A metric investigation of the cranial base and vertebrae among extant African homininae: discrimination across posturo-locomotory complexes

Abstract

Cranial base angle, vertebral dimensions, vertebral curvature, and locomotive behavior differ among Homo, Pan, and Gorilla; but many distinctions are obfuscated by dimensional and behavioral overlap among the genera and their fossil relatives. To address these issues, cranial and vertebral measurements (suites) were examined among Homo, Gorilla, and Pan as representative hominines for their posture and locomotion or positional-locomotory complexes. An additional analysis considered Australopithecus afarensis (A.L. 288-1 and A.L. 333) for comparative purposes. Using size-adjusted values with applied Bonferroni adjusted alpha levels, significant results for both the Kruskal-Wallis H-test and Mann-Whitney U-tests indicated statistically significant differences among species for cranial base angle (p = 0.000) and vertebral body dimensions with coronal and sagittal facet orientation (p = 0.000 -- 0.003). Detected significance was present for thoracic and lumbar curvature (p = 0.000) and positional-locomotory complex (p = 0.000) among species, albeit only cranial base angle was significant for the Pan-Gorilla comparison. Moreover, post hoc Spearman's rho tests indicated significant results (p = 0.000 -- 0.009) with strong positive and negative correlations throughout the column for each species. However, no pattern among vertebral measurements throughout the vertebral column was detected. Lastly, Multinomial Logistic Regression yielded a correct classification percentage with significant model fit (p = 0.000) of 86.4% for the cranial base, 82.8-97.0% for all subsequent vertebrae, and 80.3% for thoracic and lumbar curvature among species. Positional locomotory complexes were also significant (p = 0.000) and yielded a correct classification percentage of 82.2% among bipeds and the two modes of knuckle-walking practiced by Pan and Gorilla respectively. However, misclassifications between human and nonhuman primates for cranial base angle and calculated vertebral curvature suggest that these variables are not viable for assessing either genera or positional-locomotory complexes. Lastly, both Australopithecus afarensis specimens (A.L. 288-1 and A.L. 333) were incorrectly classified. The A.L. 288-1 specimen identified as Homo and the misclassification of A.L. 333 as Pan suggest either species or vertebra misidentification. Overall, the data indicate that both vertebral corpus dimensions and coronal and sagittal facet orientations differ significantly among hominine taxa and can distinguish species and their respective posturo-locomotory complex. As for the evolutionary implications, human bipedalism is distinct as related to cranial base angle and vertebral measurements; however significant differences between Pan and Gorilla suggest homoplasy among measurements and denote parallelism for the emergence of knuckle-walking.