The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa

Abstract

New ages for flowstone, sediments and fossil bones from the Dinaledi Chamber are presented. We combined optically stimulated luminescence dating of sediments with U-Th and palaeomagnetic analyses of flowstones to establish that all sediments containing Homo naledi fossils can be allocated to a single stratigraphic entity (sub-unit 3b), interpreted to be deposited between 236 ka and 414 ka. This result has been confirmed independently by dating three H. naledi teeth with combined U-series and electron spin resonance (US-ESR) dating. Two dating scenarios for the fossils were tested by varying the assumed levels of ²²²Rn loss in the encasing sediments: a maximum age scenario provides an average age for the two least altered fossil teeth of 253 +82/-70 ka, whilst a minimum age scenario yields an average age of 200 +70/-61 ka. We consider the maximum age scenario to more closely reflect conditions in the cave, and therefore, the true age of the fossils. By combining the US-ESR maximum age estimate obtained from the teeth, with the U-Th age for the oldest flowstone overlying Homo naledi fossils, we have constrained the depositional age of Homo naledi to a period between 236 ka and 335 ka. These age results demonstrate that a morphologically primitive hominin, Homo naledi, survived into the later parts of the Pleistocene in Africa, and indicate a much younger age for the Homo naledi fossils than have previously been hypothesized based on their morphology.

Keywords: Dinaledi Chamber; Homo naledi; Pleistocene; dating; evolutionary biology; genomics; hominin; none; paleoanthropology.

Figure 1.. Location of Rising Star Cave and the Dinaledi Chamber.

(a) Simplified geological map showing the position of the Rising Star Cave (in grey); (b) close-up map of the Dinaledi Chamber showing the distribution of the dating samples, including: U-Th flowstone samples (yellow dots, black text); ESR samples (purple dots, orange text); and OSL samples (red dots, blue text). Age estimates for the different samples are shown, with cross reference to Tables 1, 7 and 8. DOI: http://dx.doi.org/10.7554/eLife.24231.003

Figure 2.. Geological face map and cross-sections through the sediment pile at different locations in the Dinaledi Chamber, illustrating the relationships between the flowstone groups and sedimentary units.

The positions of the section lines are shown in (a); a face map of the entry zone of the Dinaledi Chamber (looking NE) is shown in (b); geological cross-sections through the central part of the Dinaledi Chamber near the excavation pit are shown in (c) and (d). DOI: http://dx.doi.org/10.7554/eLife.24231.004

Figure 3.. Field and close-up photographs of all flowstone samples collected for U-Th dating.

The flowstone groups (i.e., Flowstone Groups 1, 2 or 3), sample numbers, and ages (2σ uncertainty), as listed in Table 1, are shown below each sample. Ages reported here are from JCU, unless otherwise stated. DOI: http://dx.doi.org/10.7554/eLife.24231.005

Figure 4.. Location of the three H. naledi tooth samples (samples 1767, 1788 and 1810) and one baboon (cf. Papio) tooth sample (sample 1841) used for combined U-series and ESR dating.

(a) Map of the Dinaledi Chamber showing the position of the excavation pit and the position of figures (b) and (c); (b) close-up of the SE corner of the excavation pit showing the sample site for sample 1810 and the 50 cm deep sondage from which sample 1841 was recovered; (c) the area to the W of the excavation pit from which samples 1767 and 1788 were collected. DOI: http://dx.doi.org/10.7554/eLife.24231.011

Figure 5.. Samples of orange laminated mudstone of Unit 1 for OSL dating.

(a) sample OSL3 with an estimated MAM age of 231 ± 41 ka taken from sub-unit 1b; (b) sample OSL4 with an estimated MAM age of 241 ± 37 ka, taken from sub-unit 1b and covered by a flowstone sheet dated at 290 ± 6 ka (RS5); (c) sample OSL5 with an estimated MAM age of 353 ± 61 ka, taken from sub-unit 1a and covered by a flowstone sheet dated at 32.1 ± 0.4 ka (RS20). The scale bar in each of the photographs is 10 cm. DOI: http://dx.doi.org/10.7554/eLife.24231.012

Figure 6.. Photographs of H. naledi teeth used for ESR dating.

