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

Nathan M Young,Terence D Capellini,Neil T Roach,Zeresenay Alemseged

doi:10.1073/pnas.1511220112

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 ape-like 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.

Highlights

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
We found that a sustained shift in scapular shape occurred during hominin evolution from an African ape-like ancestor to a modern human-like form, first present in our genus, Homo
As in previous analyses [3, 4], Homo does not significantly overlap with any primate but instead resides in a unique position that combines primitive “quadrupedal” characteristics like a long, robust scapular spine that is perpendicular to the vertebral border with a blade shape more typical of African apes

Summary

Introduction

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. Scapular remains attributed to Australopithecus afarensis are described as similar to Gorilla [6,7,8] 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. The human fossil record is sparse, and little is known about the earliest members of our lineage This unresolved ancestor complicates reconstructions of what behavioral shifts drove major speciation events. We found that a sustained shift in scapular shape occurred during hominin evolution from an African ape-like ancestor to a modern human-like form, first present in our genus, Homo. These data suggest a long, gradual shift out of the trees and increased reliance on tools as our ancestors became more terrestrial

Methods

Results

Conclusion