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This dissertation explores the link between habitat and human evolution byexamining the mosaic habitat and its seasonal variation in rainfall at the single fossillocality of Allia Bay, Kenya (3.97±0.03 MA) using stable carbon (δ13C) and oxygen(δ18O) isotope ratios in fossil faunal tooth enamel. Serial δ18O values from 10μm spotanalyses in contrast to data from bulk powdered samples, this dissertation uses browsing and grazing faunal enamel to document prolonged periods of environmental stability with mild seasonality and periods of marked fluctuating seasonality during the drier phases at Allia Bay when Au. anamensis occupied the region.Traditional bulk δ13C and δ18O values of fossil Allia Bay compared to modernKoobi Fora fauna indicate that the local environment at Allia Bay was distinctly wetterand more closed compared to the arid open grassland of the modern region. Despiteregional interpretations of a continuously arid open Turkana Basin over the past 4 Ma,the Allia Bay fauna suggest that local habitats surrounding Lake Turkana wereecologically distinct with different microclimates. Enamel from fossil localities used forisotopic analysis to reconstruct the paleoenvironment are often considered impervious todiagenesis, however at Allia Bay mineral structure changes in enamel indicate thatdiagenesis is an issue at this fossil locality. While the complex process of enameldiagenesis is not understood completely, the high-resolution sampling of δ18Oen valuesdocumented a relationship between mineral structure change and diagenesis of δ18Oen. Confocal laser fluorescent microscopy imaging identified evidence of mineral structure change documented by far-red fluorophores that correlate with alteration of δ18Oen, while other inclusions identified by green and red fluorophores have limited impact to δ18Oen. Ultimately, this dissertation documented the first evidence of variation in seasonal rainfall patterns at Allia Bay during intra-annual cycles (~10-17 months of time recorded in enamel). The δ18Oen values documented in fossil hippopotamids and suids show a 6-8‰ baseline difference during the occupation of Allia Bay, suggesting the ecosystem shifted significantly. As rainfall patterns fluctuated, animal and plant communities were impacted and the early hominins would have required adaptive flexibility to cope with the changing habitat.