Neandertals disappeared from the fossil record around 40,000 bp, after a demographic history of small and isolated groups with high but variable levels of inbreeding, and episodes of interbreeding with other Paleolithic hominins. It is reasonable to expect that high levels of endogamy could be expressed in the skeleton of at least some Neandertal groups. Genetic studies indicate that the 13 individuals from the site of El Sidrón, Spain, dated around 49,000 bp, constituted a closely related kin group, making these Neandertals an appropriate case study for the observation of skeletal signs of inbreeding. We present the complete study of the 1674 identified skeletal specimens from El Sidrón. Altogether, 17 congenital anomalies were observed (narrowing of the internal nasal fossa, retained deciduous canine, clefts of the first cervical vertebra, unilateral hypoplasia of the second cervical vertebra, clefting of the twelfth thoracic vertebra, diminutive thoracic or lumbar rib, os centrale carpi and bipartite scaphoid, tripartite patella, left foot anomaly and cuboid-navicular coalition), with at least four individuals presenting congenital conditions (clefts of the first cervical vertebra). At 49,000 years ago, the Neandertals from El Sidrón, with genetic and skeletal evidence of inbreeding, could be representative of the beginning of the demographic collapse of this hominin phenotype.

This research was supported by a grant from the Ministerio de Economía y Competitividad of Spain (CGL2016-75-109-P and Convenio Principado de Asturias–Universidad de Oviedo CN-09–084). L.R. was founded by the Juan de la Cierva program (Spanish Government) and thanks Asier Gómez-Olivencia for providing Neandertal and modern human metric data for the second cervical vertebra and for the discussion of several aspects of the hominin cervical spine. A.E. is founded by the Juan de la Cierva program (Spanish Government). Y.Q. received support from Ministerio de Educación y Ciencia Project N° CGL2015-65387-C3-2-P, MINECO-FEDER and a Predoctoral Grant, University of Burgos. T.L.K is funded by the HP7 European Research Council Starting Grant #336301.