Does Climate or Mobility Explain the Differences in Body Proportions Between Neandertals and Their Upper Paleolithic Successors?

European Neandertals and their Upper Paleolithic modern human successors differ substantially in various proportions of their bodies. As compared to Neandertals, Upper Paleolithic Europeans tend to have longer limbs, both absolutely and relative to estimated skeletal trunk height; narrower bi-iliac breadths, both absolutely and relative to femur length; and higher brachial and crural indices.1–7 Although these differences hold generally, body proportions did change through time during the Upper Paleolithic and subsequent Mesolithic, with more recent groups approximating Neandertals more closely. In comparison with Early Upper Paleolithic humans ( 20,000 years ago), those of the Late Upper Paleolithic (20,000 to 10,000 years ago) and Mesolithic (10,000 to 5,000 years ago) have shorter limbs, both absolutely and relative to estimated skeletal trunk height.5,6 However, brachial and crural indices, which do not always reflect overall limb length, do not change much through time. Early Upper Paleolithic, Late Upper Paleolithic, and Mesolithic humans all have high brachial and crural indices; it is not until recent Europeans that lower indices appear.5,6,8 During the Upper Paleolithic and into the Mesolithic there is a shift from relatively narrow bodies with long limbs to relatively wide bodies with short limbs, but not all proportions change at once. In extant humans, as in other endothermic species, contrasts in body proportions similar to those between Neandertals and Upper Paleolithic humans appear to reflect, at least in part, population-level genetic differences produced over thousands of years by interregionally differing selection pressures due to variation in local climate.1,5,7,9–17 For effective thermoregulation, in warm climates it is advantageous to have a narrow body with long limbs to dissipate heat, and in cold climates it is better to have a wide body with short limbs to retain heat. Thus, the body proportions of Early Upper Paleolithic Europeans appear to be a genetic signature of recent warm-climate ancestry and, conversely, lack of Neandertal ancestry, which is consistent with a predominantly African origin for all modern humans.1,5,6,18 Under the “climate hypothesis,” changes in body proportions during the Upper Paleolithic and into the Mesolithic would be explained as the gradual and mosaic adaptation over time to colder climates, possibly slowed by the increased cultural buffering of selection provided by Upper Paleolithic clothing and shelters.1,2,5,6,18 However, the climate hypothesis has been questioned.8,19,20 The primary alternative is that differences in body proportions reflect adaptation to differences in mobility (see discussions in Trinkaus,1 Finlayson,19 Wolpoff,20 and Holliday and Falsetti21). One variant of the “mobility hypothesis” posits that Neandertals could have played a substantial role in the ancestry of Upper Paleolithic Europeans if differences in body proportions originated in situ through selection for increased energetic efficiency during mobile foraging with the start of the Upper Paleolithic. Under this scenario, Neandertal body proportions could also still be adaptations to cold climates but, during the Upper Paleolithic, climatic selection was relaxed by increased cultural buffering and superceded by stronger selection for mobility. Under another variant, if Neandertals and modern humans coexisted for some time in Europe, then differences in body proportions between Neandertals and Upper Paleolithic modern humans could explain whymodern humans were able to outcompete Neandertals as environmental conditions changed to those that favored mobile foraging.19 Alternatively or additionally, Neandertal body proportions, along with other features of their skeletons, could have been shaped by selection for competence in foraging activities requiring substantial mechanical power18,22 or locomotion over hilly terrain.20 Neandertal body proportions could be the result of poor nutrition and health during the growth period,20,23 but while certain anthropometric dimensions are readily affected by changes in nutrition and health, body proportions appear to be fairly stable (see discussion and references in Ruff7). The “mobility hypothesis” has less empirical support than does the climate hypothesis, because no relationship can be found between body proportions and various measures of mobility in extant hunter-gatherer groups, even when controlling for the