An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia

Abstract

Only one partial skeleton that includes both forelimb and hindlimb elements has been reported for Australopithecus afarensis. The diminutive size of this specimen (A.L. 288-1 ["Lucy"]) has hampered our understanding of the paleobiology of this species absent the potential impact of allometry. Here we describe a large-bodied (i.e., well within the range of living Homo) specimen that, at 3.58 Ma, also substantially antedates A.L. 288-1. It provides fundamental evidence of limb proportions, thoracic form, and locomotor heritage in Australopithecus afarensis. Together, these characteristics further establish that bipedality in Australopithecus was highly evolved and that thoracic form differed substantially from that of either extant African ape.

Fig. 1.

Anatomically arranged elements of KSD-VP-1/1. A list of all elements is provided in SI Appendix, Table S1.

Fig. 2.

Rib curvature index of the second rib. Assessment method is shown in SI Appendix, Fig. S17. Although one Gorilla specimen (CMNH-B1781)(n = 33) fell within the human range, KSD-VP-1/1n lies well within the human range. The platypelloidy of Australopithecus probably made its inferior thorax mediolaterally broad; but this breadth would have been unrelated to the “pyramidal form” of African ape thoraces. Box plot parameters are provided in SI Appendix, Fig. S8.

Fig. 3.

X-rays of hominoid scapulas. (A) Modern human (CMNH-HTH-2450). (B) KSD-VP-1/1g. (C) Gorilla (CMNH-B-1730). (D) Pan (CMNH-B-3551). Each specimen has been scaled to the same approximate superoinferior glenoid height and aligned with its vertebral border approximately vertical. Note the uniqueness of Pan if a line is drawn connecting each specimen's superior and inferior angles (largely vertical in D). The human's glenoid angle is among the most superior in our sample (n = 21). All specimens, save Pan, have similar glenoid orientations. Both Pan and Gorilla are distinguished from the hominids by their substantially greater inferomedial spine orientation. KSD-VP-1/1g is most similar to humans. Pan is clearly the morphological outlier.

Fig. 4.

PCA of scapular angular data. PC1 for five scapular angles (schematic is shown in SI Appendix, Fig. S23) discriminates humans from African apes but especially from Pan. KSD-VP-1/1g falls within the human cloud. The standardized distance (here the univariate equivalent of the Mahalanobis D²) between African apes and humans is very large at 4.09. PC1 is most heavily influenced by the axillary–spine and axillary–glenoid angles (Table 3). Repeating the analysis using variance-covariance rather than correlations increases D² to 4.57. PC2 and PC3 did not discriminate humans from apes. These data suggest that scapular spine orientation is a principal discriminator of pectoral function in humans and African apes, whereas glenoid orientation is a poorer discriminator.

Fig. 5.

Log-log scatterplot of tibia length versus upper limb size in KSD-VP-1/1 and A.L. 288–1. A.L. 288–1 tibia length from the mean crural index in hominoids (SI Appendix, Table S11). A reference line of slope equal to 1 is provided for reference (intercept = 2.66). Note that KSD-VP-1/1 and A.L. 288–1 fall close to this line, which passes through the human distribution.