[A]fter hundreds of thousands of years of stability, dramatic shifts occurred [in the East African Rift Valley] beginning about 400,000 years ago—extreme swings occurred between wet and dry periods, lakes shrunk and new types of vegetation periodically replaced large grasslands. Geological evidence at Olorgesailie also shows how some 400,000 years ago earthshaking tectonic activity began to reshape the region—segmenting the landscape, raising hills and cliffs, and draining huge lakes—shifts that made the area more sensitive to changes like more variable rainfall.
Without large plains to sustain them the large grazing relatives of zebras, giraffes and elephants were replaced with smaller specimens… And without the giant “lawnmowers of the Pleistocene” and their constant browsing, entirely different vegetation sprouted. This one-two punch meant that early humans had to learn new ways to gather foods, as well as ways to hunt different animals.
[Paleoanthropologist Chris] Stringer notes that early humans were completely attuned to their local environment and knew how to exploit its plant and animal resources on a daily basis. “So changes in the environment meant that they had to learn completely new patterns of behavior and that’s an obvious pressure on the human population to change,” he says. “If hunter gatherers don’t adapt to the environment, they die.”