The African ape-like foot of Ardipithecus ramidus and its implications for the origin of bipedalism.

Thomas Cody Prang

doi:10.7554/elife.44433

Abstract

The ancestral condition from which humans evolved is critical for understanding the adaptive origin of bipedal locomotion. The 4.4 million-year-old hominin partial skeleton attributed to Ardipithecus ramidus preserves a foot that purportedly shares morphometric affinities with monkeys, but this interpretation remains controversial. Here I show that the foot of Ar. ramidus is most similar to living chimpanzee and gorilla species among a large sample of anthropoid primates. The foot morphology of Ar. ramidus suggests that the evolutionary precursor of hominin bipedalism was African ape-like terrestrial quadrupedalism and climbing. The elongation of the midfoot and phalangeal reduction in Ar. ramidus relative to the African apes is consistent with hypotheses of increased propulsive capabilities associated with an early form of bipedalism. This study provides evidence that the modern human foot was derived from an ancestral form adapted to terrestrial plantigrade quadrupedalism.

Highlights

There are numerous adaptive explanations for the origin of bipedalism (Darwin, 1871; Hewes, 1961; Lovejoy, 1981; Rose, 1991; Washburn, 1960; Hunt, 1996) that are inherently difficult to test directly (Smith and Wood, 2017), but each of them depends on alternative hypothetical models for the morphology and locomotor behavior of the human-chimpanzee last common ancestor (LCA; Richmond et al, 2001)
The morphometric affinities of the Ar. ramidus foot were evaluated by constructing a morphospace based on six geometric mean-standardized variables that are preserved in the ARA-VP-6/500 foot skeleton using Principal Components Analysis (PCA, Figure 2)
The first principal component accounts for 63% of the variance and is positively loaded by the lengths of the fifth metatarsal and fourth proximal phalanx

Summary

Introduction

Terrestrial bipedalism is widely regarded as a shared-derived characteristic of the hominin clade and understanding its evolution is one of the central foci of biological anthropology (Darwin, 1871; Wasburn, 1967; Fleagle et al, 1981; Richmond et al, 2001; Gebo, 1996; Begun, 2004; Lovejoy et al, 2009a; White et al, 2015). Hypotheses for the locomotor behavior of the LCA include vertical climbing (Stern, 1975; Prost, 1980; Fleagle et al, 1981), terrestrial knuckle-walking (Wasburn, 1967; Gebo, 1992; Gebo, 1996; Pilbeam, 1996; Richmond and Strait, 2000; Richmond et al, 2001; Begun, 2004; Inouye and Shea, 2004), belowbranch suspension (Keith, 1923; Tuttle, 1969; Young et al, 2015), arboreal bipedality (Thorpe et al, 2007), and more generalized quadrupedalism with slow, deliberate climbing (Lovejoy et al, 2009a; White et al, 2009; White et al, 2015).

Methods

Results

Conclusion