in the crater is the Ngoitokitok Spring, near the eastern crater wall. There is a picnic site here open to tourists and a huge swamp fed by the spring, and the area is inhabited by hippopotamuses, elephants, lions, and many others. Many other small springs can be found around the crater's floor, and these are important water supplies for the animals and local Maasai, especially during times of drought. Masai were previously permitted to graze their cattle within the crater, but as of 2015 were restricted from doing so. Oldupai or Olduvai Gorge The Ngorongoro Conservation Area also protects Oldupai or Olduvai Gorges, situated in the plains area. It is considered to be the seat of humanity after the discovery of the earliest known specimens of the human genus, Homo habilis as well as early hominidae, such as Paranthropus boisei. The Olduvai Gorge is a steep-sided ravine in the Great Rift Valley, which stretches along eastern Africa. Olduvai is in the eastern Serengeti Plains in northern Tanzania and is about long. It lies in the rain shadow of the Ngorongoro highlands and is the driest part of the region. The gorge is named after 'Oldupaai', the Maasai word for the wild sisal plant, Sansevieria ehrenbergii. It is one of the most important prehistoric sites in the world and research there has been instrumental in furthering understanding of early human evolution. Excavation work there was pioneered by Mary and Louis Leakey in the 1950s and is continued today by their family. Some believe that 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. Wildlife Approximately 25,000 large animals, mostly ungulates, live in the crater. Large mammals in the crater include the black rhinoceros (Diceros bicornis michaeli), the local population of which declined from about 108 in 1964-66 to between 11–14 in 1995, the African buffalo or Cape buffalo (Syncerus caffer), and the hippopotamus (Hippopotamus amphibius). There also are many other ungulates: the blue wildebeest (Connochaetes taurinus) (7,000 estimated in 1994), Grant's zebra (Equus quagga boehmi) (4,000), the eland (Taurotragus oryx), and Grant's (Nanger granti) and Thomson's gazelles (Eudorcas thomsonii) (3,000). Waterbucks (Kobus ellipsiprymnus) occur mainly near Lerai Forest.