Most of Australia's largest mammals became extinct 50,000 to 45,000 years ago, shortly after humans colonized the continent. Without exceptional climate change at that time, a human cause is inferred, but a mechanism remains elusive. A 140,000-year record of dietary delta(13)C documents a permanent reduction in food sources available to the Australian emu, beginning about the time of human colonization; a change replicated at three widely separated sites and in the marsupial wombat. We speculate that human firing of landscapes rapidly converted a drought-adapted mosaic of trees, shrubs, and nutritious grasslands to the modern fire-adapted desert scrub. Animals that could adapt survived; those that could not, became extinct.

More than 85 percent of Australian terrestrial genera with a body mass exceeding 44 kilograms became extinct in the Late Pleistocene. Although most were marsupials, the list includes the large, flightless mihirung Genyornis newtoni. More than 700 dates onGenyornis eggshells from three different climate regions document the continuous presence of Genyornis from more than 100,000 years ago until their sudden disappearance 50,000 years ago, about the same time that humans arrived in Australia. Simultaneous extinction of Genyornis at all sites during an interval of modest climate change implies that human impact, not climate, was responsible.

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Our reconstructed history of Lake Eyre provides the first continuous continental proxy record of Australian monsoon intensity over the past 150 k.y. This continental record's broad correspondence to the marine isotope record demonstrates that this very large catchment, with its hydrology dependent on a planetary-scale climate element, responds to Milankovitch-scale climate forcing. Abrupt transitions from dry phases to wet phases (ca. 125 and 12 ka) coincide with Northern Hemisphere winter insolation minima rather than Southern Hemisphere summer insolation maxima, indicating that Northern Hemisphere insolation exerts a dominant control over the intensity of the Australian monsoon. Stratigraphic and dating uncertainties of other wet phases preclude conclusive correlation to specific insolation signals but, within the uncertainties, are consistent with Northern Hemisphere forcing. Regardless of the hemispheric forcing, the low intensity of the early Holocene Australian monsoon-by comparison with the last interglacial and particularly the last high-level lacustrine event at 65-60 ka when all forcing elements were modestis an enigma that can be explained by a change in boundary conditions within Australia.

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Carbon isotopes in fossil emu (Dromaius novaehollandiae) eggshell from Lake Eyre, South Australia, demonstrate that the relative abundance of C4 grasses varied substantially during the past 65,000 years. Currently, C4 grasses are more abundant in regions that are increasingly affected by warm-season precipitation. Thus, an expansion of C4 grasses likely reflects an increase in the relative effectiveness of the Australian summer monsoon, which controls summer precipitation over Lake Eyre. The data imply that the Australian monsoon was most effective between 45,000 and 65,000 years ago, least effective during the Last Glacial Maximum, and moderately effective during the Holocene.