Age and context of mid-Pliocene hominin cranium from Woranso-Mille, Ethiopia

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Abstract

A fossil hominin cranium was discovered in mid-Pliocene deltaic strata in the Godaya Valley of the northwestern Woranso-Mille study area in Ethiopia. Here we show that analyses of chemically correlated volcanic layers and the palaeomagnetic stratigraphy, combined with Bayesian modelling of dated tuffs, yield an age range of 3.804 ± 0.013 to 3.777 ± 0.014 million years old (mean ± 1σ) for the deltaic strata and the fossils that they contain. We also document deposits of a perennial lake beneath the deltaic sequence. Mammalian fossils associated with the cranium represent taxa that were widespread at the time and data from botanical remains indicate that the vegetation in the lake and delta catchment was predominantly dry shrubland with varying proportions of grassland, wetland and riparian forest. In addition, we report high rates of sediment accumulation and depositional features that are typical of a steep topographic relief and differ from younger Woranso-Mille fossil localities, reflecting the influence of active rift processes on the palaeolandscape.

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Fig. 1: Stratigraphic setting of the fossil cranium.
Fig. 2: Lake, delta and tuffaceous strata below and above the level where the hominin cranium was found.

Data availability

Any data supporting the findings of this study that are not included in the paper and Supplementary Information are available from the corresponding author upon reasonable request.

Code availability

A data analysis version of the 40Ar/39Ar dating software is available at no cost from A.D. (adeino@bgc.org). The Bayesian age–stratigraphic analysis model is available at https://github.com/brenhinkeller.

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Acknowledgements

We thank the Authority for Research and Conservation of Cultural Heritage for permission to conduct field and laboratory work, the Afar people of Woranso-Millle for their hospitality, and the project’s fieldwork crew members for their tireless support of field activities; M. Atkins and I. Van Orman for assistance with the preparation of tephrochemical samples and J. Fournelle for assistance with their analysis. This research was supported by grants from the Spanish Government CGL2016-79458-P, Catalan Government 2017-SGR 824, the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 659596 and the US National Science Foundation (1124705, 1124716, 1125157, 1125345, 1322017).

Author information

B.Z.S., L.G., M.A., A.D., N.E.L., S.M.M., M.D.P. and Y.H.-S. conducted field research. M.A. collected basalt samples and S.A.M. analysed the samples for major and trace element concentrations. B.Z.S. collected and analysed tuff samples for major and minor element abundances in glass. A.D. collected and analysed tuff samples for 40Ar/39Ar dating. L.G. collected sediment samples and analysed their palaeomagnetic polarity. N.E.L. and M.D.P. collected sediment samples for analysis of botanical remains. S.M.M. contributed to the collection and analysis of mammalian fossils. M.D.P. and S.J.F. analysed and interpreted leaf wax isotopes. B.B. and D.B. separated and identified pollen. A.N. and F.S. separated and identified phytolith and diatom components. Y.H.-S. collected and analysed the mammalian fossils and directed the field station. B.Z.S. took the lead in writing the paper with contributions from all co-authors.

Correspondence to Yohannes Haile-Selassie.

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Peer review information Nature thanks Craig S. Feibel, John W. Kappelman 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.

a, Location of the horizon of the MRD-VP-1 fossil relative to features on the Miro Dora plateau (capped by Am-Ado basalts) and the higher Guda plateau (background). b, Coarse sand matrix on MRD-VP-1/1. Additional matrix types, which are not shown, included very hard fine sand and soft silty clay. c, Lehaysule Gera hill (section S2); image shows the position of geochemically identified tuffs (Mille tuff sequence and associated basaltic tuffs, and Araskimiro tuff) and the dated T9 tuff (WM12/LHG-6) above dark deltaic siltstone and lacustrine claystone. A lower dated tuff (WM18/LHG-1) near the bottom of the claystone exposure is not discernable. d, The hill of section S5; image shows the position of the deltaic sequence relative to the geochemically identified Mille tuff and associated basaltic tuff sequence and T9 tuff. e, Large-scale cross-bedded sandstone at MRD-VP-1. Arrows show foreset dip directions, indicating the flow toward the southeast (right arrow) and toward the south-southwest (left arrow). f, Upward-coarsening delta lobe(s) along the Am-Ado Plateau (S3) with Mille tuff sequence and Am-Ado basalt above and between.

