ATE9-1 was discovered during the 2007 field season while excavation square H-29 of the Level TE9c  in Sima del Elefante Cave site in the Sierra de Atapuerca region of Burgos Province, Spain. The discovery was associated with an assemblage of Mode 1 or Oldowan lithic tools, as well as faunal remains with evidence of hominin processing.  Palaeomagnetic analyses, biostratigraphical studies, and quantitative data obtained through nuclide cosmogenic methods place ATE9-1 in the Early Pleistocene at 1.2 to 1.3 Ma [1][2]. It was the earliest evidence of a hominin in Europe until the announcement in July 2022 of a new find, still in Sima del Elefante, with an estimated age of 1.4 Ma [4][5][6].  

ATE9-1 consists of a hominin mandible’s symphyseal region and an isolated permanent left lower second premolar (LP4) from the same individual. The mandible retains a portion of the right corpus from the P3 level to the M1 alveolus with a very irregular breaking surface, as well as the basal part of the left corpus from P3 to the level of M1/M2. It preserves the roots of the LI2, the right P3, the RP4, and the auspices of both the RI2 and the RC in situ. The LP4, a partial root of LP3, the root and the crown of RI2, and the root and a portion of the crown of the RC and LC [1][2][7]. 

The specimen had various post-depositional alterations such as fractures, but these damages do not prohibit taxonomic assessment [3]. It was previously provisionally assigned to Homo antecessor [1]. However, ATE9-1 is not complete enough to make a taxonomic assignment, therefore it is currently assigned to Homo sp. [2].

Palaeopathological analyses reveal several pathological signs, including severe dental attrition in association with generalized hypercementosis, alveolar root exposure, mild periodontal disease, tooth dislocation, and an abnormal occlusal plane. Dental calculus, two cystic lesions, and an anomalous wear facet compatible with toothpick use wear were observed. These suggest compensatory eruption as a result of heavy and/or traumatic occlusion, heavy chewing, and non-chewing use, or some combination of these processes [3].