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

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.The pattern of body size evolution in hominids can provide insight into historical human ecology. Here, Grabowski and Jungers use comparative phylogenetic analysis to reconstruct the likely size of the ancestor of humans and chimpanzees and the evolutionary history of selection on body size in primates.

Fig. 1

Time-calibrated phylogenetic tree with selective regimes and estimated body size optima. a Primate phylogenetic tree including fossils with tips color-coded to denote families, major families noted on the right. Phylogeny showing complete species names shown in Supplementary Fig. 1A Colors along branches showing best-supported selective regimes for body mass evolution including convergence and are consistent between a, b and c. Two major selective regimes for primates and optimal body size for each regime shown on far right correspond to Table 2; b focus on hominoids from a including fossil taxa. Marked nodes correspond to last common ancestors of all hominoids (1), hominids (2), African hominids (3), and chimpanzees and humans (4); c body mass averages (smaller circles) and inferred primary adaptive optima (larger circles) for species in each regime for primates including fossils corresponding to a and b. Numbered LCAs match nodes in b. Also noted is the adaptive optima of chimpanzees, the earliest hominins, later early hominins, and modern humans. Named taxa are outliers to their estimated optima

Fig. 2

Alternative hypotheses for primates focused on hominoids. a Best-supported selective regimes with only well-sampled reliably attributed early hominins and without other fossil primates; complete data set with b Brownian motion; c single regime model (OU1); d chimpanzee-sized ancestor all hominoids. Colors reflect regime assignment within each figure and are not comparable between figures