recorded for Homo sapiens lies outside the range of mean values for other primates: humans have more ventrally deflected orbital axes than all other primates (Fig. 7). Modern humans also have more ventrally deflected orbital axes than expected for a hominoid with their CBA, despite the fact that the human mean was included in the calculation of the hominoid RMA (Fig. 7) ! However, the mean value for AOA among humans does not differ significantly from the mean value predicted by the nonhominid RMA regression equation (Fig. 7). Within the human sample, the correlation between AOA and CBA was moderate (r = 0.705) while the resulting RMA line (r = 0 .848~ + 19.768) is not significantly different from the RMA for AOA and CBA across hominoids.

The mean value for the angle of facial kyphosis (AFK) for the human sample is equalled or exceeded by the means of all but four other primates (Figs. 8 and 9). Thus, modern humans have comparatively ventrally deflected palates. However, the mean value for the AFK among modern humans is significantly higher (one-tailed t-test)

than the mean value estimated by the nonhominid RMA equation (Fig. 8). The correlation between AFK and CBA observed in interspecific comparisons across nonhominid primates (Ross and Ravosa, 1993) is observed within the human sample (r = 0.492) with the RMA equation (y = 0 .876~ + 24.261) not being significantly different from that for all hominoids.

Regression of AOA on AFK is illustrated in Figure 9. One-tailed t-tests reveal that, for their AFK, humans have more ventrally deflected orbital axes than predicted by the nonhominid RMA equation. AOA and AF'K are not significantly correlated within the human sample as they are among nonhominid primates.

Fossil hominids A. africanus and Archaic Homo sapiens

resemble modern humans in having more flexed basicrania than non-hominid primates. Two-tailed t-tests suggest that the CBAs of Sts 5 (114"), MLD 37/38 (126"), and Kabwe (126") are not significantly different from the mean for the Homo sapiens sample. If any of our estimates of the CBA in H. erectus is correct, then, contra Maier and Nkini (19841, OH9 also had a degree of flexion not significantly different from the human mean.

When CBA is regressed against IREl (Fig. 6) the Kabwe Skull is seen to resemble humans in having a significantly larger CBA 6 e . , a less flexed basicranium) than that predicted by the nonhominid RMA. The values for the CBA for OH9, Sts 5, and MLD 37/38 do not differ significantly from those predicted by the RMA equation for nonhominids. There are no significant differences between the slopes or y-intercepts of the RMA equations for the regression of CBA on IREl among nonhominid, nonhominoid, or hominoid primates (Table 6). Consequently, only the RMA for nonhominid primates is shown in Figure 6.

A. africanus resembles H. sapiens in having comparatively ventrally deflected orbits: the only primates with smaller AOAs are Tarsius syrichta (139") and H. sapiens (Fig. 7). Sts 5 has a higher AOA than predicted by the nonhominid RMA equation (one-tailed t- test), having more dorsally deflected orbital


axes than predicted for a nonhominid primate with its CBA.

Comparison of the RMAs for hominoids and nonhominoids reveals the slopes to be not significantly different, but the y-intercept of hominoids to be significantly higher than that of nonhominoids (Table 6). Thus, although the AOA and CBA covary across hominoids in a manner similar to that seen in nonhominoids, hominoids as a group have more dorsally deflected palates relative to their CBA than other primates.

Figure 8 illustrates the regression of AFK on CBA. Sts 5, the Kabwe Skull, and the great and lesser apes resemble modern humans in having more dorsally deflected palates than predicted by the nonhominid RMA. Comparison of the hominoid RMA regression of AFK on CBA with the RMA for nonhominoid primates reveals that the slopes are not significantly different (Table 6) but that the y-intercept for hominoids is higher than that for other primates. Thus, hominoids have more dorsally deflected palates €or their CBA than other primates.

As in humans, AOA relative to AF'K in Sts 5 is significantly smaller than that predicted by the nonhominid RMA equation (Fig. 9). Comparison of RMA regressions for hominoid and nonhominoid primates reveals that their slopes are significantly different (P < 0.05): for the range of values covered by primates, hominoids have more ventrally deflected orbital axes for their palate orientation than other primates.

