Evolutionary anatomy of the metacarpal bones from El Sidrón Neandertal sample (Asturias, Spain)

The evolutionary transformation of the hominin hand is of central interest due to its critical role in mediating interaction with both the physical and cultural environments. Beyond its functional significance, however, hand morphology can also reflect broader anatomical trends at the organism level. This study analyzes 31 metacarpal remains from the Neandertal site of El Sidrón (Asturias, Spain), using a combination of discrete morphological traits, linear measurements, and three-dimensional geometric morphometrics based on a 32-landmark template. The dataset is examined within a broad comparative and evolutionary framework. Our results confirm that Neandertal metacarpals differ markedly from those of modern Homo sapiens. They display increased robustness, more strongly developed muscle attachment sites, and enlarged heads, resulting in the characteristic 'lollipop' morphology. Significant differences are also observed in the carpometacarpal joint complex, including a flattened proximal articular facet on the first metacarpal, a reduced styloid process on the third metacarpal, and the absence of torsion in the fifth metacarpal, which also affects its articulation with the fourth metacarpal. Although these traits have often been interpreted as localized functional adaptations related to grip strength, joint loading, and manipulative performance, our results support a complementary anatomical interpretation. Comparable patterns of skeletal hypertrophy and joint reorganization (e.g., articular surface flattening and surface rotation) have been described in other Neandertal anatomical regions (including the clavicle, elbow, sacrolumbar spine, wrist, and femur), suggesting that metacarpal morphology may form part of a broader systemic pattern of skeletal organization. We therefore interpret the Neandertal metacarpals as compatible with an organism-level perspective, while emphasizing that behavioral and functional inferences require additional lines of evidence.