Olduvai Gorge is a 20 kilometer (12.4 mile) long east-west trending narrow valley in the eastern Rift Valley of northern Tanzania. The gorge cuts deep into approximately 140 meters (460 feet) of ancient Pleistocene through Holocene dated sedimentary and volcanic rocks. It was at Olduvai that paleoanthropologists Louis and Mary Leakey discovered the first recognized ancient human (or hominin) fossils in East Africa, ushering in a "golden age of paleoanthropology".
The Leakeys identified two major hominin occupation sites in Olduvai, called FLK and FLK NN, located close together on the margin of an ancient extinct lake called Paleo-Lake Olduvai. There the Leakeys identified the species Zinjanthropus (now called Paranthropus boisei) (now known to have existed between 2.3-1.2 million years ago [mya]). They also detected the first archaeological sequence in East Africa where stratigraphic transition from the Oldowan (Homo habilis 2.4 to 1.4 mya) to the Acheulean (H. erectus, 1.9 million-145,000 ya) can be seen.
Geological analysis of Olduvai"s ~140 meter thick stratigraphy was made by geologist Hans Reck in 1913; connected to the hominid discoveries by the Leakeys in the 1930s; and fully described by Richard L. Hay in his seminal 1976 book, Geology of the Olduvai Gorge. While considerable discussion of the interpretation of the different stone tools found in the different layers and how they should be classified has occurred over the past fifty years, the stratigraphic analysis has remained fairly stable.
In 1959-1960, the Leakeys recognized two localities at Olduvai in Bed I that contained the remains of ancient hominids: FLK and FLK NN. The evidence from these two sites was the first ever of the co-existence of two ancient hominin genera: P. boisei and H. habilis.
FLK Site. FLK Level 22 had a desne assemblage of stone artifacts (2566) and mammal fossils (3510), representing an ecologically diverse set of animals, including four hominin individuals OH5, 6, 35 and 44.
FLK has been interpreted as lots of things over