(a) U.W.101–1767; (b) U.W.101–1788; (c) U.W.101–1810. The order of images for each panel is: buccal, distal, lingual, mesial, and occlusal views. The scale bar in each panel is 1 cm. DOI: http://dx.doi.org/10.7554/eLife.24231.013

Figure 7.. Photographs of the baboon (cf. Papio) tooth (sample 1841), recovered from the sondage in the excavation pit, and used for ESR dating.

Views are: (a) buccal, (b) occlusal, (c) lingual, and (d) internal. DOI: http://dx.doi.org/10.7554/eLife.24231.014

Figure 8.. Cartoon illustrating the sedimentary history resulting in the deposition and redistribution of sediment of Units 2 and 3, and Flowstone Groups 1 to 3 in the Dinaledi Chamber.

Note that all hominin fossils are contained in sub-unit 3b, but that this sub-unit has been repeatedly reworked after its initial deposition. Fossil entry occurred during the initial stages of deposition of Unit 3 below the entry shaft and predated deposition of Flowstone 1c. H. naledi fossils may have continued to enter the Dinaledi Chamber as older parts of Unit 3 were eroded from below Flowstone 1c, and as remnants of all older units were reworked to be incorporated into Unit 3 sediments that accumulated along the floor of the Dinaledi Chamber. DOI: http://dx.doi.org/10.7554/eLife.24231.019

Figure 9.. Photographs illustrating the sampling approaches taken by SCU-UoW and GU-ANU in obtaining the U-Th results presented in Tables 4 and 5.

(a) Comparison of sampling grids across the enamel-dentine boundary measured by SCU-UoW (red lines) vs. GU-ANU (blue circles). SCU-UoW (red lines) measured a series of parallel, shallow (<5 μm) pits along grid lines across the teeth and averaged U concentrations across each grid. GU-ANU (blue circles) measured the average composition of the tooth in single spots that were laser-bored along profiles across the teeth, and report results for each spot. (b, c) Locations of LA-ICP-MS spot analyses for teeth samples 1788 (b) and 1810 (c) conducted by GU-ANU. The detailed transects are shown in panels (d) to (h). DOI: http://dx.doi.org/10.7554/eLife.24231.020

Figure 10.. Gamma dose rate reconstructions derived from analytical data of sediment samples collected around ESR samples 1767, 1788 and 1810 (closed circles and diamonds), combined with samples from a vertical profile in the excavation pit and sondage (open circles and diamonds).

The data show little variation in dose rate with depth (see text for explanation). DOI: http://dx.doi.org/10.7554/eLife.24231.021

Figure 11.. ESR dose response curves (DRC’s) obtained for the samples 1767, 1788, 1810 and 1841.

To facilitate comparison, all DRC’s have been normalised to the intensity of the natural point (=1). DOI: http://dx.doi.org/10.7554/eLife.24231.023

Figure 12.. Evolution of the I_max/I_min ratio vs the irradiation dose for the four tooth samples.

(see text for explanation). DOI: http://dx.doi.org/10.7554/eLife.24231.024

Figure 13.. Samples and results of palaeomagnetic analyses forFlowstone 1a.

(a) Outcrop photo of hanging erosion remnant of Flowstone 1a from which the palaeomagnetic sample was taken. The three flowstone phases separated by detrital horizons are clearly visible, and their magnetic polarity has been marked (N = normal; R = reverse). The stratigraphic top is towards the top of the photo; (b) close-up of a hand sample taken for palaeomagnetic analysis from Flowstone 1a in the Dinaledi Chamber. The sample is layered and comprises three distinct phases (from base to top: A-C marked in yellow) separated by thin clastic horizons that mark disconformities indicated with red dashed lines. The larger-scale extent of the three phases can be seen in (a); (c) intensity spectra, Zijderveld plots, and stereo plots for samples from phases A to C taken from (b). Phases B and C show normal polarity and phase A shows reversed and intermediate polarity directions. DOI: http://dx.doi.org/10.7554/eLife.24231.025

Figure 14.. Chronostratigraphic summary of radio-isotopic dating results, and interpretation of the depositional ranges of stratigraphic units, flowstones and H.

naledi fossils in the Dinaledi Chamber. Following the preferred US-ESR maximum age model and associated uncertainties for ESR samples 1788 and 1810, a maximum depositional age of 335 Ma was determined, while the minimum depositional age of 236 ka was constrained by Flowstone 1c (sample RS18), which covers H. naledi material in the entry zone. DOI: http://dx.doi.org/10.7554/eLife.24231.026