The retrotransverse foramen of the human atlas vertebra. A distinctive variant within primates.

The occurrence of a retrotransverse foramen of the atlas has been investigated in a series of 409 nonhuman primates representative of 40 genera, and in a series of 500 human atlases (dried bones). In nonhuman primates, no retrotransverse foramen was found in any of the individuals studied. In humans, the retrotransverse foramen was observed in 71 cases (14.2%). It was present bilaterally in 21 cases (29.6% of the 71 foramina), and unilaterally in 50 cases (70.4%). The foramen was present in 44 cases on the right side, and 48 on the left side. The character was noted as present only if it was complete; partial forms were not recorded. Various aspects of the retrotransverse foramen were observed. The present observations suggest that the retrotransverse foramen is a uniquely derived trait (autapomorphy) within primates which is restricted to some individuals in Homo sapiens. Appearance of this foramen in human evolution can be related to the acquisition of the erect posture and bipedal locomotion, and consecutive modifications of the regional venous circulation.