At some point around six million years ago, these new mountains became so sprawling that they separated the ape ancestors on each side of the Great Rift Valley, splitting them into two separate lineages. On the western side, in an environment still rich with forests and largely unchanged, the lineage remained similarly unchanged and became today’s chimpanzees. On the eastern side of the mountains, however, in an environment of dying trees and progressively more open grasslands, evolutionary pressures began tinkering. It was this lineage that would eventually become human.
