Bipartition of the superior articular facets of the first cervical vertebra (atlas or C1): a human variant probably specific among primates

The bipartition of the superior articular facet is one of the classical variants of the human atlas. Rare quantitative data are available in the literature on this possible bipartition in humans and this disposition remains little understood. For non-human primates, there are almost no detailed comparative data in the literature, despite their potential importance in understanding the significance of this pattern in humans and its evolution. The material used in this study consisted of 500 human atlases and of 256 atlases of non-human primates representing 37 genera. In humans, bipartition of one or both of the superior articular facets was observed in 104 individuals out of the 500 (20.8%); bilateral occurrence was observed in 46.2% of the bipartitions and unilateral occurrence in 53.8%; no significant right or left predominance was observed. In non-human primates, no case of bipartition of the superior articular facets of the atlas was found in any of the 256 individuals studied. From a phylogenetic point of view, we suggest that a single superior facet of the atlas is the primitive pattern or plesiomorphy in primates. Modification of this general pattern seems to occur only in some individuals in Homo sapiens within primates, and the bipartition of the facet could thus be interpreted as a derived characteristic restricted to the human species or autapomorphy. The appearance of the bipartition of the superior articular facet of the atlas during human evolution could be the result of functional modifications due to the acquisition of constant erect posture and bipedalism.