Thermoregulation in Homo erectus and the Neanderthals: A reassessment using a segmented model

Abstract

Thermoregulation is widely believed to have influenced body size and shape in the two best-known extinct members of genus Homo, Homo erectus and Homo neanderthalensis, and to have done so in contrasting ways. H. erectus is thought to have been warm adapted, while H. neanderthalensis is widely held to have been cold adapted. However, the methods that have been used to arrive at these conclusions ignore differences among body segments in a number of thermoregulation-related variables. We carried out a study designed to determine whether the current consensus regarding the thermoregulatory implications of the size and shape of the bodies of H. erectus and H. neanderthalensis is supported when body segment differences in surface area, skin temperature, and rate of movement are taken into account. The study involved estimating heat loss for a number of Holocene modern human skeletal samples and several fossil hominin specimens, including five Pleistocene H. sapiens, the well-known H. erectus partial skeleton KNM-WT 15000, a H. erectus specimen from Dmanisi, Georgia, and three Neanderthals. The resulting heat loss estimates were then used in two sets of comparative analyses. In the first, we focused on whole-body heat loss and tested predictions concerning heat loss in KNM-WT 15000 and European Neanderthals relative to modern humans, and within H. erectus and H. neanderthalensis. In the second set of analyses we again tested predictions concerning heat loss in H. erectus and H. neanderthalensis relative to modern humans, and within H. erectus and H. neanderthalensis, but this time we focused on the contribution of their limbs to heat loss. The results of the study do not fully support the current consensus regarding the thermoregulatory adaptations of Homo erectus and Homo neanderthalensis. The whole-body heat loss estimates were consistent with the idea that KNM-WT 15000 was warm adapted and that European Neanderthals were cold adapted, and with the notion that there are thermoregulation-related differences in body size and shape within H erectus and H. neanderthalensis. The whole-limb estimates told a similar story. In contrast, the results of our analysis of limb segment-specific heat loss were not consistent with the current consensus regarding the thermoregulatory significance of distal limb length in H. erectus and H. neanderthalensis. Contrary to expectation, differences between the proximal and distal limb segments did not follow any particular trend. The obvious implication of these results is that, while we can be more confident about the basic idea that thermoregulation influenced the evolution of body size and shape in H. erectus and H. neanderthalensis, we need to be more cautious in attributing differences in limb segment size to thermoregulation. Based on our results, it is possible that other factors influenced limb segment size in these species more than thermoregulation. Identifying these factors will require further research.