How much musculoskeletal variability did Neandertals accumulate? The study of the lower limbs entheses of the Neandertal sample from El Sidrón, Asturias, Spain

Rarely have entheses been systematically utilized in the field of human evolution, but investigation of their morphological variability (robusticity) could provide new insights on their evolutionary meaning in the European Neandertal populations. The aim of this work is to study the entheses and joint features of the lower limbs of El Sidrón Neandertals (Spain; 49kyrs), utilizing scoring standardized methods developed on modern samples. Paleobiology, growth and development of both El Sidrón juveniles and adults are explored and compared with those of Krapina Neandertals (Croatia, 130 Kyrs) and European modern humans. The morphological patterns of the gluteus maximus and vastus intermedius entheses of El Sidrón, Krapina and modern humans are different from one another. Both Neandertal groups show a definite enthesis design for the gluteus maximus, with little intrapopulation variability with respect to modern humans, that display a wider range of morphological variability. The gluteus maximus enthesis of El Sidrón shows the osseous features of fibrous entheses, as in modern humans, while the Krapina sample shows the aspects of fibrocartilaginous ones. The morphology and anatomical pattern of this enthesis is already established during growth in all of the three human groups. One out of two and three out of five adult femurs of El Sidrón and Krapina, respectively, show the imprint of m. vastus intermedius, absent in their juvenile specimens and in the modern samples. The little intra-population and evident inter-population variability of the two Neandertal samples is likely due to long-term history of small, isolate populations with high levels of inbreeding also living in different ecological conditions. The comparison of different anatomical entheseal pattern (fibruous vs. fibrocartilagineous) of the Neandertals and modern humans opens new functional perspectives.