LAUSR

It is almost 100 y since Raymond Dart described the first Australopithecus from the Taung lime quarry in South Africa (1). Since that time, discoveries of early hominins (genera Australopithecus, Sahelanthropus, Ardipithecus, and Paranthropus) have abounded across Africa, establishing the continent as our evolutionary home. However, despite major advances in chronometric techniques, controversies remain about the age of Taung and other South African sites. In PNAS, Frost et al. (2) provide new age estimates for South Africa’s most important early hominin sites, including Taung, based on the composition and evolutionary stage of their fossil monkeys. Fossil sites across the world present age-dating challenges: the problem is neither restricted to Africa nor is it new to paleontology. However, due to continental-scale geological processes, the datability of fossil sites in the African record is starkly determined by location (Fig. 1). The tectonically dynamic east of the continent, marked by the East African Rift Valley and its associated lakes, has been strewn with active volcanoes for millions of years. These regularly spew ashes and lava flows that enter the geological record and that allow for direct radiometric dating of fossil sites, namely using the K–Ar or Ar–Ar methods, which can be considered the “gold standard” for geochronological control. In contrast, southern Africa is quiescent and devoid of volcanic activity. Its terrestrial fossil sites are largely cave and breccia infills within limestones, some of which originally formed over 2 billion years ago (3). Compounding the absence of datable volcanics, the complexities of karstic limestone cave systems, namely their propensity to dissolution and redeposition, make understanding stratigraphic relationships at these sites extremely challenging (4). This has left the age of several sites open and contentious. In the absence of datable volcanics, a variety of chronometric techniques have been applied. These include paleomagnetism (on rocks), electron spin resonance (on fossil teeth), and U–Pb analyses (on both flowstones and fossil teeth) (4). The problem is that these techniques are generally more assumption-laden and give more ambiguous or less precise results than radiometric dating of volcanics. For some sites, multiple approaches have converged on a single age range, providing a basis for consensus. At other sites, however, different methods have produced age estimates that diverge wildly. In particular, the relatively novel Al–Be cosmogenic nuclide technique has produced significantly older-than-expected ages for assemblages from the famed site of Sterkfontein (5). Another approach to geochronology relies on principles as old as the field of geology itself. Biostratigraphy uses fossils to correlate the relative positions of geological layers at different sites. Layers containing similar-looking fossils are assumed to have been deposited at similar times in the past, and the relative ordering of geological deposits can be determined by their fossil content. This “principle of faunal succession” is attributed to William Smith, who used biostratigraphy to produce the first stratigraphic maps in the early 1800s. Over the last two centuries, the potential and precision of biostratigraphic applications have improved significantly with our understanding of the spatial and vertical distribution of fossils worldwide in conjunction with the development of radiometric dating techniques. Fossil taxa with well-defined lineages from well-dated contexts can provide a direct age estimate for any deposits in which they are found, an extension of biostratigraphy termed “biochronology.” For example, a trilobite informs you that you are in the Paleozoic, a tyrannosaur that you are in the Late Cretaceous, and a wooly mammoth that you are likely in the Middle to Late Pleistocene.