Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees

Abstract

Reconstructing the behavioral shifts that drove hominin evolution requires knowledge of the timing, magnitude, and direction of anatomical changes over the past ∼6-7 million years. These reconstructions depend on assumptions regarding the morphotype of the Homo-Pan last common ancestor (LCA). However, there is little consensus for the LCA, with proposed models ranging from African ape to orangutan or generalized Miocene ape-like. The ancestral state of the shoulder is of particular interest because it is functionally associated with important behavioral shifts in hominins, such as reduced arboreality, high-speed throwing, and tool use. However, previous morphometric analyses of both living and fossil taxa have yielded contradictory results. Here, we generated a 3D morphospace of ape and human scapular shape to plot evolutionary trajectories, predict ancestral morphologies, and directly test alternative evolutionary hypotheses using the hominin fossil evidence. We show that the most parsimonious model for the evolution of hominin shoulder shape starts with an African apelike ancestral state. We propose that the shoulder evolved gradually along a single morphocline, achieving modern human-like configuration and function within the genus Homo. These data are consistent with a slow, progressive loss of arboreality and increased tool use throughout human evolution.geometric morphometrics | developmental simulation | phylomorphospace | scapula | rotator cuff T he human shoulder exhibits a unique combination of traits for a primate, a fact that has complicated previous attempts to reconstruct both its functional and evolutionary history. Notably, humans are most closely related to knuckle-walking/suspensory chimpanzees and bonobos (Pan or panins) (1, 2), yet morphometric analyses suggest that our shoulders are most similar in shape to that of the highly arboreal, quadrumanous orangutan (Pongo) (3-5). The hominin fossil record is similarly complicated. Scapular remains attributed to Australopithecus afarensis are described as similar to Gorilla (6-8) whereas the more recent Australopithecus sediba (MH2) displays morphometric affinities to both African apes (gorillas, chimpanzees, and bonobos) and Pongo (9). This mix of character states raises the question of whether modern human morphology reflects evolution from a more derived African ape morphology or retention of primitive traits from an earlier ape ancestor.A critical piece of evidence for solving this puzzle is the morphotype of the hominin-panin last common ancestor (LCA) (∼6-7 Mya). Although the fossil record near the hypothesized divergence time with Pan is the most direct means of addressing what the anatomy of the LCA shoulder was like, both hominin and African ape fossils from this time period are rare, fragmentary, and poorly understood (10, 11). Despite these difficulties, competing hypotheses about the LCA make explicit and mutually exclusive predictions of the direction, magnitude, and ordering of character transformations that we should observe in the fo...

Reevaluating the functional implications of Australopithecus afarensis navicular morphology

“…All available evidence on the feet of A. afarensis suggests that terrestriality was adaptively significant due to the presence of many derived traits related to midtarsal stability, propulsion, and heel-strike (Latimer et al, 1987;Latimer andLovejoy, 1989, 1990;DeSilva, 2009DeSilva, , 2010Ward et al, 2011;Crompton et al, 2012;Prang, 2015a,b), which other hominins, such as Australopithecus sediba and Australopithecus africanus, lack (Zipfel et al, 2011;Prang, 2015aPrang, ,b, 2016. The presence of a longitudinal arch does not preclude a total locomotor repertoire that includes some degree of arboreality in A. afarensis (shown most recently by data on developmental changes in scapular morphology in juvenile apes related to climbing; Green and Alemseged, 2012;Young et al, 2015), African ape-like phalangeal morphology (Stern and Susman, 1983), and advanced climbing capabilities in some modern humans (Venkataraman et al, 2012). Although data on foot morphology (Latimer et al, 1987;Latimer and Lovejoy, 1989;DeSilva, 2009;Ward et al, 2011;Zipfel et al, 2011;Crompton et al, 2012;Prang, 2015aPrang, ,b, 2016, limb size proportions (Green et al, 2007), and hindlimb joint size (McHenry and Berger, 1998;Dobson, 2005) suggest that A. afarensis was the most terrestrial Australopithecus currently known, the potential for a locomotor repertoire including some amount of arboreality is not easily discounted.…”

An alternative interpretation of the Australopithecus scapula

“…In PNAS, Young et al (1) conclude that the scapula of the last common ancestor (LCA) of chimpanzees and humans was African apelike, supporting what they call the "African ape" model. This model was favored over the "ape convergence" model, in which the ancestral condition was more primitive and some morphological similarities shared among modern apes would reflect convergent evolution.…”

“…In their phylogenetic analyses, Young et al (1) conclude that the African ape model is more parsimonious than the ape convergence model. However, the difference between the two models is small (tree lengths: 0.085 and 0.100).…”

Resurrecting KIR2DP1: A Key Intermediate in the Evolution of Human Inhibitory NK Cell Receptors That Recognize HLA-C

“…The last common ancestor of humans and chimpanzees is considered to have been anatomically, physiologically and behaviorally more similar to modern chimpanzees than to modern humans (33–35). Thus the human line underwent greater changes and adaptations, as well as passage through narrower population bottlenecks (36), than the chimpanzee line.…”