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Teacher’s notes

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This clip shows palaeoanthropologist Dr Ron Clarke and his assistants, Stephen Motsumi and Nkwane Molefe, in a cave at Sterkfontein in South Africa where in 1997 they discovered the adult skeleton of an upright hominid, a species of early human. The narrator says that after coming across hominid foot bones that had earlier been excavated from the cave, Clarke asked Motsumi and Molefe to search for the rest of the skeleton. Clarke points out the features of the skeleton, which is embedded in the cave and which he estimates to be 3.3 million years old.

Educational value points

• The importance of the discovery detailed in this clip lies not only in the age of the skeleton but also in its completeness. Until this skeleton was discovered only partial skull or skeleton fragments of Australopithecus, an extinct genus of African hominid, were known. With this discovery scientists obtained a complete picture of this early human and an understanding of how they moved, confirming that the skeleton is the earliest evidence of an upright stance in hominids.

• The skeleton shown was initially estimated to be 3.3 million years old based on palaeomagnetic dating, which matched the magnetic elements in the sedimentary rock in which it was found to a period when the Earth’s magnetism was similar. In 2006 uranium–lead dating estimated its age to be 2.2 million years. The rock at Sterkfontein does not contain volcanic sediment, which usually provides a reliable way to date fossils.

• The clip shows the importance of examining previously discovered and, in this case, mis-catalogued bone fragments held by museums. Bones from this skeleton had previously been excavated and placed in two separate boxes labelled ‘monkey bones’ and ‘miscellaneous’. In 1924 a similar investigation of an unusually large baboon-like skull discovered as a result of mining found it to be the skull of an early hominid, Australopithecus africanus.

• The skeleton was found in the Sterkfontein Caves, which are part of the Cradle of Humankind World Heritage Site in Gauteng, South Africa, a 47,000-ha area that contains fossils dating from about 3.5 to 1.5 million years ago, a period that spans the early development of humans. In 1946 the discovery of an australopithecine skull had suggested that humans originated from this area and not, as previously thought, from Europe.

• The Sterkfontein Caves, seen here, are a series of extensive limestone caverns in the dolomite bedrock beneath the Witwatersrand Basin in South Africa, which was once an ancient seabed. About 3.5 million years ago shafts began to form in the Caves as a result of erosion. The Caves began to fill with debris from the surface, including mud, sand and stones, which turned into breccia (a sedimentary rock) and provided a fossil-rich bed of animals and plants.

• Excavation of the skeleton has been painstaking and has taken more than ten years as it is embedded in the breccia, which Dr Clarke describes as being as hard as cement. In addition the bones of the skeleton have been scattered by a rock fall. According to Dr Clarke the creature may have plunged down 10 m into the cave where it was found, and if it survived the fall it probably died of hunger and thirst.