a bed layer at Olduvai Gorge based on mineral content. Pleistocene fauna and Acheulean artifacts have been found in the Olpiro Beds. Based on a trachytic tuff which occurs within the beds, the Ngaloba Beds may therefore be dated between 120,000 and 150,000 years BP. Discoveries Hominina footprints from site G The principal discovery, made by Mary Leakey and her team in 1976 (and fully excavated by 1978), is a 75-foot (24-meter) line of Hominina fossil footprints, preserved in powdery volcanic ash originally thought to have been from an eruption of the nearby (20 km) Sadiman volcano. However, recent study of the Sadiman volcano has shown that it is not a source for the Laetoli Footprints Tuff (Zaitsev et al. 2011). Soft rain cemented the ash-layer (15 cm thick) to tuff without destroying the prints. In time, they were covered by other ash deposits. The fossil footprints were rather whimsically discovered by Yale's Andrew Hill when visiting Mary Leakey in 1976. While walking back to camp one evening, Hill fell trying to avoid a large ball of elephant dung thrown at him by a colleague. With his face only inches from the rock, he recognized footprints made by antelopes and rhinos preserved in the volcanic ash, and among these, hominid footprints. The Hominina prints were produced by three individuals, one walking in the footprints of the other, making the preceding footprints difficult to recover. As the tracks lead in the same direction, they might have been produced by a group visiting a waterhole together, but there is nothing—or very little (see below, Interpretation and significance)—to support the common assumption of a nuclear family. Roberto Sáez claims that this 27-metre trail of about 70 footprints was left by two Australopithecus walking in front, while the third hominid walked behind, superimposing its steps on the footprints left by one of the two in front. He acknowledges that it will never be possible to prove that this is true. The footprints demonstrate that the Hominina habitually walked upright as there are no knuckle-impressions. The feet do not have the mobile big toe of apes; instead, they have an arch (the bending of the sole of the foot) typical of modern humans. The hominins seem to have moved in a leisurely stroll. Computer simulations based on information from A. afarensis fossil skeletons and the spacing of the footprints indicate that the Hominina were walking