SUMMARY OF RESULTS Homo erectus, Australopithecus africanus,

Archaic and Modern Homo sapiens have more flexed basicrania than other primates.

When relative brain size is taken into account, Archaic and Modern Homo sapiens have significantly less flexed basicrania than predicted by the nonhominid RMA. H. erectus and A. africanus have the degree of flexion predicted by the RMA regression equation for nonhominids (Fig. 6).

Modern humans have more ventrally deflected orbits than all other primates and, for their CBA, have more ventrally deflected orbits than predicted by the RMA for hominoids. However, they have the orbit orienta-

tion expected for a nonhominid primate with their degree of basicranial flexion. The orbit orientation ofA. africanus is as predicted for hominoids with their CBA, but higher than predicted for a nonhominid (Fig. 7). Compar- ison of RMA regression lines indicates that hominoids have more dorsally deflected palates than nonhominoids when CBA is taken into account.

All hominoids have more airorhynch palates than predicted for nonhominoids (and nonhominids) with their degree of basicranial flexion (Fig. 8). Modern humans have fairly kyphotic palates in comparison with other primates, however. Comparison of RMA regression lines indicates that hominoids have more dorsally deflected palates than nonhominoids when CBA is taken into account.

Hominoids, including humans, have more ventrally deflected orbital axes for their palate orientation than other primates (Fig. 9).

DISCUSSION Basicranial flexion

Humans have long been known to have extreme degrees of basicranial flexion and australopithecines have often been assumed to have a degree of flexion intermediate between that of humans and the great apes (e.g., Biegert, 1963). The results of the present study suggest that this is not the case: Australopithecus africanus, represented by Sts 5 and MLD 37/38, has the degree of basicranial flexion seen in Modern and Archaic Homo sapiens. If our estimates of basicranial flexion in OH9 are correct, then this is also true of Homo erectus. These results confirm observations by Ashton et al. (1975) that basicranial flexion measured endocranially is similar in Sts 5 to that in Homo sapiens. However, previous studies of basicranial flexion using measurements on the exterior of the skull found no similarity between Sts 5 and Homo sapiens (Laitman et al., 1979). This suggests that endocranial and exocra- nial flexion of the basicranium bear little if any relationship to each other.

Although A. africanus, H. erectus, and Ar- chaic and Modern humans have similar degrees of basicranial flexion, relative brain


size in the four groups is very different: the humans have a much greater brain size relative to basicranial length than A. africanus and H. erectus. In the context of non-human primates, A. africanus and H. erectus have the degree of flexion for their relative brain size predicted by the RMA, but Homo sapiens have less flexed basicrania than predicted. Thus, anatomically modern humans do not follow the nonhominid primate trend €or basicranial flexion to increase with increasing brain size relative to basicranial length. Be- fore addressing the deviation of humans from the non-hominid primate trend, why does this trend exist in other primates?

As noted by Ross and Ravosa (1993), increasing flexion of the basicranium effec- tively increases the volume of the skull utilized as neurocranium without increasing the length of the skull “in a fashion analo- gous to increasing the proportion of a sphere which is utilized without increasing the sphere’s diameter” (Ross and Ravosa, 1993, p. 319). Basicranial flexion thereby enables the brain to be enlarged without changing its “spherical” shape and without increasing the diameter of the skull. This is advanta- geous for two reasons. First, although the brain becomes larger, distances between the different parts of the brain are minimized. If the brain is enlarged by becoming longer, then the distance between rostra1 and caudal poles of the cerebrum, for example, would expand, increasing the time that it takes for neuronal impulses to travel between the two points. Given that many brain functions are not localized but are dispersed over various areas of cortex, this may be an important functional constraint on brain shape. Sec- ond, a spherical brain results in a spherical brain case. Spherical shell structures not only enclose the greatest amount of volume for a given surface area, but they also provide greater strength than cylindrical shell structures, enabling them to be thinner- walled (Demes, 1985). For example, Demes (1985) has argued that the thick wall of the Homo erectus skull may be necessary for strengthening a skull that is elongated in shape; the more spherical braincase of Homo sapiens facilitates a thinner-walled calvaria. Thus, there is reason to suggest that selec- tion for a strong and economical brain case

may select for increased basicranial flexion with increasing relative brain size.

