Morphometric analysis of modern human crania: a framework for assessing early Pleistocene hominids E.J. Odes1,2 & J.F. Thackeray1* 1Institute for Human Evolution, University of the Witwatersrand, P.O. WITS, Johannesburg, 2050 South Africa 2School of Geosciences, University of the Witwatersrand, P.O. WITS, Johannesburg, 2050 South Africa Received 16 January 2012. Accepted12 October 2012 INTRODUCTION Thackeray (2005) has previously examined cranial varia- tion in modern hominoid primates, excluding modern humans, in the context of variation in skulls of extinct African hominins. In this study we obtain cranial data from a sample of modern humans, using more than 100 landmarks, to provide a frame of reference for assess- ing Plio-Pleistocene hominins. MATERIALS AND METHODS The modern comparative human sample in the Dart Collection at the University of the Witwatersrand, from cadavers, includes 24 African crania: Ndebele (two male, two female), Shangaan (two male, two female), Sotho (two male, two female), Swazi (two male, two female), Tswana (one male, one female), Xhosa (two female, two male), Zulu (one male), and one African human cranium; and five crania catalogued as Europeans (Caucasian). The choice of sample size was arbitrary, with the objective of securing a total sample of at least 25 individuals, recogniz- ing that this represents only part of the range of variation that would be expected from a global sample of H. sapiens. The measurements on the modern crania (Table 1) were based on landmarks previously used in Wood’s (1991) study of fossil hominins. The landmarks include Prosthion (pr); Nasospinale (ns); Nasion (n); Glabella (g); Bregma (b); Vertex (v); Apex (ap); Lambda (l); Opistocranion (op); Inion (i) Opisthion (o); Basion (ba); Alveolon ( alv); Staphylion (sta); Orale (ol); Euryon (eu); Porion (po); Mastoidale (ms); Alare (al); Orbitale (or); Zygion (zy); Ectoconchion (ec); Ectomalare (ecm); Pterion (pt); Endomalare (enm); Frontotemporale (ft). Measurements were obtained using (1) digital callipers manufactured by the Mitutoyo Corporation (product name Digimatic Caliper; model no: CD-6 inch CX; code no: 500-171-20; serial no: 09093312; measuring range 0–150 mm; minimum indication 0.01 mm); (2) a two- button digital calliper manufactured by Mitutoyo Corpo- ration, with the same technical specifications (resolution: 0.01 mm); and (3) a Mitutoyo digital linear spreading calliper with a 300 mm digital scale and a throat depth of 150 mm. The modern crania were positioned on a foam base for stability and protection. Measurements were taken using the same calipers throughout the study to minimize measurement error. Curved regions were measured using chord distance. Statistical method The method that is used in this study has been devel- oped by Thackeray et al. (1997, 2007). Morphological varia- tion within a species can be quantified using least squares linear regression analysis of measurements of pairs of specimens. The degree of similarity between two speci- mens of the same species can be expressed by comparing measurements of a reference specimen A (associated with the x-axis) and conspecific specimen B (associated with the y-axis). In such cases there is generally little scatter around the regression line, associated with the linear regression equation y = mx + c, where m represents the slope of the regression line, and c represents the constant; the log transformed standard error of the slope m is referred to as log s.e.m. The limited scatter around the regression line is associated with similarity in shape of the two conspecific specimens. The degree of scatter around the regression line is quantified by the standard error of the m-coefficient (s.e.m). The s.e.m value for pairs of conspecific pairs is relatively low. By contrast, when measurements of two specimens representing two different species are compared, there is ISSN 0078-8554 Palaeont. afr. (December 2012) 47: 25–28 25 *Author for correspondence. E-mail: francis.thackeray@wits.ac.za Craniofacial measurements have been obtained from modern human skulls from cadavers representing several southern African population groups including Ndebele, Shangaan, Sotho, Swazi, Tswana, Xhosa and Zulu, in addition to European Homo sapiens. The measurements were obtained from crania in the Dart Collection housed at the School of Anatomical Sciences of the University of the Witwatersrand. Pairwise comparisons, using least squares linear regression analysis of cranial measurements, were used to calculate the standard error of the m-coefficient associated