Age of the oldest known Homo sapiens from eastern Africa

The oldest known human is from eastern Africa.

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This article wasCited by Nature.

Efforts to date the oldest modern human fossils in eastern Africa, from Omo-Kibish1,2,3 and Herto4,5 in Ethiopia, have drawn on a variety of chronometric evidence, including 40Ar/39Ar ages of tuffs. The Herto hominins are reported to have been around 160– 155 thousand years old. The relationships that underpin these estimates have been challenged. The Kamoya's Hominid site Tuff9 has been linked to the member of the Omo-Kibish Formation that contains Omo I, with a major eruption of Shala volcano. The minimum age for the fossils of 233 22 kyr is obtained by dating the deposits of this eruption. We show that the KHS Tuff does not correlate with the Waidedo VItric Tuff, which means that the minimum age for the Herto fossils cannot be established. The age of the oldest known Homo sapiens fossils in eastern Africa is consistent with independent evidence for greater antiquity of the modern human group.

There are only eight sites in Africa that have yielded Homo sapiens fossils. Most of these have age uncertainty. The use of single-crystal 40Ar/39Ar isotope dating applied to volcanic ash beds is a principal method for constraining the fossil ages. Many of the tephra deposits lack suitable crystals for dating. In this case, the tephra layer can be compared to the larger deposits with larger phenocrysts. The Omo I1 and Herto specimen are two of the most widely accepted fossils that are thought to be modern cranial apomorphies. The evidence that constrains their ages is a topic of considerable controversy.

In the late 1960s, the remains of the Omo I were found at the bottom of a siltstone near the top of the Member I of the Omo-Kibish Formation. The maximum age of Omo I was derived from the 40Ar/39Ar age of 196 4 kyr (2)3,6,17 obtained for alkali feldspar phenocrysts from the three youngest pumice clasts. The Nakaa'kire Tuff age was calculated using a more widely adopted age of 28.199 million years. There is uncertainty about the relationship between the tuff and the hominin fossils, so much attention has been focused on dating it. Member I was retrieved around 1.4 m lower down section, and is younger than the fossils. The Nakaa'kire Tuff was not found in the same section as the KHS Tuff because the dated pumice clasts correlated with the Nakaa'kire Tuff. The fine grain size of the KHS Tuff precludes direct 40Ar/39Ar dating, and no correlation to a source volcano has been made previously. It has been correlated with both the Konso Formation and the directly 40Ar/39Ar-dated 184 10 kyr unit D22 of the Gademotta Formation. A slightly younger age for the KHS Tuff of around 172 kyr has also been proposed. The proposed age for Omo I is 197 4 kyr.

The Main Ethiopia Rift is the subject of a tephrostratigraphy in this picture.

The Map of the MER shows the late Middle Pleistocene formations and tephra units. There are white boxes with blue edges that show correlations between the KHS Tuff6 and the Alyio Tuff6 in the late Middle Pleistocene formations. The upper Herto Member is LHM, while the lower Herto Member is UHM.

The Herto H. sapiens fossils were found in the Middle Awash5 in the late 1990s. The upper Herto Member of the Bouri Formation contains sandstone. This sandstone is found across western Afar and is also present at Gona 24, 50 km north of Herto. The WAVT's age was found to be around 160 Kyr, but it was not possible to determine the exact age due to crystalcontamination. 5). The WAVT was found to be a correlation of tephra TA-55 on the basis of major element analysis of individual grains and major and trace element analysis of purified bulk separates. The Herto fossils are thought to be around 160– 155 kyr, which is below the 155 14 kyr Silver Tuff5 in Konso. There are 4 This finding was challenged in a study that correlated the Kibish KHS with KonsoTA-55 and the Herto WAVT. The argument suggested that the WAVT was older than Herto's. The WAVT above the Herto fossils challenged the age of the KHS by corroborating their original stratigraphy. They concluded that the KHS, Konso unit TA-555, Gademotta unit D, and WAVT5 could all be a single tephrostratigraphic marker. The age of the KHS Tuff is critical to the chronostratigraphy of the sites.

We re-analyzed the ash deposits at Konso and the KHS Tuff to see if the ages of the oldest modern human fossils can be inferred. While visiting the sampling locality of the KHS Tuff, we found another tephra layer in an outcrop about 100 m from the KS type section. The unit is 40 cm above the KHS Tuff and is made of 15 cm-thick fine sand grey tephra. It is ubiquitous between the KHS section and the Chibele section, and may correspond to unit CRF-23 previously identified above the KHS Tuff at the CB section9.

We included samples of ignimbrites from the caldera-forming eruptions of Shala and Corbetti volcanoes in our attempt to identify and date the eruption that generated the KHS tuff. Major eruptions between 170 ka and 250 ka26 have been seen in Shala and Corbetti. The largest caldera in the central MER is at Shala. We analysed glass from a welded ignimbrite that was attributed to the formation of Corbetti caldera. 26). There is a challenge of correlation between the tephra deposits in the region and the volcano in the MER28. The correlations should be based on a detailed suite of major, minor and trace element analyses.

The age of the Shala Qi2 ignimbrite is shown in the second figure.

