From these measurements, the researchers traced what kinds of plants grew in the gorge over time and compared them with how the climate changed, constructing a continuous record of plant and water fluctuations. “What we find is that the period between 2 million and 1.8 million years ago is associated with extreme environmental variability,” Magill said.

Grasslands gave way to woody forests and back again while water levels in the gorge rose and fell, often very quickly by geological time scales. “There was evidence that there was contraction and expansion over time,” said Katherine Freeman, a co-author and a geosciences professor at Penn State. “What we show is a repeated transition from the driest to the wettest on the scale of a few thousand years.”

Though not as dramatic as a towering black monolith, these changes may have spurred human evolution by forcing early hominids to adapt to a rapidly changing climate, driving them to develop new strategies to hunt, gather and survive with changes in food and fresh water. “That’s where the connection has been; the development of the brain, food gathering, might have been triggered by the continually changing climate,” Ashley said. These changes also created selection pressures in other species like birds and reptiles.