Extended Data Fig. 2 Tephrochemical data and age–depth models.

a, Bivariate plots of oxide abundances in volcanic glass normalized for differences in water content (Supplementary Table 2). Symbol colours and shapes are as in Fig. 1 with closed, smaller symbols indicating newly reported analyses of individual glass shards from samples from the study area, differentiated by symbol border colour and size. Open symbols indicate previously published22 sample means with 1σ variation for named tuffs near the Waki-Mille confluence, specifically: Kilaytoli tuff (WM-KSD-3, n = 19, 11 high iron and 8 low iron), Mesgid Dora tuff (MSD-08-5, n = 12), Araskimiro tuff (MSD-08-3R, n = 12 and WM-MD5 P, n = 10, duplicate samples from same location), bimodal tuff (WMC-08-3, n = 16, n = 10, duplicate analyses of same sample), Mille tuff sequence (ARI-08-3, n = 16, ARI-08-4, n = 11, two chemically similar, vertically adjacent tuffs in the sequence). The Mille tuff sequence, the rhyolitic component of the basaltic and rhyolitic ‘bimodal tuff’ and their correlates are distinguished from other pumice tuffs in the area by their low CaO abundances. The Araskimiro tuff and its correlates are distinguished from chemically similar pumice tuffs in the area, including the Mesgid Dora tuff, by position above the Mille tuff/basaltic tuff sequence and below the Waki tuff, which is present below the Mesgid Dora tuff in a drainage approximately 300 m to the east of S2 (Fig. 1d). b, Bayesian age–stratigraphic model44 for n = 3 tuffs at S2, including the lowermost dated tuff (WM18/LHG-1), the T9 tuff (WM12/LHG-6) and the bottom of the Mille tuff sequence (around 2 m below LLG-12-10, which is chemically similar to the Mille/basaltic tuff sequence), and a similar model correlating these horizons to S6.

Extended Data Fig. 3 Detailed stratigraphic sections with positions of samples that were investigated and analysed for leaf wax, pollen and phytolith content.

All sections are in centimetres except for MRD18-03. Names of sections are listed at the top of each column and equivalent section names reported in Fig. 2 are listed in parentheses. The number associated with each sample identifier is listed next to the dot that indicates the stratigraphic position of samples. If sample numbers are equivalent stratigraphically to other samples, this is noted in parentheses. Tuff numbering is included on the left of each column. Dashed lines provide ties between stratigraphic and lithologic equivalent horizons for non-tuffaceous units. The tuff numbering scheme also indicates tie points. Facies classification abbreviations are from a previously published study45. F, S and G indicate grain size (clay or silt size, sand size and gravel size, respectively) and the descriptors m, h, l, p and t describe the primary bedding as massive, horizontal, laminated, planar and trough, respectively. Listed latitudes and longitudes are in reference to the WGS-84 datum and indicate the position of the base of the section unless otherwise noted. The stratigraphic positions for samples WM18-MRD-210, -413, -414, -416 (section S4) and WM18-LHG-258 (section S2) can be found in Fig. 2.

Extended Data Fig. 4 Incremental heating release spectra from single-grain 40Ar/39Ar analyses.

The x axis of each subplot is the cumulative percentage release of 39Ar during the course of the experiment. ‘Int. Age’ is the ‘integrated’ or total-gas age, obtained by isotopically summing the individual steps weighted by the inverse variance, with the error calculated as the modified standard error at 1σ (standard error of the weighted mean multiplied by the square root of the MSWD when MSWD > 1). The apparent age plateau—defined as a sequence of at least three or more consecutive steps representing at least half of the total 39Ar release, and for which analytical error alone is sufficient to explain the observed age dispersion at a probability of 95%—is calculated as the mean age weighted by the inverse variance, with the error shown as the 1σ modified standard error. Neither the integrated age nor plateau age incorporated the error in J at this stage in the data reduction; this systematic error is included in the final weighted-mean age calculated from the isochron results. Results for sample WM12/LHG-6 indicated n = 10 plateaus identified out of a total of n = 13 single-crystal incremental heating experiments; for sample WM18/LHG-1, n = 8 plateaus were found in n = 8 heating experiments.

Extended Data Fig. 5 Inverse-isochron correlation plots of plateau steps from incremental heating experiments.

Plots show isotope correlation diagrams for 36Ar/40Ar and 39Ar/40Ar ratios Each grain is plotted separately. The age is obtained from the x-axis intercept and is shown with a 1σ modified standard error. ‘40Ar/39Ar Int.’ refers to the ‘trapped’ non-radiogenic 40Ar/36Ar ratio derived from the y-axis intercept of the isochron. ‘P’ refers to the probability that the observed scatter can be explained by analytical errors alone; a value below 0.05 indicates that analytical errors are insufficient to explain the magnitude of the dispersion (that is, geological scatter is present). ‘n’ indicates the included number of analyses.

