Excavation methods
ThI-GH SU4 and SU5 have been systematically excavated since 1994. A 0.5/1-m-deep sequence of an area of 48 m2 was excavated (Fig. 3a and Extended Data Fig. 1b). Excavation was performed according to the stratigraphic sediment deposition, and stratigraphic units were subsequently numbered from 1 to 7 from top to bottom. We established an arbitrary excavation 1 m × 1 m grid, and spatial data (x, y, z) of all finds (worked and unworked lithics, as well as faunal and human remains) were recorded (Extended Data Table 1). From 1994 to 2005, single finds were assigned unique IDs consisting of the quarry acronym (ThI), the site acronym (GH), the name of the square and a progressive number (for example, ThI-GH-SA26-88). From 2006 onwards, spatial data measurement was carried out with the total station. The code for each find consists of the quarry acronym, the site acronym and a number from 10000 (for example, ThI-GH-10634). We documented layers, special features and profiles in 3D models using total station measurements, digital photographs and drawings. The 3D models were referenced with control points recorded with the total station to align them to the excavation grid. Sediments have been collected for every m2, dissociated with diluted formic acid and wet-screened to recover lithic and faunal small fragments.
Stratigraphy of the OHF
The chronostratigraphic framework of the OHF exposed at ThI is based on the direct observations of sedimentary formations, stratigraphic boundaries and facies. The successions and associations of facies have been used to characterize the depositional environments, their evolution and to infer sea level changes. According to the sequence stratigraphy concept, an allostratigraphic unit is defined by a sedimentary sequence characterized by a succession of deposits attributed to intertidal, supratidal and aeolian/continental environments, bounded at its base and top by unconformities. This sequence is essentially deposited during phases of marine transgression and sea-level high stands. According to the international stratigraphic guide, the allostratigraphic units were formalized as members of the OHF. Microfacies analysis was carried out on large thin sections prepared from blocks of oriented sediments vacuum-impregnated with polyester resin. These analyses provided specific information about diagenetic processes occurring during and after deposition.
Sedimentology of ThI-GH infilling
Stratigraphic units SU5 to SU3 were studied using a geoarchaeological approach, integrating field observations (sedimentary structure, colour, discontinuities and so on), micromorphology and analyses (particle size distribution, magnetic susceptibility measurements and energy-dispersive X-ray fluorescence (ED-XRF) analyses). Micromorphology was based on the observation of large thin sections taken continuously in stratigraphic order. Particle size analyses were performed on bulk samples after decarbonatation. Volume magnetic susceptibility was measured along the section using a Bartington MS2K sensor, with a vertical resolution of 2 cm. Air-dried and crushed bulk samples (<2 mm) were analysed by ED-XRF using a calibrated portable spectrometer (SPECTRO X-SORT) (Supplementary Note 1).
Magnetostratigraphy and rock magnetic properties
Magnetostratigraphic data were obtained from a population of 119 oriented core-samples retrieved from members OH3, OH4 and GHCCC in 2022 and 2023 and integrated with 62 samples previously analysed in 2018–201913 from the same members plus member OH5. The sampling of ThI-GH infilling (from SU6 to SU3) was conducted along 5 sections, A and B (68 samples), C (14 samples), D (6 samples) and composite section E (19 samples) (Fig. 1d and Extended Data Fig. 2b), yielding a total of 107 oriented core-samples taken to better anchor the hominin bearing site ThI-GH to the MBT. Furthermore, 13 samples were retrieved from member OH3 to refine the record of the Jaramillo subchron previously observed13.
Magnetostratigraphic samples were thermally demagnetized from room temperature up to a maximum of 690 °C with a TD48 ASC furnace. Alternating-field (AF) demagnetization up to 200 mT performed on two test samples with an LDA5 AF demagnetizer resulted inadequate to resolve the magnetic remanence of the samples. After each thermal demagnetization step, the initial magnetic susceptibility was measured using a Bartington susceptibility bridge. The natural remanent magnetization was measured on a 2G DC-SQUID cryogenic magnetometer located in a magnetically shielded room. Standard least-squares analysis was used to calculate ChRM component directions from vector end-point demagnetization diagrams, from which VGP latitudes were derived (positive VGP values for normal polarity, negative values for reverse polarity). Great circles were used to assess qualitatively the ChRM orientation in absence of stable end points. The magnetic mineralogy was investigated using hysteresis experiments from −1.5 T to +1.5 T, low-resolution first-order reversal curves (FORCs, n = 76) interpreted with FORCinel53, stepwise acquisition of an isothermal remanent magnetization (IRM) up to 1.5 T, AF decay of a 1 T IRM in AF peak fields from 50 mT to 1.5 T and thermomagnetic decay of a 1 T magnetization performed in Ar atmosphere from room temperature to 680 °C. These experiments were performed using a MicroSense EZ7 Vibrating Sample Magnetometer with heating ability. Additional samples were also subjected to thermal demagnetization of a three-component IRM using orthogonal fields of 1.5 T, 0.4 T and 0.12 T imparted with an ASC pulse magnetizer. Details are provided in the main text and Supplementary Note 5.
Geometric morphometric analysis of ThI-GH-1 and ThI-GH-10717 mandibles
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