This is part one of an introduction to the natural history of Sydney's Wolli Creek Valley. The Valley contains the last substantial remnants of the natural environment in the city's inner south-west. Even though it is quite small it contains a variety of habitats, ranging from tidal mudflats, saltmarsh and mangroves to reed-lands, open woodland and even fragments of rainforest. They, in turn, provide refuge for an impressive range of native wildlife, especially birds and reptiles. Part I, 'Prehistory', gives an account of the Valley's geological history, its fossil record and human occupation before 1788.