Extended Data Fig. 6 Age-probability density plot of ages derived from isochron analyses.

a, Mean Ca/K atomic ratio of individual grains estimated from 37Ar/39Ar measurements. b, Rank-order plot of single-crystal isochron results with 1σ analytical uncertainty. c, The vertical scale is the relative likelihood of a given age occurring in the sample. The weighted-mean ages of the population are shown, the 1σ error includes the error in J, the neutron-fluence parameter. The MSWD and number of included analyses are included after the mean age. Values in purple were omitted from analyses based on distance from the median age, with the cut-off level of 2.0 normalized median absolute deviations46. For sample WM12/LHG-6, eight out of ten isochron results are included in the weighted mean; for sample WM18/LHG-1, six out of eight results are included.

Extended Data Fig. 7 Palaeomagnetic results.

a, Magnetostratigraphy for sections S6, S4 and S2 with results expressed as the angle to expected normal direction (Δ). The reverse–normal–reverse polarity sequence in S6 is correlated with the GPTS24 on the basis of dated and geochemically identified tuffs in S2. White circles are mean directions of specimens (black), open circles are ambiguous specimen directions. vf, very fine sand; m, medium sand; C, coarse sand. b, Stereoplots and orthogonal demagnetization diagrams for a reverse (MRD-19a) and a normal (MRD-22b) sample. dec, declination; inc, inclination; int, intensity; NRM, natural remanent magnetization. The plots show the stepwise demagnetization process and the characteristic remanent magnetization directions (red lines) calculated using a principal component analysis47. In the stereonet plots (left), open circles show negative inclinations and solid dots show positive inclinations. In the orthogonal plots (right), solid dots indicate declination values and empty circles inclination values. Intensity units are in 10 A m−1. mad, maximum angle deviation. c, Stereographic projection, referred to magnetic North of characteristic remanent magnetization directions for specimens collected at S6, S4 and S2.Circles indicate individual specimens. The plot shows the associated normal and reversed mean directions (squares) of the specimens, as determined by Fisher statistics, and the position of the present field (triangle). Only those samples showing non-ambiguous directions are plotted. In total, 48 specimens show reverse polarity and 10 samples show normal polarity.

Extended Data Fig. 8 Pollen diagrams for sample WM18-MRD-210 and diatom diagram for WM18-LHG-248.

a, Relative abundances of pollen taxa (percentage) in WM18-MRD-210 calculated using the total number of pollen grains and fern spores. The asterisk indicate ‘-type’. Numbers in brackets refer to the number of pollen grains. b, Relative abundance of arboreal pollen (AP; trees and shrubs), non-arboreal pollen (NAP; herbs and forbs) and non-classified habitus (NC) in WM18-MRD-210. Numbers in brackets refer to the number of pollen grains. aqua., aquatic. c, Distribution of diatoms in WN18-LGH 248 according their ecological affinity: coastal–marine (yellow) versus brackish lacustrine (blue) environments. Among the species with coastal–marine affinities, Coscinodiscus lineatus and Coscinodiscus excentricus are typically observed in the Indian Ocean and Mozambique Channel48,49. Numbers in brackets refer to the number of diatom samples.

Extended Data Fig. 9 Examples of fossil pollen grains (sample WM18-MRD-210) and biogenic silica remains (sample WM18-LHG-248).

a–d, Pollen grains. a, Aerva-type javanica. b, Euclea. c, Acacia. d, Tribulus. e–g, Phytolith bodies. e, A spherical/globular psilate body, probably an indicator of dicot tree/shrub taxa. f, g, Two unidentified phytolith remains. h, A sponge spicule. i–l, Diatom frustules. i, Cyclotella meneghiniana. j, Actinoptychus splendens. k, Denticula valida. l, Cyclotella stylorum. Scale bars, 10 μm.

Extended Data Fig. 10 Shaded relief map of west central Afar.

Volcanic and sedimentary strata of the Sullu Adu Range (including the Guda Plateau) are sub-horizontal, 4 Myr old and younger, and stratigraphically above Oligocene and Miocene basalts and rhyolites that are tilted to the east-northeast, indicating periods of extension32 before approximately 4 Myr ago. Nearby areas with well-studied exposures13,32 of these Oligocene and Miocene formations are labelled in bold. Areas with lake deposits10,13 in the lower Hadar or upper Sagantole formations are labelled in italics. The base map is from http://www.geomapapp.org using the global multi-resolution topography synthesis50.

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