The hominid fossil record and recent mtDNA data increasingly support the theory thatÂ HomoÂ species first evolved in east Africa and later evolved into differentÂ HomoÂ species, includingÂ 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., â€œrangelandâ€) 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 inÂ Australopithecus, and later an enlargement of cranial capacity with severalÂ HomoÂ species. Current theory suggestsÂ AustralopithecusÂ 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 firstÂ HomoÂ species) that may have provided greater cognitive abilities to cope with life in the new environment. An alternate natural selection strategy exhibited byÂ Paranthropus, which also branched off ofÂ Australopithecus, was the development of large molars to eat the tougher plant foods found on rangelands. ParanthropusÂ 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.
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