Did Australopithecus africanus live 2 or 4 million years ago?

Prof. Pieter Vermeesh, University College London, UK

Abstract

In 1871, Charles Darwin published his landmark book The Descent of Man, in which he argued that humans are closely related to the great apes of Africa (gorillas and chimpanzees) and Asia (orangutans and gibbons). Although Darwin favoured an African origin for our species, many of his contemporaries believed that humans originated in Asia. The debate was largely settled by the discovery of the Taung Child in South Africa in 1924. As the holotype of Australopithecus africanus, the Taung Child is one of the most important fossils ever discovered. Yet, more than a century later, its precise age remains uncertain. Since the discovery of Taung, numerous other australopithecine fossils have been found in southern and eastern Africa. The eastern African fossils (including Australopithecus afarensis, represented most famously by "Lucy") can be dated reliably using the 40Ar/39Ar method on associated volcanic rocks. In contrast, dating the southern African fossils is much more challenging because there has been little volcanic activity south of Tanzania. Two Quaternary geochronological methods currently yield conflicting age estimates. Cosmogenic 26Al/10Be burial dating of cave sediments suggests that Australopithecus africanus lived around 4 Ma, whereas U-Pb dating of flowstones from the same cave systems yields ages closer to 2 Ma. This discrepancy may result from incorrect uncertainty propagation in disequilibrium-corrected 206Pb/238U dates. The much less commonly used 207Pb/235U method offers a potential solution to this problem. Preliminary results from carbonates at three key South African fossil sites (Taung, Hoogland, and Malapa) are encouraging.