Evidence of a chimpanzee-sized ancestor of humans but a gibbon-sized ancestor of apes

Mark Grabowski,William L Jungers

doi:10.1038/s41467-017-00997-4

Abstract

Body mass directly affects how an animal relates to its environment and has a wide range of biological implications. However, little is known about the mass of the last common ancestor (LCA) of humans and chimpanzees, hominids (great apes and humans), or hominoids (all apes and humans), which is needed to evaluate numerous paleobiological hypotheses at and prior to the root of our lineage. Here we use phylogenetic comparative methods and data from primates including humans, fossil hominins, and a wide sample of fossil primates including Miocene apes from Africa, Europe, and Asia to test alternative hypotheses of body mass evolution. Our results suggest, contrary to previous suggestions, that the LCA of all hominoids lived in an environment that favored a gibbon-like size, but a series of selective regime shifts, possibly due to resource availability, led to a decrease and then increase in body mass in early hominins from a chimpanzee-sized LCA.

Highlights

Body mass directly affects how an animal relates to its environment and has a wide range of biological implications
While body mass predictions for the earliest undisputed hominin, Australopithecus anamensis (46.3 kg)[3], and the earliest putative hominins O. tugenensis (35–50 kg)[36], and the later Ar. ramidus (~50 kg), are all in the range of common chimpanzees, these estimates are based on single fossils[3], and overall these findings argue that the pattern of body mass evolution in our own lineage may be more complicated than either stasis or a steady increase in body mass from a chimpanzee-like ancestor
Our results show that the earliest putative hominins (O. tugenensis, Ar. ramidus), and the early australopith Au. anamensis shared an selective regime with both species of Pan (Fig. 1a, b; Regime “m” in Table 2), and along with evidence that these fossil taxa were the mass of a chimpanzee, argue that the last common ancestor (LCA) of chimpanzees and humans was chimpanzee-sized

Summary

Introduction

Body mass directly affects how an animal relates to its environment and has a wide range of biological implications. Issues with body mass variation in Miocene hominoids are evident in the description of Ar. ramidus, which argues that the chimpanzee-human and African hominid LCAs were likely to be “equal to or smaller than Ar. ramidus, possibly even substantially so”7—a range extending down from the fossil’s predicted body mass (~50 kg)[7] and encompassing almost all primates from chimpanzees to diminutive monkeys. We test hypotheses of when shifts in selective regimes occurred along a phylogeny and estimate new optima (i.e., the optimal body mass of the new regime) that coincide with regime shifts by reconstructing the macroevolutionary adaptive landscape for primate and hominin body size This approach provides a new and novel source of information on a trait that directly influences numerous hypotheses on the paleobiology of the human lineage

Methods

Results

Conclusion