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The hominid fossil record and recent mtDNA data increasingly support the theory that Homo species first evolved in east Africa and later evolved into different Homo species, including H. sapiens, that migrated from Africa to other continents. Isotopic ?13C analysis of fossil herbivore molar enamel indicates a shift from C3 tropical forest to C4 grassland savanna (i.e., “rangeland”) vegetation beginning about 10 Ma in many regions, including east-central Africa, as a result of global cooling and drying. This coincides with the two most significant shifts in hominid evolution, first to a bipedal locomotion in Australopithecus, and later an enlargement of cranial capacity with several Homo species. Current theory suggests Australopithecus evolved bipedal locomotion as an adaptation to life away from the forest as forests declined, but their existence was constantly threatened by predatory mammals until natural selection increased cranial capacity at about 2 Ma (the first Homo species) that may have provided greater cognitive abilities to cope with life in the new environment. An alternate natural selection strategy exhibited by Paranthropus, which also branched off of Australopithecus, was the development of large molars to eat the tougher plant foods found on rangelands. Paranthropus had some success but ultimately became extinct. This paper will review current data on hominid evolution, hominid cranial capacity, and paleo-climate and paleo-vegetation analysis that support the argument that were it not for the shift in central African vegetation toward rangeland, the evolution of modern humans may never have occurred.