Interpretation of Homoplasies in Oreopithecus and Their Relevance for Elucidating the Chimpanzee-Human Last Common Ancestor
- Author(s): Carlson, Joshua
- Advisor(s): White, Tim D
- et al.
A new species of hominid, Ardipithecus ramidus, was published in 1994 and 1995. The discovery team initially diagnosed and assigned the 4.4-million-year-old Ardipithecus ramidus to the hominid branch of the hominoid family tree based on the limited available dental, cranial, and postcranial characters exclusively shared with later hominids (i.e., all species derived in the human direction after the last common ancestor shared with chimpanzees; equivalent to “hominin”). A subsequent and limited parsimony-based cladistics analysis in 2006 again aligned Ardipithecus with later Australopithecus based on shared, derived characters of the dentition, as did other independent efforts based on the 1994 publication.
A more comprehensive exposition of the species followed in 2009 when a suite of 11 research papers comprising ~600 manuscript pages of description, data, and analysis was published in Science. The additional anatomy revealed in these publications employed the numerous additional shared derived characters observable on newly recovered fossils. These characters are spread across developmentally separate parts of the body. The combined evidence for phylogenetic placement reinforced the team’s earlier inference that Ardipithecus ramidus was a hominid exclusively related to Australopithecus and Homo. By illuminating the 4-6 Ma time period, the Ardipithecus fossils allowed practitioners to contextualize both the chronologically younger Australopithecus afarensis, despite the latter’s many derivations, and the Late Miocene common ancestor that we humans once shared with chimpanzees (CLCA).
After the 2009 publication of Ardipithecus several analysts doubted its hominid phylogenetic status based on arguments invoking the Late Miocene (8.3 - 6.7 Ma) Tusco-Sardinian hominoid Oreopithecus. These claims were based on several anatomical characters of its dentition, cranium, and postcranium that were argued to have been evolved in parallel with Ardipithecus. Such parallel acquisitions (homoplasies, or traits evolved in parallel from a last common ancestor that did not share the trait) in Oreopithecus potentially confound the recognition of phylogenetically primitive hominids, and might falsify claims of derived characters argued to exclusively link Ardipithecus with later hominids.
Since its discovery in 1872, Oreopithecus has inspired 145 years of field and laboratory research, hundreds of primary publications, and countless mentions in scientific and popular literature. This taxon has been proposed by some, since its discovery, as a mysterious member of the family Hominidae or as a distant relative ape taxon that evolved several features claimed to be hominid-like. Despite the extremely distorted and damaged condition of the fossils, many of these features were originally alleged to be synapomorphic between Oreopithecus and later hominids, but with the better resolution of the phylogenetic position of Oreopithecus, these are today widely promulgated as homoplasies.
This dissertation aims to evaluate whether features are actually shared between Oreopithecus and hominids, and if so, whether they are homoplastic (evolved in parallel), synapomorphic (shared derived), or symplesiomorphic (the shared primitive condition). Such a comprehensive examination and evaluation of claims of hominid homoplasy in Oreopithecus, conducted with
original fossils in comparison with original Ardipithecus fossils, has not been undertaken until now.
The objectives of this dissertation are to determine whether the claimed morphological features in Oreopithecus are present/observable and if so, whether the alleged characters can be diagnosed as hominid homoplasies. The following three research questions are addressed:
(1) Is the claimed morphological feature present/observable, or can it be reasonably inferred on the basis of the actual fossil evidence?
(2) If the claimed morphological feature is verified as present/observable, has the feature been accurately diagnosed in hominids (Australopithecus, Ardipithecus, and others)?
(3) Can allegedly shared characters that remain after testing per (1) and (2) be diagnosed as symplesiomorphic, synapomorphic, autapomorphic (uniquely derived), or)?
Key character complexes and their subdivided characters claimed to be hominid synapomorphies or homoplasies in Oreopithecus were examined and evaluated by comparing Ardipithecus, Australopithecus, Homo sapiens, extant hominoids, and fossil hominoids. Claims of hominid homoplasy in five (5) of the character complexes and eighteen (18) of their subdivided characters of Oreopithecus skeletal anatomy were falsified because relevant Oreopithecus anatomy is absent or has been misinterpreted. Claims of homoplasy in two (2) character complexes were falsified because the similarities in Oreopithecus and Ardipithecus anatomy are probably the primitive condition for the two taxa. The claim of homoplasy in one (1) subdivided character (vertical implantation of upper and lower incisors) of the character complex of an anteroposteriorly short face in Oreopithecus could not be falsified and represents a probable homoplasy. Of the six (6) singular characters claimed as hominid homoplasies in Oreopithecus, two (2) are revealed to have been based on Oreopithecus anatomy that is absent or has been misinterpreted and four (4) represent possible homoplasies but further fossils and outgroup analysis will be required to confirm them.
The results of this study form a foundation for future comparative work to test the potential hominid homoplasies in Oreopithecus when further fossils suffering from comparatively less distortion of relevant anatomy become available. In the meantime, future studies illuminating the underlying developmental genetic relationship of potentially homoplastic morphological characters in Hominoidea is warranted.