Earliest axial fossils from the genus Australopithecus.

Area of Science:

• Paleoanthropology
• Human Evolution
• Vertebrate Paleontology

Background:

• Australopithecus anamensis fossils indicate a more primitive craniodental and postcranial anatomy compared to Australopithecus afarensis.
• Previous evidence suggested habitual bipedality with arboreal adaptations in A. anamensis.

Purpose of the Study:

• To report on the oldest known Australopithecus axial remains from the Assa Issie locality in Ethiopia, dated to approximately 4.2 million years ago.
• To analyze the vertebral morphology of A. anamensis for insights into its adaptation, behavior, and evolutionary significance.

Main Methods:

• Analysis of newly discovered axial skeletal remains of A. anamensis, including the atlas (C1), C2, C6, and T1 vertebrae.
• Comparative analysis of fossil morphology with extant human and great ape anatomy.

Main Results:

• The atlas (C1) shows similarities to Homo sapiens, suggesting the absence of the atlantoclavicularis muscle and reduced climbing capacity.
• The T1 vertebra exhibits derived features in shape and size, indicative of adaptations for mitigating compressive loads from bipedality, representing the oldest evidence of the univertebral pattern in hominins.
• Vertebral morphology, particularly the T1, suggests more derived traits in A. anamensis than in A. afarensis, contrasting with craniodental evidence.

Conclusions:

• The vertebral fossils support habitual bipedality in A. anamensis, despite some primitive craniofacial traits.
• Certain vertebral aspects of A. anamensis are more derived than those of its successor, A. afarensis.
• These findings enhance our understanding of early hominin locomotion and adaptation.