Avoidance of overheating and selection for both hair loss and bipedality in hominins

Abstract

Two frequently debated aspects of hominin evolution are the development of upright bipedal stance and reduction in body hair. It has long been argued, on the basis of heat-balance models, that thermoregulation might have been important in the evolution of both of these traits. Previous models were based on a stationary individual standing in direct sunlight; here we extend this approach to consider a walking hominin, having argued that walking is more thermally challenging than remaining still. Further, stationary activities may be more compatible with shade seeking than activities (such as foraging) involving travel across the landscape. Our model predictions suggest that upright stance probably evolved for nonthermoregulatory reasons. However, the thermoregulatory explanation for hair loss was supported. Specifically, we postulate progressive hair loss being selected and this allowing individuals to be active in hot, open environments initially around dusk and dawn without overheating. Then, as our ancestors' hair loss increased and sweating ability improved over evolutionary time, the fraction of the day when they could remain active in such environments extended. Our model suggests that only when hair loss and sweating ability reach near-modern human levels could hominins have been active in the heat of the day in hot, open environments.T wo frequently debated aspects of hominin evolution are the development of upright stance and the reduction in body hair. Indeed, a good case can be made that upright stance was "the first novelty of our lineage" (1), and lack of hair is one of the visually most obvious aspects of modern humans compared with other similar-sized mammals (2). One line of reasoning posits that thermoregulation may have been important in the evolutionary development of both of these traits. Highly supportive of such discussions has been a series of papers by Wheeler that used simple heat-balance models to quantify the potential effects of such adaptations (3-10). Recently, we presented a revised version of Wheeler's model to explore thermoregulatory aspects of putative endurance running in extinct hominids (11). The key modification in our revised model was a change from a stationary individual, as considered by Wheeler, to a moving one. Movement both adds to the thermal challenge on an individual by increasing their metabolically generated heat production and allows greater heat loss to the air by forced convection. Here we apply a further development of this model to the issues in which Wheeler was originally interested: bipedalism and hair loss.We believe that consideration of a walking individual should be particularly informative, because it is easier to envisage ecological reasons why an individual would be required to walk through areas that offer no shade compared with situations where an individual would stand still for long periods in direct sunlight. Vision is likely to have been an important sense in both food finding (e.g., searching for and pursuing mobile animal prey) and predator...

Aridity and hominin environments

“…Aridity influences the distribution and abundance of vegetation in African environments (3), and changes in aridity over both long and short time scales have been suggested to drive changes in hominin environments leading to adaptation, dispersal, speciation, and extinction (2,4,5). The notion that aridity may have driven certain adaptations has been fundamental to discussions of hominin evolution since 1925 (6), and continues to feature prominently in studies addressing changes in hominin locomotion, body proportions, thermoregulation, food acquisition, tool use, and social organization (7)(8)(9)(10).…”

Book review

“…If this were true indeed, one wonders why adaptive stories that rely on an open plain assumption, themselves often no more than just-so stories (such as: achieving big brains by digging up tubers, standing tall to overlook the savannah grass or for postural feeding or throwing -see Table 1, Chapter 3 of the eBook), continue to be published in high impact journals, whereas our experience during the last decades shows that almost every comment referring to an aquatic past fails to get published. While we agree with the general critique of Langdon that adaptive explanations in palaeo-anthropology are intrinsically based on assumptions, we keep wondering why is it that generally only manuscripts based on terrestrial origin assumptions (e.g., Bramble and Lieberman, 2004;Hunt, 1996;Ruxton and Wilkinson, 2011;Young, 2003) are accepted for publication. In other words, the Us versus Them mentality is a reality, but it is not AH proponents that are to be blamed for this situation.…”

A prostaglandin d‐synthase‐positive mast cell gradient characterizes scalp patterning

“…Evaporative heat loss is dramatically favored by a smooth skin surface unencumbered by hair or feathers. Indeed, current hominid evolutionary theory holds that genetic mutations favoring progressive hair loss have been selected for in bipedal hunters and gatherers, thus allowing for effective thermoregulation in hot and open environments …”

“…Indeed, current hominid evolutionary theory holds that genetic mutations favoring progressive hair loss have been selected for in bipedal hunters and gatherers, thus allowing for effective thermoregulation in hot and open environments. 15 In this respect, although it is highly conjectural and data is limited, it is intriguing to note that mast cell lines are thermosensitive with regard to experimentally-induced mediator release, 16 and sweating has long been associated with mast cell degranulation in the form of cholinergic urticaria. Thus, for a bipedal hominim active under the heat of direct sunlight, the highly vascularized vertex skin of the scalp would serve as an effective evaporative cooler in the setting of pattern hair loss, and thus this trait could be regarded as having a potential evolutionary advantage that might influence the developmental anatomy of scalp skin.…”