Postcranial morphology of the middle Pleistocene humans from Sima de los Huesos, Spain

Abstract

Current knowledge of the evolution of the postcranial skeleton in the genus Homo is hampered by a geographically and chronologically scattered fossil record. Here we present a complete characterization of the postcranium of the middle Pleistocene paleodeme from the Sima de los Huesos (SH) and its paleobiological implications. The SH hominins show the following: (i) wide bodies, a plesiomorphic character in the genus Homo inherited from their early hominin ancestors; (ii) statures that can be found in modern human middle-latitude populations that first appeared 1.6-1.5 Mya; and (iii) large femoral heads in some individuals, a trait that first appeared during the middle Pleistocene in Africa and Europe. The intrapopulational size variation in SH shows that the level of dimorphism was similar to modern humans (MH), but the SH hominins were less encephalized than Neandertals. SH shares many postcranial anatomical features with Neandertals. Although most of these features appear to be either plesiomorphic retentions or are of uncertain phylogenetic polarity, a few represent Neandertal apomorphies. Nevertheless, the full suite of Neandertal-derived features is not yet present in the SH population. The postcranial evidence is consistent with the hypothesis based on the cranial morphology that the SH hominins are a sister group to the later Neandertals. Comparison of the SH postcranial skeleton to other hominins suggests that the evolution of the postcranium occurred in a mosaic mode, both at a general and at a detailed level.

Keywords: Sierra de Atapuerca; bauplan; human evolution; phylogeny; postcranial anatomy.

Fig. 1.

SH-selected measurements compared with other hominin groups. (A) Bi-iliac breadth. (B) Femoral total length. (C) Femoral head diameter. (D) Femoral neck index (biomechanical length of the neck following ref. /femoral maximum length × 100). (E) Percentage of cortical area in the right and left humeri and femora. (F) Palmar projection of the trapezium tubercle. EP1: 2.0–1.8 Mya early Pleistocene Homo; EP2: 1.7–0.8 Mya early Pleistocene Homo; MP: non-SH middle Pleistocene Homo; Ne: Neandertals; MH: modern humans. See SI Appendix for raw data. Boxes: SD; whiskers: range.

Fig. 2.

SH-selected postcranial traits. (A) Third lumbar vertebra (L3). Cranial view of VL2, presenting a very long and dorso-laterally oriented transverse process (arrowhead). (B) Humerus. Subadult (H-IV, Left) and adult (H-VI, Right) specimens showing the thin medial pillar and broad and deep olecranon fossa. (C) First metacarpal (MC1). Palmar view of juvenile (AT-3104, Left) and adult (AT-5565, Right) specimens, both showing a strong attachment for the opponens pollicis muscle (arrowheads). (D) Os coxae. Ventral view of AT-1000, displaying a strongly twisted anterior inferior iliac spine (white arrow) and a deep iliopsoas groove (black arrow). (E) Femur. F-X (Left) and F-XI (Right) proximal femora in posterior view, showing a low neck angle, large gluteal ridges, and well-developed hypotrochanteric fossae. Midshaft section (Middle, CT-scan image) is rounded and shows an absence of a pilaster. (F) Talus. Dorsal view of AT-2803 that shows an expanded lateral malleolar facet (arrowhead) and parallel edges of the trochlea.