with the general equation y = mx + c, where m is the slope of the regression line. The stan- dard error of the m-coefficient is a measure of the degree of similarity between specimens. Log transformed s.e.m values (log s.e.m) show a normal distribution with a mean value of –1.84 ± 0.087 (n = 384 pairwise comparisons). These results can be used as a frame of reference for comparing Early Pleistocene specimens. For example, a comparison between KNM-ER 1813 (attributed to H. habilis) and KNM-ER 3733 (attributed to H. erectus or H. ergaster) is associated with a log s.e.m value of –1.844. Despite differences in size, these two penecontemporary hominid fossils are associated with a high probability of conspecificity, since the log s.e.m value is identical to the mean log s.e.m value of –1.84 obtained for pairwise comparisons of modern Homo sapiens. Keywords: Homo sapiens, Homo ergaster, Homo habilis, morphology, craniofacial. a greater degree of scatter around the regression line and the s.e.m value is relatively high (Thackeray et al. 1997; Aiello et.al., 2000). The distribution of s.e.m values obtained from pairwise comparisons of extant conspecific pairs of specimens has been examined. Thackeray et.al. (1997) showed there is a log normal distribution of s.e.m values when pairwise comparisons are made between conspecific specimens of extant vertebrates including mammals, birds, reptiles and extant invertebrates. In a larger study (Thackeray, 2007), log s.e.m values are inclined to be centrally distributed around a mean log s.e.m value of –1.61 ± 0.23 (including vertebrates and invertebrates where n = 1424 specimens), not significantly different from a mean log s.e.m value of –1.66 ± 0.20 for inverte- brates (n = 172 specimens). Thackeray (2007) proposed that when comparisons are made between any two specimens of the same species, the log s.e.m approximates a ‘biological species constant’ (T = –1.61) which is considered to prevail over evolution- ary time and geographical space. The advantage of this approach is that the mean log s.e.m value of –1.61 ± 0.23 facilitates a definition of a species based on morphometric analysis. It allows for the assessment of probabilities of conspecificity associated with fossil specimens including hominins from South and East Africa, taking into account the fact that there is morphological variation in time and space, and recognizing that there is no clear boundary between hominin species in space and time, and no clear boundary between Australopithecus and Homo. The method has been used in this study to assess the degree of similarity between a sample of modern specimens of Homo sapiens, representing only part of the range of varia- tion that would be expected in a global sample. As an example of the application of this approach, comparisons 26 ISSN 0078-8554 Palaeont. afr. (December 2012) 47: 25–28 Table 1. List of craniofacial measurements. Number Measurement Number Measurement 1 Glabella–opisthocranion 2 Posthion–inion 3 Posterior cranial length 4 Basion–bregma 5 Basion–nasion 6 Porion height 7 Mastoid length 8 Minimum frontal breadth 9 Maximum parietal breadth 10 Maximum temporal breadth 11 Biporionic breadth 12 Supramastoid breadth 13 Maximum breadth across mastoid process 14 Biaterionic breadth 15 Interentoglenoid breadth 16 Entoglenoid breadth 17 Glabella–bregma 18 Glabella–bregma 19 Postglabellar sulcus–bregma 20 Postglabellar sulcus–bregma 21 Bregma–left pterion 22 Bregma–left pterion 23 Bregma–right pterion 24 Bregma–right pterion 25 Parietal sagittal length 26 Parietal sagittal length 27 Parietal temporal length 28 Parietal temporal length 29 Parietal coronal breadth 30 Parietal coronal breadth 31 Parietal lambdoid length 32 Parietal lambdoid length 33 Bregma–asterion 34 Bregma–asterion 35 Lambda–inion 36 Lambda–inion 37 Inion–opisthion 38 Inion–opisthio 39 Occipital sagittal length 40 Occipital sagittal length 41 Biasterionic breadth 42 Biasterionic breadth 43 Superior facial height 44 Superior facial length 45 Alveolar height 46 Subnasale–prosthion 47 Subnasale–prosthion (horizontal projection) 48 Subnasale–prosthion (vertical projection) 49 Superior facial breadth 50 Biorbital breadth 51 Bijugal breadth 52 Bizygomatic breadth 53 Bimaxillary breadth 54 Outer alveolar breadth 55 Anterior interorbital breadth 56 Orbital breadth 57 Orbital height 58 Orbitale–zygomaxillare 59 Minimum malar height 60 Malar thickness 61 Width temporal gutter 62 Vertical thickness of supraorbital torus 63 Anteroposterior thickness of supraorbital torus 64 Anteroposterior thickness of supraorbital