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Olduvai Gorge

It is one of the most important prehistoric sites in the world and has been instrumental in furthering understanding of early human development. Excavation work there was pioneered by Louis Leakey in the 1950s and is continued today by his family. Millions of years ago, 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.
The stratigraphy is extremely deep and layers of volcanic ashes and stones allow radiometric dating of the embedded artifacts, mostly through potassium-argon dating. The first artifacts in Olduvai date to 2 million years ago but fossil remains of human ancestors have been found from as long as 2.5 million years ago.
The earliest archaeological deposit, known as Bed I, has produced evidence of campsites and living floors along with flint tools made on flakes. Bones from this layer are not of modern humans but primitive hominid forms of Australopithecus boisei and the first discovered specimens of Homo habilis.
Above this, in Bed II, pebble tools begin to be replaced by more sophisticated handaxes of the Abbevillian industry and made by Homo erectus. This layer dates to around 1.5 million years ago. 
Beds III and IV have produced Acheulean tools and fossil bones with Neandertal characteristics which were used until around 600,000 years ago.
Beds above these contained tools from a Kenya-Capsian industry made by modern humans and are termed the Masek Beds (600,000 to 400,000 years ago), the Ndutu Beds (400,000 to 32,000 years ago), and the Naisiusiu Beds (22,000 to 15,000 years ago).