An early form of terrestrial hominine bipedalism in the Late Miocene of Bulgaria

Abstract Fossils of Orrorin record the first convincing evidence of hominid terrestrial bipedalism in the Late Miocene, at about six million years ago. Bipedalism in the slightly older (7 Ma old) Sahelanthropus has recently been called into question. Here we present the first known hominine postcranial element from Azmaka (Bulgaria), a 7.2 Ma old nearly complete femur, which we tentatively attribute to cf. Graecopithecus. The Azmaka hominine represents a candidate for the ancestral form of positional behaviour from which bipedalism documented in later hominins evolved. The Azmaka femur lacks many of the specialised attributes of arboreal quadrupeds. At the same time the combination of locomotor features of this femur indicates a complex locomotor repertoire. Qualitative and quantitative morphological analyses demonstrate that the Azmaka femur combines certain attributes of terrestrial quadrupeds and bipeds and clusters mostly with early bipeds and partially with African apes. The morphology indicates a transitional form of bipedalism. The wooded-grassland savanna environment of the early Messinian locality of Azmaka suggests that terrestrial bipedalism likely evolved in a non-forested setting. The early Messinian age is critical to our understanding of mammalian palaeobiogeography and the intercontinental dispersals between Eurasia and Africa. We hypothesise that the descendants of the Azmaka hominine may have dispersed from Eurasia into Africa under the influence of climatic and environmental changes in the eastern Mediterranean. If such dispersal occurred, it may have been associated with subsequent re-occupation of more forested settings in both the ancestors of African apes and hominins.