The proximal extremity of the humerus: morphology and adaptation. Application to neandertal remains

The post-cranial skeleton is not similar in neandertal and modern human, especially for the proximal extremity of the humerus. However, interpretation of these differences is still subject of discussion. This work compares proximal extremity of the humerus in hominoids (including neandertal remains) and spider monkeys. Each variable is not studied separately, but instead globally by PCA (Principal Component Analysis). The PCA revealed six morphological groups for the proximal extremity of the humerus: 1) African great apes, 2) gibbon, 3) orang-utan, 4) spider monkey, 5) modern human and 6) neandertal. The human plot includes a large part of the spider monkeys, gibbon and orang-utan plots but not the one of African great apes and not the one of neandertal. These differences show that within hominoids the morphology of the proximal extremity of the humerus expresses differences in locomotor behaviour. The great variability observed inside modern human is associated to the loss of the locomotor function of the upper limbs. The peculiar morphology of the proximal extremity of the neandertal humerus can be explained by two hypotheses: 1) it reflects functional differences between this human group and modern human, 2) it represents a genetic drift in an isolated population without any functional aspect. It may be more accurate to consider that these differences reveal, at the same time, a genetic drift and a peculiar functional adaptation. Whatever the answer is, the proximal extremity of the humerus is characteristic enough for distinguishing modern human remains from those of neandertals.