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| The origin of the hominins takes place in Africa. The earliest presumed hominins (Sahelanthropus, Orrorin, and Ardipithecus) date back 6 million years or more, coinciding with the suggested time for hominin divergence based on genetic studies. These earliest hominins were very primitive, and debate continues as to whether all of them are actually hominins. Between roughly 4 million and 3 million years ago, two additional species of primitive hominin (Australopithecus anamensis and Australopithecus afarensis) appeared that were definitely bipeds but still retained some apelike anatomy, particularly in certain dental features. The evolutionary significance of a newly discovered early hominid, Kenyanthropus platyops, is not clear at this time. Starting about 3 million years ago, several lines of hominins, including three species of robust australopiths, emerged, characterized by large back teeth and huge chewing muscles adapted for a hard-to-chew diet. Two other hominins, Australopithecus africanus and Australopithecus garhi, were less robust, and one of them may represent the ancestor of Homo. Preliminary analysis suggests that A. garhi made stone tools for scavenging animal flesh and bone marrow. All of these early hominins had small brains but were bipedal. The origin of bipedalism is a major event in hominin evolution for which a number of hypotheses have been proposed. New evidence on ancient environments shows that the first hominids lived in woodlands and forests. Once bipedalism evolved, hominins were able to adapt to the open grasslands of Africa. |