It was in the great African rift valley that hominids, or human ancestors, arose. Hominid fossils of the genus Australopithicus dating 3-4 million years ago have been unearthed in Ethiopia and Tanzania. And the remains of a more direct ancestor of man, Homo erectus, who was using fire 500,000 years ago, have been found in Olduvai Gorge in Tanzania as well as in Morocco, Algeria, and Chad.
Paleontologists, who study fossil remains, employ radioisotope dating techniques to determine the age of hominid and other species' fossil remains. This technique measures the decay of short-lived radioactive isotopes like carbon and argon to determine a fossil's age. This is based on the radioscope's atomic half-life, or the time required for half of a sample of a radioisotope to undergo radioactive decay. Dating is typically done on volcanic ash layers and charred wood associated with hominid fossils rather than the fossils themselves, which usually do not contain significant amounts of radioactive isotopes.
Science EncyclopediaScience & Philosophy: Adrenoceptor (adrenoreceptor; adrenergic receptor) to AmbientAfrica - Origin Of Africa, Continental Drift, General Features, East African Rift System, Human Evolution