torus 65 Anteropoterior thickness of glabella 66 Frontal torus breadth 67 Frontal torus breadth 68 Maximum nasal width 69 Nasal height 70 Rhinion–nasospinale 71 Sagittal length of nasal bones 72 Superior breadth of nasal bones 74 Inferior breadth of nasal bones 75 Infratemporal fossa depth 76 Foramen magnum length 77 Foramen magnum maximum width 78 Occipital condyle maximum length 79 Occipital condyle maximum width 80 Mandibular fossa length 81 Mandibular fossa length 82 Mandibular fossa breadth 83 Mandibular fossa breadth 84 Mandibular fossa depth 85 Depth of postglenoid process 86 Depth of articular eminence 87 Maxillo-alveolar length 88 Maxillo-alveolar breadth 89 Palate length (orale-staphylion) 90 Palate length (orale-palatomaxillary suture) 91 Palate breadth 92 Incisive canal-palatomaxillary suture 93 Internal alveolar breadth at M3 94 I1-I2 alveolar length 95 Canine alveolus breadth 96 P3-P4 alveolar length 97 M1-M3 alveolar length 98 Intercanine distance 99 P3 interalveolar distance 100 P4 interalveolar distance 101 M2 interalveolar distance 102 M3 interalveolar distance 103 Palatal height are between two penecontemporary Early Pleistocene hominin specimens, KNM-ER 1813 (attributed to H. habilis) and KNM-3733 (attributed to H. ergaster or H. erectus), dated around 1.6 million years, from Koobi Fora (East Turkana) in Kenya (Wood, 1991). RESULTS Results obtained from pair-wise comparisons of crania of modern humans (Table 2) are presented in Fig. 1. COMPARISON BETWEEN FOSSIL CRANIA A log s.e.m value of –1.84 has been obtained from a comparison of two early Pleistocene hominid crania, KNM-ER 1813 (attributed to H. habilis) and KNM-ER 3733 (attributed to H. ergaster or H. erectus). DISCUSSION AND CONCLUSION The results of this study show that the log s.e.m values obtained from 384 pairwise comparisons of human crania display a normal distribution, similar to results obtained from previous studies of both vertebrates and inverte- brates (Thackeray et al. 1997; Thackeray 2007). The mean log s.e.m value of –1.84 ± 0.09 obtained from the human crania provides a frame of reference for assessing Early Pleistocene hominids. Thackeray et al. (1997) compared conspecific specimens of extant vertebrates including mammals such as primates, rodents and ungulates; birds; reptiles; and invertebrates (Coleoptera and Lepidoptera). In a preliminary study the mean log-transformed s.e.m value calculated by Thackeray et al (1997) from comparisons of measurements of conspecific pairs of vertebrates and invertebrates was –1.78 ± 0.27 for 1260 specimens (70 extant species). Extending this approach Thackeray (2007) provided a statistical definition of a species, taking advantage of a substantially larger sample of vertebrates. Using that enlarged sample Thackeray (2007) calculated a mean log s.e.m of –1.61 ± 0.23 (n = 1424 specimens), which is not significantly different from the mean value obtained from the initial study. The result of –1.84 obtained from Homo sapiens crania in this study is lower than the mean log s.e.m value of –1.78 (Thackeray et al. 1997) and –1.61 (Thackeray, 2007) obtained for large samples of a diversity of vertebrates and invertebrates. Despite differences in size, KNM-ER 1813 and KNM-ER 3733 are examples of early Pleistocene hominid fossils which are associated with a high degree of similarity. The log s.e.m. value of –1.84 obtained from the comparison of these two specimens is identical to the mean log s.e.m. ISSN 0078-8554 Palaeont. afr. (December 2012) 47: 25–28 27 Figure 1. Normal distribution of log s.e.m values obtained from pair-wise comparisons of Homo sapiens crania in the Dart Collection. The mean log-transformed s.e.m value based on comparisons of conspecific pairs of Homo sapiens is –1.84 ± 0.09 ( n= 384 comparisons). Table 2. Human specimens from the Dart Collection, School of Anatomi- cal Sciences, University of the Witwatersrand. This list includes cata- logue numbers, population affinities, age and sex. Catalogue Population Age Sex number affinity 4011 Tswana 54 m 4069 Tswana 20 f 3955 Zulu 56 m 1532 Ndebele 69 f 1549 Ndebele 28 f 1274 Ndebele 39 m 1535 Ndebele 35 m 172 Shangaan 60 m 58 Shangaan 38 m 263 Shangaan 30 f 3057 Shangaan 30 f 2492 Sotho 21 f 2307 Sotho 60 f 2248 Sotho 48 m 2077 Sotho 48 m 1360 Swazi 19 f 1534 Swazi 24 f 2014 Swazi 42 m 1362 Swazi 49 m 1333 Xhosa 48 m 400 Xhosa 36 m 22 Xhosa 30 f 761 Xhosa 37 f 4035 African 22 f 2179 Caucasian 19 f 3902 Caucasian 40 m 3129 Caucasian 29 f 2186 Caucasian 57 m 3545 Caucasian 40 m value of –1.84 obtained for pairwise comparisons of