The Cradle of Humankind (Cradle) in South Africa preserves a rich collection of fossil hominins representing Australopithecus, Paranthropus and Homo1. The ages of these fossils are contentious2,3,4 and have compromised the degree to which the South African hominin record can be used to test hypotheses of human evolution. However, uranium–lead (U–Pb) analyses of horizontally bedded layers of calcium carbonate (flowstone) provide a potential opportunity to obtain a robust chronology5. Flowstones are ubiquitous cave features and provide a palaeoclimatic context, because they grow only during phases of increased effective precipitation6,7, ideally in closed caves. Here we show that flowstones from eight Cradle caves date to six narrow time intervals between 3.2 and 1.3 million years ago. We use a kernel density estimate to combine 29 U–Pb ages into a single record of flowstone growth intervals. We interpret these as major wet phases, when an increased water supply, more extensive vegetation cover and at least partially closed caves allowed for undisturbed, semi-continuous growth of the flowstones. The intervening times represent substantially drier phases, during which fossils of hominins and other fossils accumulated in open caves. Fossil preservation, restricted to drier intervals, thus biases the view of hominin evolutionary history and behaviour, and places the hominins in a community of comparatively dry-adapted fauna. Although the periods of cave closure leave temporal gaps in the South African fossil record, the flowstones themselves provide valuable insights into both local and pan-African climate variability.
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The authors declare that all data supporting the findings of this study are available within the paper and the Supplementary Information (see Supplementary Information and Supplementary Table 1).
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We thank D. Braun, R. Potts, B. Wood and W. L. S. Joe for their insightful discussion. Site access granted by C. Steininger, R. Clarke, T. Pickering, C. Menter, S. Potze, J. Adams and L. Berger; permits provided by South African Heritage Resource Agency. This work was supported by Australian Research Council DECRA DE120102504 (to R.P.), University of Melbourne McKenzie Post-Doctoral Fellowship 0023249 (to R.P.), Australian Research Council Future Fellowship FT120100399 (to A.I.R.H.) and Discovery Project DP170100056 (to A.I.R.H. and D.S.S.) and National Science Foundation Grant BCS 0962564 (to A.I.R.H.) .
Nature thanks C. Feibel, T. Rasbury and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Extended Data Fig. 1 Field photographs showing U–Pb-dated flowstone from the indicated sites, all of which record some variation of an alternating stack of flowstones and fossil-bearing sediments.
a, The basal flowstone from Cooper’s Cave. b, c, Flowstones at Haasgat are preserved in the now-deroofed section of the deposits (b) and inside the cave (c). d, The flowstone capping the MB1 Lower Bank at Swartkrans. e, f, Flowstone from Bolt’s Farm Pit 7 at the base (e) and top (f) of the sequence. g, Flowstone capping Member 4 at Sterkfontein. h, Flowstone underlying fossil bearing sediments at Malapa. i, Flowstone sandwiched between fossil-bearing sediments at Drimolen. j, Massive flowstone at the base of Member 4 at Sterkfontein is exposed in a borehole. k, Flowstone underlying fossil-bearing sediments at Hoogland.
Extended Data Fig. 2 U–Pb ages plotted against time and by site, additional un-U–Pb dated Cradle sites and non-Cradle hominin cave sites included.
a–e, A variation of Fig. 2. U–Pb ages of Cradle sites are shown, with Cradle sites not dated with U–Pb (Gondolin and Kromdraai) included, as well as the non-Cradle hominin cave sites (Makapansgat and Taung) shown for comparison. All U–Pb ages are plotted against time and by site, n = 29, diamonds represent individual ages and 2σ errors are shown as whiskers. Also included here are four records of climate and variability derived from orbital parameters for East African sites (d), specifically arid phases from soil carbonates34, periods of deep rift valley lakes35, phases of extreme climate variability36 and key phases of variability as described previously37. Again, there is no clear relationship between these records and the new South African flowstone record. Indicated data were obtained from previous studies28,34,35,36,37.
Detailed discussion about the ages of the sedimentary fossil bearing units preserved in the Cradle caves.
All uranium and lead isotope concentration and ratio data needed for age calculations.
Animation showing the location and age distribution of speleothem samples at hominin bearing cave sites in the Cradle of Humankind between 3.200 and 1.332 million years ago.