If these explanations for the correlation between increasing relative brain size and increasing flexion are correct, why do humans not obey the nonhominid primate trend? Basicranial flexion is correlated with orbit and palate orientation, both across all primates and among modern humans, i.e., increasing basicranial flexion is associated with increased ventral deflection of the orbital axes and palate relative to clivus. If humans had the CBA predicted for a primate of their relative brain size, and values for AFK and AOA predicted for a primate with that CBA, their faces and basicrania would be so closely approximated as to occlude the airway and disturb the functional relationships in the masticatory apparatus. This is clearly untenable. It would seem that, given the relationships between basicranial flexion and the orientation of the upper and lower face demonstrated here, basicranial flexion cannot be reduced much below 90” without radical changes in the entire cranial architecture.

It is therefore likely that, because of spa- tial-packing problems, the degrees of basicranial flexion seen in A. africanus and H. erectus are close to the extreme value possible and that modern humans do not exhibit significantly more flexed basicrania because it would be functionally impossible. This im- plies that brain expansion in the human lineage beyond that seen in A. africanus must have been accommodated in the skull via mechanisms other than basicranial flexion. One possible mechanism is suggested below (see Facial Orientation).

There are also good reasons for believing that there are upper limits to the angle of flexion measured by the CBA. Retroflexion of the basicranium would not only limit the amount of space available for the brain, but it would also reverse the primary flexure of the brain that occurs during primate ontogeny. Thus, the few values for CBA that ex- ceed 180“ (Cheirogaleus major 181“, Loris tardigradus 181”, Alouatta belzebul 186“, A. palliata 188”, and Pithecia pithecia 182”) probably approach the upper limits for the CBA. If this is correct, then the line most appropriate for describing the relationship


100 1 I I I

0.5 1 .O I .5 2.0

Index of relative encephalization 1

Fig. 10. Plot of cranial base angle (CBA) against neurocranial volumehasicranial length (IRE11 with Loess (Lowess) regression line added.

between CBA and IRE1 is not a reduced major-axis (or least squares regression), but a logistic curve with asymptotes of approxi- mately 90" and 180". At present, however, only the flattening out of the bottom part of the distribution can be demonstrated. Fig- ure 10 illustrates a Loess regression line fit- ted to all the data from Figure 6 except OH9, this fossil being excluded because of its frag- mentary condition. The decrease in the slope of the curve as it approaches Homo sapiens is apparent. It is notable that the most flexed basicranium recorded among humans has a CBA of 92", slightly above the proposed lower limit of 90".

Facial orientation When interpreting these results, it is im-

portant to note that the angle of orbital axis orientation, the angle of facial kyphosis, and the cranial base angle measure the orientation of different planes (orbit, palate, planum sphenoideum) to the same plane-clivus ossis occipitalis. Consequently, changes in the orientation of clivus alone will result in apparently correlated changes in these angles. However, changes in the cranial base angle are not necessarily accompanied by changes in orbit orientation; across strepsirhines, CBA and AOA are not correlated

(Ross and Ravosa, 1993). Moreover, measures of orbit orientation that do not use clivus as a reference plane confirm that orbit orientation across haplorhines covaries with the degree of flexion; the degree of orbital frontation, or the degree of verticality of the orbital margins relative to a line from nasion to inion, covaries with the the CBA across haplorhines but not strepsirhines (Ross, 1995). Finally, although AOA and AFK covary with each other across primates with an RMA slope not significantly different from 1.00, as expected ifchanges in clivus orientation account for the correlation between the two (Table 6) , the nature of this relationship can change; the slope of the hominoid RMA is significantly different not only from the nonhominoid line, but also from a slope of 1.00. Thus, although it is possible for changes in clivus orientation alone to pro- duce correlated changes in the CBA, AOA, and AFK, it is clear that more complex morphological changes are being described by our measurements.

