THE RETROTRANSVERSE FORAMEN OF THE HUMAN ATLAS VERTEBRA - A DISTINCTIVE VARIANT WITHIN PRIMATES

The occurrence of a retrotransverse foramen of the atlas has been inve stigated in a series of 409 nonhuman primates representative of 40 gen era, and in a series of 500 human atlases (dried bones). In nonhuman p rimates, no retrotransverse foramen was found in any of the individual s studied. In humans, the retrotransverse foramen was observed in 71 c ases (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 prese nt in 44 cases on the right side, and 48 on the left side. The charact er was noted as present only if it was complete; partial forms were no t recorded. Various aspects of the retrotransverse foramen were observ ed. The present observations suggest that the retrotransverse foremen is a uniquely derived trait (autapomorphy) within primates which is re stricted to some individuals in Homo sapiens. Appearance of this foram en in human evolution can be related to the acquisition of the erect p osture and bipedal locomotion, and consecutive modifications of the re gional venous circulation.