Taxonomic assignments for the 3.4 Ma to 1.1 Ma hominin postcanine teeth from the Usno and Shungura Formations, Lower Omo Valley, Ethiopia

  1. Leslea J. Hlusko
  2. Franck Guy
  3. Mario Modesto-Mata
  4. Marina Martínez de Pinillos
  5. Marianne F. Brasil
  6. Ian Towle
  7. Arthur Thiebaut
  8. Jean-Renaud Boisserie

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Abstract

Sediments of the Usno Formation and the Shungura Formation date from ca. 3.75 Ma to 1.09 Ma, during which time the genus Homo appeared, Australopithecus disappeared, and the eastern African robust hominins did both. We performed an in-depth analysis of size, shape, and morphology of 108 Omo permanent postcanine dental specimens. First, we compared linear dimensions against a comparative dataset of 809 teeth from 359 specimens representing 10 hominin taxa. We then developed a linear discriminant function to predict the most-likely taxon for the Omo specimens based on the crown dimensions. We also ran cluster analyses on morphological scores and 2-dimensional cusp areas from the Omo sample to test whether these statistical clusters align with the taxa predicted by the discriminant function analysis. We found that variation in morphological scores does not align well with the taxonomic groups predicted from linear data. Cluster analyses of mandibular molar cusp areas categorize robust and non-robust groups fairly well, but maxillary molar cusp areas do not. We conclude that only Au. afarensis is found in the earliest part of the stratigraphy (3.4 Ma). By 2.95 Ma, there are two hominin lineages, Paranthropus and a more dentally gracile group in which Au. afarensis and Homo are not distinguishable until higher in the stratigraphic sequence. Paranthropus increased in frequency from 2.27 Ma until 1.9 Ma. After this time, Homo is the dominant taxon. The geologically youngest evidence of Paranthropus occurs at 1.37 Ma, after which, only teeth attributable to Homo are observed.

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  1. Peer Community in Paleontology

    The evolutionary history of hominins is increasingly viewed as a branching rather than a linear process, with multiple species and genera coexisting in Africa during the Pliocene and Pleistocene. This pattern reflects a high degree of morphological diversity, which remains the subject of much debate, particularly for periods prior to 3 million years ago (e.g., White, 2003; Haile-Selassie et al., 2016, Hanegraef et al., 2024) and between 3 and 2.5 Ma, for which a rigorous taxonomic assessment of the fossils is essential. In this context, Leslea Hlusko et al. (2026) undertook a major study of specimens recovered from the Lower Omo Valley in south-western Ethiopia. These fossils originate from the Omo-Turkana Basin, which preserves approximately one-third of African hominin fossils dating from 7 Ma to 780 ka (Marchal et al., 2025), with the Lower Omo Valley assemblage representing a significant contribution to this collection. The material was recovered during two international research programs: the International Omo Research Expedition (IORE, 1967–1976; e.g., Coppens and Howell, 1976) and the more recent Omo Group Research Expedition (OGRE; e.g., Boisserie et al., 2008), launched in 2006. The specimens date from approximately 3.75 to 1.09 Ma, covering a pivotal period in hominin evolution characterised by the decline of the genus Australopithecus and the emergence of the genera Paranthropus and Homo.

    Particular emphasis should be placed on the considerable scope of the work carried out by Hlusko et al. (2026) and on the significant challenge involved in testing the taxonomic attribution of dental remains from the Lower Omo Valley.

    This major contribution significantly enriches the hominin fossil record and represents a significant advance in the study of hominin evolution in the Omo-Turkana Basin. In this study, Hlusko et al. (2026) analysed 116 post-canine teeth from the Shungura and Usno Formations and provide detailed specimen-by-specimen descriptions. Using a rigorous analytical framework, the authors attribute the oldest specimens (approximately 3.3 Ma) to "Australopithecus Late Pliocene" and identify two groups of hominins by 2.95 Ma: Paranthropus and a group with a more gracile dentition, within which "Australopithecus Late Pliocene" and Homo remain indistinguishable until the upper levels of the stratigraphic sequence. The relative abundance of Paranthropus increases between 2.27 Ma and 1.9 Ma, with its most recent occurrence dated to 1.37 Ma. After 1.9 Ma, Homo becomes the dominant taxon and remains the only genus represented after 1.37 Ma in the Lower Omo Valley.

    The taxonomic classification of certain specimens — particularly those attributed to Australopithecus garhi — is likely to be a subject of debate. Hlusko et al. (2026) therefore suggest that Au. garhi was not restricted to the Awash region. As the authors acknowledge, further fossil discoveries will be required to test this hypothesis more robustly. Conversely, recent research suggests that the range and geographical distribution of Paranthropus extended northwards to the Afar region, following the discovery of a mandible at Mille-Logya (Alemseged et al., 2026).

    As they point out in their conclusion, and echoing Robinson (1956), who had expressed scepticism regarding taxonomic inferences based solely on dental variations, Hlusko et al. (2026) urge caution when interpreting taxonomic identifications.

    Beyond its taxonomic implications, this study is also notable for its methodological contribution, which is made available within an open science framework, thereby enabling its replication and application by other researchers. Hlusko et al. (2026) developed an integrative methodological approach based on a large comparative sample of well-established eastern hominin taxa. This framework combines both qualitative and quantitative parameters — including morphological traits, linear measurements, linear discriminant function analyses, cluster analyses, and two-dimensional measurements of cusp surface area — to assign the most probable taxonomic identification to isolated post-canine teeth.

    This article makes a major contribution to the study of hominin evolutionary history, as it increases for example the number of known post-canine teeth dating from the period between 3.0 and 2.5 Ma by approximately 70%. This time interval is particularly crucial in eastern Africa, as it encompasses the last representatives of Australopithecus as well as the emergence of Paranthropus and Homo, and is therefore of paramount importance for understanding the evolutionary trajectories of these two lineages.

    More broadly, this study constitutes a major reference not only because of the significance of the newly described material and the detailed descriptions of the specimens provided, but also because of its analysis of taxonomic attribution and the proposed methodological framework. Its critical evaluation of analytical approaches further highlights the importance of combining multiple data sources in order to refine taxonomic classifications as rigorously as possible.

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