The location of the site is near Lake Shala in the MER. The field observations show that deposits 14A and 14B are part of the same eruption. Weighted means are used for data. The bars show data and results. 40Ar, radiogenic 40Ar, mean square of weighted deviates, P, probability that residuals are explained by measurement errors exclusively, n, number of accepted grains.

The KHS glass shards are made from pantelleritic rhyolite with a total Fe of 0.1 wt%. The abundances of FeO*, CaO, Al2O3 and TiO2 correspond with the glasses from the eruption of Shala volcano. Supplementary Table 1, Supplementary Information are included. The correlations are confirmed by comparing trace element ratios for the glasses. Supplementary Table 2 contains supplementary information.

There aregeochemical fingerprints of MER tephra.

The major element abundances and trace element ratios of glasses are from the Shala Qi2 ignimbrite, the Corbetti ignimbrite, the Gademotta unit D, and the Kibish KHS. The major element data is normalized. The error bars are derived from repeat measurements of matrix match glass secondary standards STH-S6 and ATHO-G. They are plotted in the top right corner of the plot for clarity. Error propagation has been applied in the case of element ratios. There are additional observations and biplots presented.

The rhyolite glass from the 177 8 kyr is from the COI2E pantelleritic rhyolite glass. Corbetti ignimbrite has a 0.2 wt% SiO2, 9.1 0.1 wt% Al2O3, and 0.2 wt% FeO. Supplementary Table 2 contains supplementary information.

We used the 40Ar/39Ar dating method to analyse the sanidine crystals from the Shala Qi2 deposits. Six grains with ages exceeding 1 Myr were excluded from the data because they were significantly older than the mean of the dataset. The ages from each sample were not different at 2 uncertainty. The weighted mean of 233 22 kyr at 2 was derived from the analyses of both pumice samples.

The age of 233 22 kyr is consistent with the age of 177 8 tephra. It casts doubt on the correlation between deposition in the Omo basin and large in-flows of fresh water from the Nile River system into the Mediterranean sea. The formation of Mediterranean Sapropel S6 at 172 ka6 is in line with our KHS age. Although the age of the basin is consistent with the formation of sapropel S6 (172 ka)29, only a mudstone unit of around 40 cm thickness separates KHS from ETH18-8, which cannot account for the rapid deposition in the basin.

The revision of the Omo-Kibish stratigraphy is incompatible with the age reported for the Nakaa'kire Tuff, which is found in Member I of the formation. Three out of five dated pumice clasts were found in a sandy tuffaceous matrix. The samples had similar elements but they were collected from a different outcrop. The 233 22 ka Qi2 eruption of Shala was identified as the source of the Nakaa'kire Tuff because of the uncertainty in the age and location of the Tuff.

Our glass composition data, source correlation and age estimate allow us to re-assess its identification at other archaeological sites in Ethiopia. After density separation, the pedogenically altered unit TA-55 at Konso failed to identify glass shards. This precluded the evaluation of the reported correlation. The underlying unit is now correlated with the other units. It's clear that the age of the person is younger than 177 8 kyr.

The major and trace element abundances are not clearly overlap in the unit D. The match was precluding. Immobile trace element ratios and principal component analysis show that unit D is different from the others. Supplementary information.

The Herto WAVT and Konso unit were correlated, leading investigators to accept the age of the SVT at Konso as the end of the Herto fossils. The correlation is reinforced by additional data. The age of the Konso tuff was not found in our sample, but the tephrostratigraphic correlations proposed between the Omo-Kibish, Gademotta and Konso formations were not supported by our results. The Herto H. sapiens fossils are considerably older than this age group, and this correlation with the WAVT at Herto should be confirmed in the future using grain-discrete single-point glass analyses.

Our new age constraints are in line with most models for the evolution of modern humans, which estimate the origin of H. sapiens at around 350–200 ka. 16,31,32) The Herto fossils do not lie beneath the same tephra horizon as the Kibish fossils, so the challenge remains to obtain a robust maximum age for Omo I. Further data is needed to clarify the relationship between the WAVT and other MER tephra, and may lead to the identification of the WAVT source, promising a more reliable minimum age for the Herto fossils. Efforts to develop the tephrochronological framework for eastern Africa will help in addressing a range of interrelated volcanological, palaeoenvironmental and palaeoanthropological questions.

Sampling and descriptions were done during two field seasons. We looked at the Konso20,21, Omo-Kibish3,6,9 and Gademotta22,34 formations after sampling the previously described Shala volcano. At each site, we described the outcrops and the thickness of the units and samples.

40Ar/39Ar dating.

The Departments of Geography and Earth Sciences at the University of Cambridge have Feldspars. A 250–500-m size fraction was obtained by crushing rocks in a jaw crushing machine and then sieved and passed through a Frantz magnetic barrier laboratory separator to be isolated from the groundmass. The 100–200 sanidine grains were further handpicked and then removed from the crystals.

