Dissertations, Theses, and Capstone Projects

Date of Degree


Document Type


Degree Name





Thomas W. Plummer


Timothy G. Bromage

Committee Members

Timothy G. Bromage

Jessica M. Rothman

Thure E. Cerling

Subject Categories

Biological and Physical Anthropology | Geochemistry | Paleontology


Paleoanthropology, Human Evolution, Paleoecology, Stable Isotopes, East Africa


A major goal of paleoanthropology is to identify the selective pressures associated with hominin biological and behavioral evolution, yet establishing cause-effect relationships between climate, ecology, and human evolution remains problematic. This dissertation seeks to investigate hominin paleoecology in eastern Africa by reconstructing aspects of climate and ecology using stable isotope analysis.

The first part of this dissertation is focused on the ecology of primates and hominins. Modern tropical African ecosystems provide a useful model for understanding the ecological correlates of isotopic variation in the fossil record, and living primates provide a useful model for understanding the ecological significance of isotopic variation in fossil hominin tissues. Isotope data from modern primate excreta and plants shows that isotopic variation primarily relates to diet. The magnitude of isotopic variation in gorilla feces, which relates to intra-annual dietary variability, demonstrates that African apes have less isotopically variable diets than fossil hominins. Carbon isotope data from large mammalian tooth enamel in three areas (southern Kenya & northern Tanzania, northern Kenya, and northern Ethiopia) demonstrate a long-term increase in the prevalence of C4-grazing taxa over time, primarily at the expense of C3-C4 mixed feeding taxa. Pleistocene and Holocene ecosystems in southwestern Kenya, where there is the most complete record over the past 2 million years, are persistently dominated by C4-grazers.

The second part of this dissertation is focused on climate and human evolution. The notion that aridity and seasonality have increased over the Pliocene and Pleistocene has remained central to models of environmental change (i.e. C4 grass expansion) and human evolution; however, few empirical records of terrestrial climate are available to assess this claim. An existing tooth enamel oxygen isotope aridity proxy is revised using a large compilation of new and existing oxygen isotope values in modern herbivore teeth from eastern and central Africa. The application of the aridity index to fossil teeth from eastern Africa, primarily the Turkana Basin, demonstrate that mesic and arid conditions were both prevalent during periods of fossil preservation. There is no long-term trend in aridity, which demonstrates that ecological changes associated with the expansion of C4 vegetation and C4 grazers may not have been driven by changes in water availability. A survey of extant equids from eastern Africa demonstrates that intra-tooth oxygen isotopic variability relates to intra-annual oxygen isotope variability in precipitation. This approach is applied to fossils from the Homa Peninsula, Kenya, demonstrating that Pleistocene hominins in this region would have experienced seasonal rainfall, similar to modern eastern African climates. No trend over time was detected in fossil intra-tooth δ18O range, although the number of localities and time periods included in this analysis are limited.

Isotopic records presented in this dissertation demonstrate new ways to investigate relationships between climate, ecology, and human evolution. These findings provide new detail on hominin environments, and provide a critical test of long-assumed relationships between long-term changes in climate and ecology.