Paleoenvironmental change in the late Middle Pleistocene–Holocene Kibish Formation, southern Ethiopia: Evidence from ungulate isotopic ecology

• New stable isotope data for the Kibish Formation ungulates are presented.
• Oxygen isotope data indicate wetter conditions over time.
• Mesowear and oxygen, but not carbon, data are correlated.
• The Holocene aged Member IV is interpreted to be the most mesic.
• These data have implications for the origins of Homo sapiens.

East Africa has produced the earliest record of Homo sapiens ~ 200 ka and a punctuated record of Middle Stone Age and Later Stone Age behaviors. We lack, however, a detailed late Quaternary paleoenvironmental record for the region, particularly during humid periods. Without a regional record, hypotheses about the evolution and ecology of early Homo sapiens in East Africa remain vague and untestable. The Kibish Formation of southern Ethiopia presents a long, albeit punctuated, record of late Middle Pleistocene to Holocene faunal change in East Africa, which was deposited during humid periods. Here, we present oxygen and carbon stable isotope data of the Kibish ungulates and test whether there are environmental changes within the Kibish Formation. Significant differences in δ18O enamel isotopes are consistent with more humid conditions during the Holocene-age Member IV (~ 13–4 ka) than either Pleistocene-age Member I (~ 196 ka) or Member III (~ 104 ka). Mesowear data document a shift toward more attritional wear among grazers in Member IV and are correlated with more depleted δ18O enamel values, suggesting that the wear pattern shift is linked to the onset of more humid conditions during the Holocene. δ13C enamel values show subtle variations through time, but do not suggest any major changes in diets. We propose that the paleoenvironmental differences evident in Member IV, based on δ18O enamel values, mesowear, and bovid abundances, may be explained by cooler and wetter conditions at the beginning of the Holocene in the lower Omo Valley. The evidence suggesting that the Holocene humid phase is more pronounced than earlier humid phases may explain why arid-adapted grassland ungulates became extinct in East Africa by the Pleistocene–Holocene transition, but persisted through previous humid phases of the late Quaternary.