Out of Afar: the first hominin migration? Long-term landscape changes in the Afar region and implications for hominin bipedalism

Ian George Smith , Sally Christine Reynolds

Climate change in Africa has long been considered to be a key driver in hominin behavioural adaptations such as bipedalism. The prevailing argument holds that open grasslands favoured the adoption of bipedalism, which can be defined as one of three types: facultative, habitual, and obligate. Across Africa, only one region, Ethiopia, shows evidence of hominins showing all three types of bipedalism: Ardipithecus (facultative), followed by Australopithecus (habitual), and finally, Homo (obligate bipedalism). Was this simply a chance development, a pattern shaped purely by preservation bias, or was there something specific about the Ethiopian region that facilitated the adoption of bipedal behaviours?
This paper introduces the Refugial Bipedalism Model (RBM), proposing that the tectonically complex and relatively isolated Afar region functioned as a long-term evolutionary refugium. We evaluate the unique landscape properties of the Afar region, by reviewing its geological/geomorphological evolution as part of the wider East African Rift System (EARS) and the role it could have played in hominin bipedal locomotion. We suggest that limited carnivoran biodiversity, and lack of migration routes to enter the region may have provided protection (lowered predation and competition) for hominins over millions of years and allowed increasingly upright locomotor behaviours to emerge. The dominance of rifts and cliffs in this tectonically active area might also have facilitated the adoption of new bipedal behaviour that that would later prove evolutionarily advantageous.
Around 4.2 Ma, the opening of the Main Ethiopian Rift (MER) enabled southward dispersal, coinciding with the first appearance of facultative bipedal hominins in the Turkana fossil record. Their sudden emergence contrasts with earlier regional absences and may reflect the earliest documented hominin migration “out of Afar” and into wider Africa.