Background: Major biological and cultural innovations in late Pliocene hominin evolution are frequently linked to the spread or fluctuating presence of C₄ vegetation with a shift towards a cooler, drier and more variable global climatic regime beginning approximately 3 million years ago (Ma), evidence for grassland-dominated ecosystems in continental Africa and hominin activies within such ecosystems have been lacking. Methodology/Principal Findings: We report stable isotopic analyses of pedogenic carbonates and ungulate enamel, as well as faunal data from ~2.0 Ma archeological occurrences at Kanjera South, Kenya. These documente repeated hominin activities within a grassland-dominated ecosystem. Conclusions/Significance: These data demonstrate what hitherto had been speculated based on indirect evidence: that grassland-dominated ecosystems did in fact exist during the Plio-Pleistocene, and that early <em>Homo</em> was active in open settings. Comparison with other Oldowan occurrences indicates that by 2.0 Ma hominins, almost certainly of the genus <em>Homo</em>, used a broad spectrum of habitats in East Africa, from open grassland to riparian forest. This strongly contrasts with the habitat usage of <em>Australopithecus</em>, and may signal an important shift in hominin landscape usage.Citation: Plummer, T. W. et al. (2009). 'Oldest evidence of toolmaking hominins in a grassland-dominated ecosystem', PLoS ONE, 4(9): e7199. © 2009 Plummer et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
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