Like Father, Like Son: Assessment of the Morphological Affinities of A.L. 288–1 (A. afarensis), Sts 7 (A. africanus) and Omo 119–73–2718 (Australopithecus sp.) through a Three-Dimensional Shape Analysis of the Shoulder Joint

The postcranial evidence for the Australopithecus genus indicates that australopiths were able bipeds; however, the morphology of the forelimbs and particularly that of the shoulder girdle suggests that they were partially adapted to an arboreal lifestyle. The nature of such arboreal adaptations is still unclear, as are the kind of arboreal behaviors in which australopiths might have engaged. In this study we analyzed the shape of the shoulder joint (proximal humerus and glenoid cavity of the scapula) of three australopith specimens: A.L. 288–1 (A. afarensis), Sts 7 (A. africanus) and Omo 119–73–2718 (Australopithecus sp.) with three-dimensional geometric morphometrics. The morphology of the specimens was compared with that of a wide array of living anthropoid taxa and some additional fossil hominins (the Homo erectus specimen KNM-WT 15000 and the H. neanderthalensis specimen Tabun 1). Our results indicate that A.L. 288–1 shows mosaic traits resembling H. sapiens and Pongo, whereas the Sts 7 shoulder is most similar to the arboreal apes and does not present affinities with H. sapiens. Omo 119–73–2718 exhibits morphological affinities with the more arboreal and partially suspensory New World monkey Lagothrix. The shoulder of the australopith specimens thus shows a combination of primitive and derived traits (humeral globularity, enhancement of internal and external rotation of the joint), related to use of the arm in overhead positions. The genus Homo specimens show overall affinities with H. sapiens at the shoulder, indicating full correspondence of these hominin shoulders with the modern human morphotype.针对南方古猿属（Australopithecus）的颅后骨骼证据表明，南方古猿能够进行两足行走；然而其前肢形态，尤其是肩带形态，显示它们已部分适应树栖生活方式。这类树栖适应的本质尚不明确，南方古猿可能存在的树栖行为类型亦然。本研究采用三维几何形态测量学（three-dimensional geometric morphometrics），对三件南方古猿标本的肩关节形态——即肱骨近端与肩胛骨关节盂——进行了分析：A.L. 288–1（阿法南方古猿，A. afarensis）、Sts 7（非洲南方古猿，A. africanus）以及Omo 119–73–2718（未定种南方古猿，Australopithecus sp.）。研究将上述标本的形态与大量现生类人猿类群，以及部分化石人属成员——直立人（Homo erectus）标本KNM-WT 15000、尼安德特人（H. neanderthalensis）标本Tabun 1——进行了对比。研究结果显示，A.L. 288–1呈现出与智人（H. sapiens）和红毛猩猩属（Pongo）相似的镶嵌性状；而Sts 7的肩关节形态与树栖类人猿最为接近，未表现出与智人的形态相似性。Omo 119–73–2718则与更偏向树栖、具备部分悬垂行为的新世界猴绒毛猴属（Lagothrix）存在形态相似性。综上，三件南方古猿标本的肩关节兼具祖征与衍征特征——包括肱骨球状度提升、关节内外旋转功能增强——这些特征与手臂上举姿势的使用相关。人属（Homo）标本的肩关节整体与智人存在形态相似性，表明这些古人类的肩关节完全符合现代人类的形态型。