The pectoral girdle of StW 573 ('Little Foot') and its implications for shoulder evolution in the Hominina

Kristian J Carlson; David J Green; Tea Jashashvili; Travis R Pickering; Jason L Heaton; Amélie Beaudet; Dominic Stratford; Robin Crompton; Kathleen Kuman; Laurent Bruxelles; Ronald J Clarke

doi:10.1016/j.jhevol.2021.102983

The pectoral girdle of StW 573 ('Little Foot') and its implications for shoulder evolution in the Hominina

J Hum Evol. 2021 Sep:158:102983. doi: 10.1016/j.jhevol.2021.102983. Epub 2021 Apr 20.

Authors

Affiliations

¹ Department of Integrative Anatomical Sciences, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033, USA; Evolutionary Studies Institute, University of the Witwatersrand, Private Bag 3, Johannesburg WITS 2050, South Africa. Electronic address: kristian.carlson@usc.edu.
² Department of Anatomy, Campbell University School of Osteopathic Medicine, Buies Creek, NC 27506, USA; Evolutionary Studies Institute, University of the Witwatersrand, Private Bag 3, Johannesburg WITS 2050, South Africa.
³ Department of Integrative Anatomical Sciences, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033, USA; Department of Geology and Paleontology, Georgian National Museum, Tbilisi 0105, Georgia.
⁴ Department of Anthropology, University of Wisconsin, Madison, WI 53706, USA; Evolutionary Studies Institute, University of the Witwatersrand, Private Bag 3, Johannesburg WITS 2050, South Africa; Plio-Pleistocene Palaeontology Section, Department of Vertebrates, Ditsong National Museum, Pretoria 0001, South Africa.
⁵ Department of Biology, Birmingham-Southern College, Birmingham, AL 35254, USA; Evolutionary Studies Institute, University of the Witwatersrand, Private Bag 3, Johannesburg WITS 2050, South Africa; Plio-Pleistocene Palaeontology Section, Department of Vertebrates, Ditsong National Museum, Pretoria 0001, South Africa.
⁶ School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Private Bag 3, Johannesburg WITS 2050 South Africa; Department of Anatomy, University of Pretoria, PO Box 2034, Pretoria 0001, South Africa.
⁷ School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Private Bag 3, Johannesburg WITS 2050 South Africa.
⁸ Department of Musculoskeletal Biology, Institute of Life Course and Medical Sciences, University of Liverpool, Liverpool L7 8TX, UK.
⁹ TRACES, UMR 5608 of the French National Centre for Scientific Research, Jean Jaurès University, 31058 Toulouse, France; French National Institute for Preventive Archaeological Researches (INRAP), 30900 Nîmes, France; School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Private Bag 3, Johannesburg WITS 2050 South Africa.
¹⁰ Evolutionary Studies Institute, University of the Witwatersrand, Private Bag 3, Johannesburg WITS 2050, South Africa.

PMID: 33888323
DOI: 10.1016/j.jhevol.2021.102983

Abstract

The ca. 3.67 Ma adult skeleton known as 'Little Foot' (StW 573), recovered from Sterkfontein Member 2 breccia in the Silberberg Grotto, is remarkable for its morphology and completeness. Preservation of clavicles and scapulae, including essentially complete right-side elements, offers opportunities to assess morphological and functional aspects of a nearly complete Australopithecus pectoral girdle. Here we describe the StW 573 pectoral girdle and offer quantitative comparisons to those of extant hominoids and selected homininans. The StW 573 pectoral girdle combines features intermediate between those of humans and other apes: a long and curved clavicle, suggesting a relatively dorsally positioned scapula; an enlarged and uniquely proportioned supraspinous fossa; a relatively cranially oriented glenoid fossa; and ape-like reinforcement of the axillary margin by a stout ventral bar. StW 573 scapulae are as follows: smaller than those of some homininans (i.e., KSD-VP-1/1 and KNM-ER 47000A), larger than others (i.e., A.L. 288-1, Sts 7, and MH2), and most similar in size to another australopith from Sterkfontein, StW 431. Moreover, StW 573 and StW 431 exhibit similar structural features along their axillary margins and inferior angles. As the StW 573 pectoral girdle (e.g., scapular configuration) has a greater affinity to that of apes-Gorilla in particular-rather than modern humans, we suggest that the StW 573 morphological pattern appears to reflect adaptations to arboreal behaviors, especially those with the hand positioned above the head, more than human-like manipulatory capabilities. When compared with less complete pectoral girdles from middle/late Miocene apes and that of the penecontemporaneous KSD-VP-1/1 (Australopithecus afarensis), and mindful of consensus views on the adaptiveness of arboreal positional behaviors soliciting abducted glenohumeral joints in early Pliocene taxa, we propose that the StW 573 pectoral girdle is a reasonable model for hypothesizing pectoral girdle configuration of the crown hominin last common ancestor.

Keywords: Australopithecus; Clavicle; Locomotion; Scapula; Sterkfontein; Upper limb.

Publication types

Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

Animals
Biological Evolution*
Female
Fossils*
Gorilla gorilla / anatomy & histology
Hominidae / anatomy & histology*
Humans
Male
Scapula / anatomy & histology
Shoulder / anatomy & histology*

Grants and funding

BCS-0824552/National Science Foundation