an almost perfectly preserved fossil jaw of the hominin (of human lineage) species Paranthropus boisei (Australopithecus) containing a complete set of adult teeth. It was found in 1964 at Peninj, a locale in Tanzania to the west of Lake Natron (Natron, Lake) and about 80 km (50 miles) from Olduvai Gorge, a major paleoanthropological site.

The Peninj mandible is robust and deep, with tiny incisors and canines and larger premolars and molars. Only one tooth was damaged, though the rest were heavily worn. The Peninj site is also important to the study of human evolution because about 120 artifacts were unearthed near the fossil. These Stone Age implements belong to the Acheulean industry and include stone cleavers and hand axes. The sandstone in which the fossil was found has been dated to about 1.5 million years ago, but the artifacts came from a somewhat later period.