OLDUVAI GORGE

Olduvai also spelled Olduwai, archaeological site in the eastern Serengeti Plains, within the boundaries of the Ngorongoro Conservation Area in northern Tanzania. It is a steep-sided ravine about 30 miles (48 km) long and 295 feet (90 metres) deep, with subsidiary valleys. Deposits exposed in the sides of the gorge cover a time span from about 2,100,000 to 15,000 years ago. The deposits have yielded the fossil remains of more than 50 hominids, providing the most continuous known record of human presence during the past two million years, as well as the longest known archaeological record of the development of stone-tool industries. Olduvai Gorge was designated part of a UNESCO World Heritage site in 1979. The Olduvai fossil beds accumulated in a lake basin about 16 miles (25 km) in diameter, which is underlain by volcanic rocks of the Pliocene Epoch (1.6 to 5.3 million years old) and, further below, by metamorphic deposits of Precambrian age (540 million to 3.8 billion years old). Relatively continuous rift-valley fault movements and volcanic action left Olduvai deeply incised. Water flow through the gorge further eroded the rock, exposing a delineated sequence of strata from which evolutionary events could be traced. Seven major stratigraphic units, or formations, have been distinguished. From the oldest to the youngest they are: Bed I (about 1,700,000 to 2,100,000 years old), Bed II (1,150,0001,700,000 years old), Bed III (800,0001,150,000 years old), Bed IV (600,000800,000 years old), the Masek Beds (400,000600,000 years old), the Ndutu Beds (32,000400,000 years old), and the Naisiusiu Beds (15,00022,000 years old). Bed I is at most 197 feet (60 metres) thick. It consists largely of lava flows, volcanic ash deposits, and detrital sediments. The upper part of the bed (1,700,000 to 1,850,000 years old) contains a rich and varied fauna and archaeological sites of the Oldowan industry. It was there in 1959 that the English-born archaeologist Mary Leakey discovered a skull fragment belonging to an early hominid that her husband, Louis Leakey, named Zinjanthropus boisei (later reclassified as Australopithecus boisei). Dubbed Nutcracker Man because of its huge molars, indicative of a vegetarian diet, the skull was dated to about 1.75 million years ago. The discovery indicated that hominids had evolved in Africa. Specimens of Homo habilis, a more advanced hominid, were also found there. Lower jaw of OH 7, found at Olduvai Gorge, Tanzania; the type specimen of Homo habilis. The hominid living sites of Bed I are found principally where streams from the volcanic highlands brought fresh water to the southern margin of the alkaline Olduvai lake. Conditions for preservation were unusually favourable at these sites because ashfalls from nearby volcanoes and fluctuations of the lake led to rapid burial of the hominid and associated remains. Other finds include Oldowan tools and the bones and teeth of various animals, notably medium-size antelopes. Long animal bones and others containing marrow generally have been split and broken. Living sites in Beds II, III, and IV generally are found in former river or stream channels. Bed II is 6698 feet (2030 metres) thick and consists of different rock formations separated by a disconformity, or erosional break. Only the Oldowan industry occurs below the disconformity; the so-called Developed Oldowan industry and the Acheulean industry occur above. Homo habilis remains were found in the lower one-third of Bed II, and a cranium of Homo erectus was collected near the top of Bed II. Australopithecus boisei occurs both in upper and lower parts of Bed II. Beds III and IV were deposited on an alluvial plain. These two units are distinct only in the eastern part of the gorge and are combined elsewhere into a single unit. The two beds have a maximum aggregate thickness of about 98 feet (30 metres) and consist almost entirely of stream-laid detrital sediment. Archaeological sites in Beds III and IV represent the Developed Oldowan and Acheulean industries. Hominid remains there are assigned to H. erectus and other species of Homo. The Masek Beds accumulated during a period of major faulting and explosive volcanism. They are some 82 feet (25 metres) thick and consist of about equal amounts of stream-laid detrital sediment and aeolian (wind-worked) tuff. Only one archaeological site, of the Acheulean industry, is known in these beds. The Ndutu Beds were deposited during intermittent faulting, erosion, and partial filling of the gorge. They consist largely of aeolian tuffs, and their maximum thickness is 79 feet (24 metres). The Naisiusiu Beds were deposited on the sides and in the bottom of the gorge after it had been eroded to very near its present level. These deposits are as much as 33 feet (10 metres) thick and consist largely of aeolian tuff. They contain one archaeological site consisting of a microlithic tool assemblage and a Homo sapiens skeleton, both of which have an age of about 17,000 years.