Formation of the Great Rift Valley

It has been created through the rifting and separation of the African and Arabian tectonic plates that began around 35 mya in the north, and by the ongoing separation of East Africa from the rest of Africa along the East African Rift, which began about 15 mya. —Origins of the social mind

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With the advent of the rift during the oligocene, the domain of the apes became restricted to the west of the continent, ending at the rift valey. Those apes that found themselves on the east side of the rift were forced to adapt to the loss of the jungle. The major competitors of the apes were the monkeys, who were better able to hide from predators in the short bush and scramble for pieces of food. The challenge to the apes of East Africa was to make use of the new, drier savannah habitat in a way that did not compete with the monkeys. They did this by adopting a bipedal gait. (see Bipedal locomotion) —Origins of the social mind