The packets of samples and the monitors were packaged in copper foil and stacked in tubes with relative positions of the packets precisely measured. The sample package was irradiated in the Oregon State University reactor. Fish Canyon sanidine was found one million years ago. The 39Ar production was monitored and the J values for the samples were established. A CO2 laser with a non-gaussian, uniform energy profile and a 1.5mm beam diameter was used for the step-heating of the gas. The samples were loaded into a planchette with wells and were housed in a doubly pumped ZnS-window laser cell. The active gases CO2, H2O, H2, N2 and CH4 were liquefied using three Zr-Al getters, one at 16 C and two at 400 C. Time-intensity data was adjusted to inlet time with second-order fits to the data. The runs were corrected using the standard deviation of blanks. Mass discrimination was monitored on a daily basis, between and within sample runs, by analysis of an air standard aliquot delivered by an automated pipette system. The MassSpec software package was used to perform all the blank, interference and mass discrimination calculations. The approach of Renne et al. 201036 and the parameters of Renne et al. 201135 were used to make decay constants and corrections.

The samples with low radiogenic yields were rejected. The peak age distributions were determined by determining the youngest population of individual grain analyses that fit into a Gaussian distribution with the expected scatter as indicated by the value of the mean square of weighted deviates.

Two sigma errors are reported in Supplementary Table 3 with the raw data in Supplementary Table 4. The two sub-samples from the top and bottom of the same unit are indistinguishable in age and can be combined into a single sample. The accepted age for this population is 231 22 kyr. It's relative to ref. 18). The age of 219 27 kyr (40Ar/36Ar(i)) is indistinguishable from the accepted age.

We are using the Kuiper et al. Calibration, the Renne et al. 2011. Calibration has quantifiable uncertainties and is our preferred age for the sample. For consistency with previous work, the last age is used throughout the manuscript.

Prepare a sample for the analysis.

The samples were prepared in the Cambridge Tephra Laboratory in line with the protocols of the International Focus Group on Tephrochronology. Pumice samples were washed in a bath of water and hydrochloric acid. Glass grains from the 125–250-m fraction were sectioned and polished. The tephra samples were washed through a sieve in 80 degree water. The density of the altered samples of the Konso formation was used to find volcanic glass. The sample was sieved at 125, 80 and 25 m, and then inspected under the microscope, but no glass was found.

The major element analysis was done.

The Department of Earth Sciences at the University of Cambridge analysed the samples with a SX 100 CAMECA electron microprobe. Major elements were measured with a 10-nA defocused beam. 10 s, 20 s, 20 s, 60 s, 90 s, P and 120 s were counted. The first thing to measure was the amount of sodium. The analytical accuracy was checked against international standards. Standard deviations and replicas are reported in Supplementary Table 6. The standard deviation of each sample is affected by their natural variability, so error bars are used instead. We analysed 40–50 points per sample. The analyses are reported in Supplementary Table 1.

A trace element analysis is done.

The element compositions of individual tephra shards were analysed by the iCRAG laboratory at Trinity College Dublin. The instrument used was a thermo iCAPQ coupled to a laser and a cell. We used a spot size of 40 m and a repetition rate of 6 s and a count time of 33 s on the sample and 30 s on the gas blank. We used the average Ca concentration of each sample as a correction factor when we analysed large-enough glass shards. The internal standard was used to calibrate the concentrations. Data reduction and a secondary Ca correction factor were done. The accuracies of ATHO-G and StHs6/80-G MPI-DING glass analyses are usually better than the rest. The standard deviations of replicate standard analyses are reflected in the error bars on the Supplementary figs. Standard deviations of trace element ratios are taken into account. Supplementary Table 2 contains detailed compositions of samples.

A summary of the report.

The Nature Research Reporting Summary contains further information on research design.

The paper and its supplementary information files contain all the data supporting the study. Settlements, lakes and other features are from the data from the NASA Land Processes distributed Active Archive Center. The background image for the top left corner is from the US Geological Survey.

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References can be downloaded.

The study was supported by the Leverhulme Trust and the Cambridge-AfricaALBORADA Research Fund. The UK Natural Environment Research Council supported Ar-Ar dating. The iCRAG LA-ICP-MS facility at Trinity College Dublin is supported by a SFI award. We acknowledge the local and regional authorities in Ethiopia for facilitating the export of samples. We are grateful to Y. Beyene, Ethioder and their drivers, and field assistants Alex in the Konso tephra localities. We would like to thank D. Colby for facilitating access to the Corbetti sample. The manuscript has been helped by comments from W. Hart and C. Feibel.

The authors do not have competing or financial interests.

Nature thanks William Hart, Craig Feibel, and other anonymous reviewers for their contribution to the peer review of this work.

There is information in this file on tephra. There are additional references.

The tephra samples have a major element normalized composition.

The tephra samples have element abundances.

40Ar/39Ar ages for unit A and unit C of the Qi2 ignimbrite of Shala.

The data for the samples are from the rosin.

There are decay constants and correction factors.

The average compositions of secondary standards for the last two years.

Standard compositions are used for LA-ICP-MS analyses.

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Page 2

The Map of the MER shows the late Middle Pleistocene formations and tephra units. There are white boxes with blue edges that show correlations between the KHS Tuff6 and the Alyio Tuff6 in the late Middle Pleistocene formations. The upper Herto Member is LHM, while the lower Herto Member is UHM.