EVIDENCE FROM PLANT WAX BIOMARKERS FOR ECOLOGICALLY-DRIVEN EVOLUTIONARY EVENTS AT 2.8 MA IN THE LOWER OMO VALLEY, ETHIOPIA

A long-standing challenge in human evolution studies is linking climate and vegetation change to East African mammalian evolutionary events. We can test whether there is a relationship between vegetation change and faunal evolution in places where we can pair rich fossil assemblages and continuous, high-resolution paleoecological records. One such locality is the Shungura Formation in the Lower Omo Valley of Ethiopia (Omo). More than 50,000 vertebrate fossils have been recovered from this Formation, including 40% of hominin fossils collected in the Turkana Basin. Furthermore, over 25 radiometric and magnetostratigraphic ages provide exceptional age control. We evaluate the relationship between vegetation and faunal change using new carbon isotope data from plant wax biomarkers in the Shungura Formation, where isotopes reflect the proportion of C₃ to C₄vegetation on the landscape. Focusing upon a wide range of lithologies from the Shungura Formation allows us to directly correlate vegetation changes to the rich fossil record in the same sedimentary sequence.

Carbon isotope data from n‑alkanoic acids in Shungura Formation sediments indicate highly variable vegetation throughout the record. Against this variability, the most prominent feature of the vegetation record is a marked increase in C₄ vegetation from 2.8 to 2.6 Ma. Two major evolutionary events accompany this vegetation shift. First, trageliphin bovids, kolpochoere suids, and hippos all shift towards diets rich in C₄ vegetation. The second major event is the first appearance of the megadont hominin Paranthropus aethiopicus­ around 2.7 Ma, whose diet also relied heavily on C₄ resources. We propose that vegetation change at 2.8 Ma drove the evolution of herbivore diets and directly preceded the first appearance P. aethiopicus in the Omo.