modern Homo sapiens. The implication is that there is a high probability that KNM-ER 1813 and KNM-ER 3733 are conspecifics, despite the fact that they have previously been attributed to different species of Homo (H. habilis and H. erectus/ergaster). This serves as one example demonstrating the applicability of a morphometric approach to assess probabilities of conspecificity in fossil hominids. Thackeray (2005) has previously suggested that KNM-ER 1813 is a small female of a species also repre- sented by KNM-ER-3733, considered to be a large male. Both are dated to about 1.6 million years before present (BP). We are grateful to the NRF, the University of the Witwatersrand, the Andrew Mellon Foundation, and the French Embassy in South Africa for financial support. We thank Brendon Billings for access to material in the Dart Collection. We also thank Alan Morris and an anonymous referee for helpful comments. REFERENCES THACKERAY, J.F. 2005. Probabilities of conspecificity: an application of a morphometric approach to extant and extinct hominoids. In: �trkalj, G., Pather, N. & Kramer, B. (eds), Voyages in Science. Essays by South African Anatomists in Honour of Phillip V. Tobias’s 80th Birthday, 85–97. Pretoria, Content Solutions. THACKERAY, J.F., 2007. Approximation of a biological species constant? South African Journal of Science 103, 489. THACKERAY, J.F., BELLAMY, C.L., BELLARS, D., BRONNER, G., CHIMIMBA, C., FOURIE, H., KEMP, A., KRUGER M., PLUG, I., PRINSLOO, S., TOMS, R., VAN ZYL, A.J., & WHITING, M.J. 1997. Probabilities of conspecificity: application of a morphometric technique to modern taxa and fossil specimens attributed to Australopithecus and Homo. South African Journal of Science 93 : 195–196. WOOD, B. 1991. Koobi Fora Research Project, Vol. 4. Hominid Cranial remains. Oxford, Clarendon Press. 28 ISSN 0078-8554 Palaeont. afr. (December 2012) 47: 25–28 << /ASCII85EncodePages false /AllowTransparency false /AutoPositionEPSFiles true /AutoRotatePages /All /Binding /Left /CalGrayProfile (Dot Gain 20%) /CalRGBProfile (sRGB IEC61966-2.1) /CalCMYKProfile (U.S. Web Coated \050SWOP\051 v2) /sRGBProfile (sRGB IEC61966-2.1) /CannotEmbedFontPolicy /Warning /CompatibilityLevel 1.3 /CompressObjects /Tags /CompressPages true /ConvertImagesToIndexed true /PassThroughJPEGImages true /CreateJDFFile false /CreateJobTicket false /DefaultRenderingIntent /Default /DetectBlends true /ColorConversionStrategy /LeaveColorUnchanged /DoThumbnails false /EmbedAllFonts true /EmbedJobOptions true /DSCReportingLevel 0 /EmitDSCWarnings false /EndPage -1 /ImageMemory 1048576 /LockDistillerParams false /MaxSubsetPct 100 /Optimize true /OPM 1 /ParseDSCComments true /ParseDSCCommentsForDocInfo true /PreserveCopyPage true /PreserveEPSInfo true /PreserveHalftoneInfo false /PreserveOPIComments false /PreserveOverprintSettings true /StartPage 1 /SubsetFonts false /TransferFunctionInfo /Apply /UCRandBGInfo /Preserve /UsePrologue false /ColorSettingsFile () /AlwaysEmbed [ true /AmazoneBT-Regular /Humanist521BT-Bold /Humanist521BT-BoldItalic ] /NeverEmbed [ true ] /AntiAliasColorImages false /DownsampleColorImages true /ColorImageDownsampleType /Bicubic /ColorImageResolution 200 /ColorImageDepth -1 /ColorImageDownsampleThreshold 1.09500 /EncodeColorImages true /ColorImageFilter /DCTEncode /AutoFilterColorImages true /ColorImageAutoFilterStrategy /JPEG /ColorACSImageDict << /QFactor 0.40 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /ColorImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000ColorACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000ColorImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasGrayImages false /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 200 /GrayImageDepth -1 /GrayImageDownsampleThreshold 1.00500 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /GrayImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000GrayACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000GrayImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasMonoImages false /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 700 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.14286 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict << /K -1 >> /AllowPSXObjects false /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputCondition () /PDFXRegistryName (http://www.color.org) /PDFXTrapped /Unknown /SyntheticBoldness 1.000000 /Description << /FRA /JPN /DEU /PTB /DAN /NLD /ESP /SUO /ITA /NOR /SVE /ENU >> >> setdistillerparams << /HWResolution [1200 1200] /PageSize [5952.756 8418.897] >> setpagedevice