THE PRESENT IS NOT THE KEY TO THE PAST: HERBIVORE DIETS, CLIMATE CHANGE AND THE REIGN OF C4 GRASSES IN SOUTHERN KENYAN RIFT VALLEY FOR THE LAST 1 MILLION YEARS

Global climate change is often invoked to explain major events in human evolution; however, in eastern Africa, basin-scale biotic and abiotic processes may exert greater evolutionary pressure on mammalian communities than global climate. The southern Kenyan Rift Valley preserves a record of evolutionary trends over the last one million years, including evidence for the earliest known Middle Stone Age (MSA) technology and the emergence of the taxonomically modern, large herbivore communities in eastern Africa by ~300 ka. Evidence of these evolutionary events coincided with a time when wet-dry climate variability was likely heightened in eastern Africa; however, we need more data to establish the links between climate change and evolutionary pressures on early humans and mammalian herbivores. Here we report carbon and oxygen isotope data from fossil teeth of large herbivores living in the Olorgesailie Basin and nearby at Lainyamok between 1.0 to 0.3 Ma. Herbivore diets were largely invariant through time, even after a significant faunal turnover that marks the appearance of species that are characteristic of the region today. Over 85% of individuals sampled were committed C₄-grazers with δ¹³C_enamel values > 0 ‰, including notably, every elephant in our sample population. This is in contrast to dietary behaviors of these same species today, many of which include considerable amounts of C₃ browse in their diets, such as elephants. The dominance of grazers and near-absence of browsing herbivores means that functional modernity of the herbivore community did not arise in the southern Kenya Rift Valley until after ~300 ka. This result adds to the growing understanding that present-day and historical conditions may not be appropriate analogues for understanding environments and ecosystems of even the very recent past. These observations also mean that the reorganization of mammalian communities by 300 ka cannot be used to infer a shift in resource availability in the southern Kenya Rift during the periods of fossil accumulation. The continued usage of C₄ grasses as a food resource may indicate that these fossil assemblages represent environments that were buffered from the effects of regional climate events.