It is one of the most important prehistoric sites in the world. Research there has greatly developed our understanding of early human evolution. Excavation work there was pioneered by Mary and Louis Leakey in the 1950s. It is still continued today by their family. During the Pleistocene, the site was that of a large lake, the shores of which were covered with successive deposits of volcanic ash. Around 500,000 years ago seismic activity diverted a nearby stream which began to cut down into the sediments, revealing seven main layers in the walls of the gorge.