Hominoid primates (great apes, lesser apes, and hominids) follow the general trends for palate and orbital axis orientation to covary with basicranial flexion. However, they generally have more airorhynch faces (palates and orbits) than other primates, as reflected in the transposition of the hominoid RMA lines above that for other primates in Figures 7 and 8. These data support the hypothesis that this is the primitive condition for hominoids (Shea, 1985).

Humans are a significant exception to these trends. Their orbits are more ventrally deflected than is expected for a nonhominid primate with their degree of palatal kyphosis and more ventrally deflected than expected for a hominoid with their degree of flexion. However, they do have the palate orientation expected for a hominoid with their degree of basicranial flexion. Why do humans have such ventrally deflected orbits in comparison with other primates with similar degrees of flexion (i.e., Australopithecus africanus)? Why do they display the orbit orientation expected for a non-hominoid primate with their degree of flexion, rather than that expected for a hominoid?

Brain expansion in the post-australopithecine lineage leading to Homo sapiens proba-


bly occurred primarily via addition of mass to the periphery of the brain, to the neocortex. The brain ofHomo habilis, which is “ap- preciably” more encephalized than that of the australopithecines (Tobias, 19871, exhib- its “marked transverse expansion of the cerebrum, especially the frontal and parieto- occipital parts . . . and increased bulk of the frontal and parietal lobes” (%bias, 1987, p. 741). We hypothesize that addition of cortex to the frontal lobes of the brain was accom- plished by an anterior and inferior move- ment of the frontal pole relative to basicranial and facial structures. As a result, the orbits became ventrally deflected relative to the basicranium, explaining why humans have the value for the orbital axis orientation predicted for non-hominid primates rather than hominoids. In other words, modern humans have more ventrally deflected orbital axes than predicted for a hominoid with their CBA because of the need to accommodate brain expansion without basicranial flexion.

This may also explain why the hominoid RMA for the regression of AOA on AFK is steeper than that for nonhominids (Fig. 9). It was noted above that although palate and orbit orientation covary across hominoids, they do so in a manner different from that in other primates; i.e., hominoids tend to have more ventrally deflected orbital axes than nonhominoids with equivalent degrees of palatal kyphosis. It is possible that this enables brain expansion to be accommodated via ventral deflection of the upper face, while the associated ventral rotation of the palate is minimized to avoid possibly detrimental effects to the masticatory apparatus and airway, Humans already have one of the most kyphotic palates relative to clivus orientation of any primates (Fig. 81, confirming Lait- man et al.’s (1979) measurements using exo- cranial landmarks. It is difficult to imagine how further restriction of the oropharynx- due to approximation of clivus and palate- might be accommodated.

CONCLUSIONS Gould (1977) hypothesized that increased

brain size relative to basicranial length is the most important cause of the extreme ba-

sicranial flexion and other “paedomorphic” features characteristic of the human skull. Ross and Ravosa (1993) found, as predicted by Gould‘s hypothesis, that increasing basicranial flexion among nonhominid primates is significantly correlated with increasing brain size relative to basicranial length. The present study demonstrates that living and fossil hominids do indeed have larger brains relative to basicranial length than other primates, a s well a s having more flexed basicrania. However, when the values for hominid basicranial flexion are compared with those predicted by the nonhominid primate reduced major-axis equation, it is clear that Archaic and Modern Homo sapiens have less flexed basicrania than predicted for their relative brain sizes. Moreover, the considerable variability in relative brain size among hominids is not correlated with differences in the degree of basicranial flexion.

These results suggest that although Gould‘s (1977) hypothesis is generally true across all primates, increased basicranial flexion in post-australopithecine hominids is not attributable to increases in relative brain size. Hominids probably achieved the most extreme basicranial flexion possible early in their evolution; subsequent expansion in relative brain size was not accompanied by increases in basicranial flexion. This suggests that if basicranial flexion serves as a mechanism for accommodating an expanding brain, then brain expansion among hominids must have been accommodated by mechanisms other than basicranial flexion. One possible mechanism is ventral deflection of the upper face relative to both the basicranium and the palate; another is lateral expansion of the braincase above the petrous parts of the temporal bone.

