Rearfoot posture of Australopithecus sediba and the evolution of the hominin longitudinal arch

Abstract

The longitudinal arch is one of the hallmarks of the human foot but its evolutionary history remains controversial due to the fragmentary nature of the fossil record. In modern humans, the presence of a longitudinal arch is reflected in the angular relationships among the major surfaces of the human talus and calcaneus complex, which is also known as the rearfoot. A complete talus and calcaneus of Australopithecus sediba provide the opportunity to evaluate rearfoot posture in an early hominin for the first time. Here I show that A. sediba is indistinguishable from extant African apes in the angular configuration of its rearfoot, which strongly suggests that it lacked a longitudinal arch. Inferences made from isolated fossils support the hypothesis that Australopithecus afarensis possessed an arched foot. However, tali attributed to temporally younger taxa like Australopithecus africanus and Homo floresiensis are more similar to those of A. sediba. The inferred absence of a longitudinal arch in A. sediba would be biomechanically consistent with prior suggestions of increased midtarsal mobility in this taxon. The morphological patterns in talus and calcaneus angular relationships among fossil hominins suggest that there was diversity in traits associated with the longitudinal arch in the Plio-Pleistocene.

Figure 1. Rearfoot posture in humans, apes, and fossil hominins.

(A) Angular relationships among the major surfaces of the hominoid talus and calcaneus. Note the dissimilarity between A. sediba and H. sapiens in talar head declination and other angular relationships. (B) Canonical Variates Analysis (CVA) of talus and calcaneus variables showing 90% of the total sample variance (left). CVA of reduced dataset of talus and calcaneus variables showing 89.3% of the total sample variance (right). Homo = plus, Pan = purple triangles, Gorilla = green squares, Pongo = orange diamonds, Hylobates = red circles. Humans are completely distinct from non-humans and A. sediba (MH2) is indistinguishable from an African ape, whereas A. afarensis is human-like.

Figure 2. Talonavicular joint angle in extant hominoids and fossil hominins.

Bars represent the mean, box represents plus/minus one standard error of the mean, whiskers represent the bootstrapped 95% confidence interval, and circles represent individual data points. Light blue bar represents the range of modern human variation. From smallest to largest values, A. africanus = StW 88, StW 363, Homo/Paranthropus = Omo 323-76-898, KNM-ER 1464, OH 8, KNM-ER 5428, KNM-ER 813, A. afarensis = A.L. 288–1, A.L. 333–147, A. sediba = U.W. 88–98. Note that all fossil hominins fall outside of the range of variation of modern humans except for Australopithecus afarensis specimens and Omo 323-76-898, which fall within the ranges of variation of Homo, Pan, and Gorilla.