Date of Degree

2-2020

Document Type

Dissertation

Degree Name

Ph.D.

Program

Anthropology

Advisor

Christopher C. Gilbert

Committee Members

Eric Delson

Michael E. Steiper

David M. Alba

Subject Categories

Biological and Physical Anthropology

Keywords

Miocene apes, Hominidae, Human Evolution, Early hominins, Phylogenetics

Abstract

The living great apes, humans, and their fossil relatives (family Hominidae) are among the most intensively studied mammalian groups, yet many aspects of their shared evolutionary history are not well understood. Phylogenetic relationships of fossil great apes are poorly resolved and the positions of many fossil taxa relative to crown ape clades are debated. Moreover, the relationships of Sahelanthropus, Orrorin, and Ardipithecus to hominins are disputed, with some authors suggesting that alternative positions within Hominidae are more likely. Analyzing the position of these taxa within the broader context of the Miocene ape fossil record is thus necessary to fully test competing phylogenetic hypotheses. Phylogenetic uncertainty impedes our ability to address evolutionary and paleobiological questions about apes and humans, so an updated and clearer understanding of phylogenetic relationships is crucial in order to gain a better understanding of ape evolution.

This study builds upon previous phylogenetic studies of hominoids by adding novel craniodental and postcranial characters and quantifying traditional characters where possible, and by expanding sampled taxa to include all species that have been inferred to be stem or crown hominids, including the earliest potential hominins Sahelanthropus, Orrorin, and Ardipithecus. Parsimony and Bayesian inference methods were used to infer phylogeny. Bayesian hypothesis testing methods were then used to explicitly test competing phylogenetic hypotheses for the positions of Sahelanthropus, Orrorin, and Ardipithecus, as well as for Ouranopithecus and Graecopithecus, which have also been suggested to be early hominins. In contrast to previous studies, the results of the phylogenetic analyses presented here suggest that European apes, with the exception of Ouranopithecus, Graecopithecus, and perhaps Oreopithecus, are stem great apes. Ouranopithecus and Graecopithecus are recovered as stem members of the African ape and human clade and there is mixed support for the hominid status of Oreopithecus. Most Asian fossil apes are inferred to be closely related to orangutans, with the exception of Lufengpithecus hudienensis. Nakalipithecus, from the Late Miocene of Africa, is supported as a member of the African ape and human clade, while Samburupithecus, also from the Late Miocene of Africa, consistently groups with Oreopithecus. The results of both the phylogenetic analyses and the Bayesian hypothesis testing strongly support Sahelanthropus, Orrorin, and Ardipithecus as hominins; alternative phylogenetic positions for these taxa can be rejected.

The relationships inferred here were then used to examine the evolution of orthogrady, suspensory locomotion, and the impact of the Ardipithecus ramidus on reconstructions of the last common ancestor of chimpanzees and humans (LCA) using parsimony ancestral state reconstruction methods. Morphological features related to suspensory locomotion have long been considered synapomorphies of living apes; however, there are a number of fossil great ape taxa that lack suspensory adaptations. Based on the relationships inferred here, suspensory locomotion is reconstructed as having evolved independently multiple times in hominoids, including in hylobatids, pongines, and hominines. Predictions based on morphology observed or inferred for Ar. ramidus have been used to suggest that the LCA was very unlike living apes, and specifically that it lacked “advanced” orthogrady. Using currently available evidence from Miocene apes, extant apes, and fossil hominins, it is shown that features related to body plan in Ardipithecus ramidus do not influence reconstructions of the LCA, which is reconstructed as short-backed, dorsostable, and orthograde. Instead, if Ar. ramidus was “multigrade” and had a long lumbar spine, as has been hypothesized, then then these features are derived in this taxon.

Finally, biogeographic scenarios for apes are investigated by reconstructing ancestral area for hominoid clades using parsimony ancestral state reconstruction methods and by estimating the timing of evolutionary events using a Bayesian tip-dating approach. Notably, the results of this analysis suggest that there were more than two dispersals of apes between Africa and Eurasia and that the African ape and human clade is likely to have originated in Eurasia. The latter scenario is supported by successive Eurasian stem hominid branches preceding the crown hominid node and a reconstructed Eurasian origin for the ancestor of crown hominids. Thus, current evidence suggests that there was an initial stem great ape expansion out of Africa, likely in the Middle Miocene, that gave rise to the ancestor of all crown great apes; hominines then dispersed back into Africa in the Late Miocene preceding the origin of crown hominines.

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