Historical changes in patterns of morphological correlation

Despite the admittedly restricted sample of fossil hominids used here, we feel that the patterns of correlation discussed here (and in Ross and Ravosa, 1993) can be hypothesized to have arisen as the result of certain evolutionary changes in anthropoid skull morphology. In Figure 11, these changes are mapped onto an accepted phylogeny of the Hominoidea; their consequences for patterns


Apes A. africanus Homo sapiens

Other

0

Lower limit of

’ anthropoids

Fig. 11. Phylogeny ofhominoid primates with evolutionary events discussed in the text mapped onto it.

of interspecific and evolutionary correlation are depicted in the “correlation-line’’ diagram to the right. Extensive approximation of the bony orbits below the olfactory tract in the haplorhine stem lineage resulted in morphological integration of the orbits and anterior basicranium (Ross and Ravosa, 1993; Ross, 1994). As a result, interspecific changes in orbital axis orientation among anthropoids were accompanied by correlated increases in basicranial flexion (and vice versa). Subsequently, the hominoid stem lineage-or a predecessor-acquired increased airorhynchy (dorsal deflection of the face relative to the basicranium), accounting for the higher y-intercept of the hominoid RMA in Figure 7 (Shea, 1985, 1988). This is symbol- ized in the right side of Figure 11 by the transposition of the hominoid “correlation line” above that of other anthropoids. It is notable, however, that the correlation between AOA and CBA still obtains across hominoids. Increases in flexion in the hominoid lineage continued until the lower limit for the CBA (90”) was reached, possibly by some members of the A. africanus popula-

tion. Further increases in relative brain size in the lineage leading to modern humans- which could not be accommodated by increased basicranial flexion-were accommodated by ventral deflection of the orbital axes, giving Homo sapiens the orbit orientation expected for a nonhominid primate with their degree of basicranial flexion.

If this sequence of events is correct, the basicranium of Homo habilis should not be significantly different from that of humans (or A. africanus) in its degree of flexion. Whether the increased ventral deflection of the orbits posited for the humans lineage had already occurred in H. erectus or H. habilis remains to be determined. However, given the expansion of the neocortex that was well underway in Homo habilis, it might be expected that this taxon would exhibit some ventral deflection of the orbits relative to the basicranium.

If the sequence of events postulated in Fig- ure 11 is correct, then i t is important to explain why hominoids as a group have more airorhynch faces than other primates; it is this increased airorhynchy that enabled


Homo sapiens to accommodate subsequent neural expansion via ventral deflection of the orbital axes. It is also important to un- derstand how palate and orbit orientation might become dissociated during evolution; it is only via this dissociation that humans could ventrally deflect their orbits without also ventrally deflecting their palates. Such questions regarding morphological integration and “disintegration” during hominoid evolution remain to be addressed.

ACKNOWLEDGMENTS We thank Robin Van Der Riet for providing

access to the facilities in the Department of Radiology at Johannesburg General Hospi- tal, Elspeth Kruger for assistance in ob- taining images of Sts 5 , and Berneice Eales for the lateral radiographs of the Dart Col- lection skulls. Dr. Frans Zonneveld kindly provided hospitality and assistance in ob- taining access to CT scans in the curatorship of the Foundation for Hominid Palaeoradiol- ogy. At the Transvaal Museum, Dr. Francis Thackeray kindly granted permission to study Sts 5 and Mr. David Panagos expertly removed the calotte of Sts 5 . Thanks are also offered to Brigitte Demes and Charles Lockwood, whose comments improved the manuscript; Bill Jungers, who assisted with Loess regression; and David Strait, for pro- ductive discussions on basicranial evolution. C.R. was supported by a J.J. Smieszek Re- search Fellowship in the Department of Anatomy and Human Biology and by a Post- doctoral Research Fellowship from the Uni- versity of the Witwatersrand. This research was supported by a SAFRD Research Grant to M.H.

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