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Ardipithecus ramidus and the evolution of the human cranial base
William H Kimbel1, Gen Suwa, Berhane Asfaw
1Institute of Human Origins and School of Human Evolution and Social Change, Arizona State University, Tempe, AZ 85287.
Summary
Ardipithecus ramidus exhibits derived basicranial features, specifically a reorganized central cranial base, aligning it with the Australopithecus + Homo clade. This suggests early hominoid evolution involved fundamental cranial base changes.
Area of Science:
- Paleoanthropology
- Primate Evolution
- Human Origins
Background:
- Ardipithecus ramidus's phylogenetic position is debated, with some traits suggesting a link to Australopithecus and Homo, while others indicate significant arboreality.
- Previous analyses focused on dental and postcranial features, but basicranial morphology offers further insights into early hominoid evolution.
Purpose of the Study:
- To investigate the basicranial morphology of Ardipithecus ramidus to clarify its phylogenetic relationship with African apes, Australopithecus, and Homo.
- To identify derived cranial base traits indicative of the early hominoid lineage leading to humans.
Main Methods:
- Comparative analysis of basicranial features in Ardipithecus ramidus, Australopithecus, Homo, and African apes.
- Estimation of missing basicranial length using proportional relationships from preserved fossil remains (ARA-VP 1/500).
- Examination of key features: foramen magnum position, carotid foramina, tympanic bone, and basioccipital element.
Main Results:
- Ardipithecus ramidus shares derived human-like basicranial modifications with Australopithecus, including a relatively short basicranium and a reorganized central cranial base.
- Specific shared traits include a relatively anterior foramen magnum, lateral shift of carotid foramina, mediolateral abbreviation of the lateral tympanic, and a shortened basioccipital.
- These findings confirm Ardipithecus ramidus possesses a short basicranium, consistent with Australopithecus and Homo.
Conclusions:
- Reorganization of the central cranial base is an early morphological marker of the Ardipithecus + Australopithecus + Homo clade.
- The basicranial evidence supports a phylogenetic placement of Ardipithecus ramidus closer to the human lineage than previously suggested by some interpretations.
- Cranial base morphology provides crucial evidence for understanding the divergence and early evolution of hominoids.
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