Body mass estimates of the earliest possible hominins and implications for the last common ancestor

Abstract

Many hypotheses regarding the paleobiology of the earliest possible hominins, Orrorin tugenensis and Ardipithecus ramidus, are dependent upon accurate body mass estimates for these taxa. While we have previously published body mass predictions for Orrorin and Ardipithecus, the accuracies of those estimates depend on the assumption that the postcranial skeletal dimensions and body masses of these taxa followed scaling patterns that were similar to those observed in modern humans. This assumption may not be correct because certain aspects of postcranial morphology in Orrorin and Ardipithecus differ from modern humans, and suggest that their overall body plans might be unique but more similar to modern non-human great apes than to modern humans. Here we present individual body mass predictions for O. tugenensis and Ar. ramidus assuming that they followed postcranial scaling patterns similar to those of chimpanzees. All estimates include individual prediction intervals as measures of uncertainty. In addition, we provide equations for predicting body mass from univariate postcranial measurements based on the largest sample (n = 25) yet compiled of common chimpanzee skeletons with known body masses, which is vital for calculating prediction intervals for individual fossils. Our results show that estimated body masses in Orrorin and Ardipithecus are generally larger when derived from a chimpanzee-like scaling pattern compared to estimates that assume a human-like pattern, though the prediction intervals of the two sets of estimates overlap. In addition, the more complete of the two known Orrorin femora has an overall scaling pattern that is more similar to common chimpanzees than to modern humans, supporting the application of a non-human great ape comparative model. Our new estimates fall near the male (Ardipithecus) average and in between the male and female averages (Orrorin) for wild-caught common chimpanzees. If a chimpanzee-like pattern of scaling between postcranial dimensions and body mass did exist in these earliest hominins, our results suggest the large body masses found in some early australopiths were already present in taxa near the origins of our lineage, and perhaps also in the Pan-Homo last common ancestor.

Divergence-time estimates for hominins provide insight into encephalization and body mass trends in human evolution

“…Furthermore, our ACSR results are consistent with previous studies that show that body size in hominins has not been a simple linear increment since the divergence with P. troglodytes 26,27,39 . The observed decreasing trend in body size from the root of the tree is in agreement with previous studies that suggest a chimpanzee-sized common ancestor with P. troglodytes 40,41 . Our ACSR results also displayed a general trend towards greater body size that started after the emergence of the last common ancestor of Homo, contrary to a previous claim stating that there were no clear body mass temporal trends in hominin evolution 26 .…”

Ardipithecus hand provides evidence that humans and chimpanzees evolved from an ancestor with suspensory adaptations

“…Therefore, suspension, vertical climbing, and knuckle walking are naturally linked from both functional and evolutionary perspectives. Ultimately, the definitive resolution of the knuckle-walking hypothesis relies on the recovery of direct fossil evidence of chimpanzee and gorilla postcranial evolutionary history, but we interpret the preponderance of the available fossil and comparative evidence to support hypotheses of a large-bodied, semiterrestrial, knuckle-walking LCA with adaptations to climbing, suspension, and heel-strike plantigrady (1,5,40,(53)(54)(55)81).…”

“…afarensis, 39.1 kg; Au. sediba, 25.8 kg; H. naledi, 37.4 kg; H. neanderthalensis, 72 kg; H. laietanus, 33.5 kg; P. catalaunicus, 32.5 kg; D. guggenmosi, 31 kg) were culled from the literature (20,40,(79)(80)(81)(82)(83). The body mass estimates for the ARA-VP-6/500 partial skeleton vary substantially, so we included both small (32.1 kg) and large (50.8 kg) estimates, as in previous studies (22,40).…”

Reevaluation of the body mass estimate for the KNM‐ER 5428Homo erectustalus

“…We speculate that the overestimation of KNM‐ER 5428 by McHenry () and Boyle and DeSilva () is due to unknown error(s) in McHenry's () TTML equations primarily affecting body mass estimates for large‐bodied individuals. This may be because (a) McHenry's () equations are based on a mixture of very small‐ and large‐bodied individuals from different populations with dissimilar scaling relationships between TTML and body mass, and (b) McHenry () used population means, not individuals, to generate regression equations, thereby relying on only a few data points (Grabowski et al, ; Ruff et al, ). We recommend that these TTML equations not be used for future estimates of body mass in large‐bodied individuals.…”