In conclusion, humans have a surprisingly impressive ability to run long distances at relatively high speeds and in extremely hot conditions compared to other specialized cursors. In many respects, these capabilities can also exceed those of the few other mammals - all social carnivores -known to engage in ER. In addition, human ER capabilities are all the more special because other primates generally eschew running other than occasional sprinting, and they lack endurance capabilities.
If humans are so good at ER, then why have these capabilities received so little attention in the history of research on human evolution? There have been countless articles and numerous books on the evolution of bipedalism in hominins, yet, with the exceptions of Carrier (1984) and Bramble and Lieberman (2004), none have considered running in any depth (see also Bortz, 1985; Heinrich, 2002). There are three major reasons for this lack of attention. First, what is out of sight is often out of mind: humans no longer need to run very much, and do so now primarily for pleasure or health. Second, we consistently underrate our abilities as athletes, primarily because we tend to focus on aspects of athleticism related to speed and power in which humans are pathetic compared to most mammals. The idea that brains have triumphed over brawn is so deeply engrained that it rarely receives much consideration.
Finally, students of the fossil record of human evolution have, understandably, focused on the origins of walking. There is substantial evidence that the earliest hominins were bipedal (Haile-Selassie, 2001; Galik et al., 2004; Zollikofer et al., 2005; Richmond and Jungers, 2008), and that walking was a key part of the transition that set early hominins off on a strikingly different evolutionary trajectory than chimpanzees (Darwin, 1871). There has been much debate over the extent to which early hominins were arboreal and whether these capabilities compromised their ability to walk optimally (Lovejoy, 1988; Stern, 2000; Ward, 2002), but few doubt that australopithecines were capable, habitual bipeds. However, it is important to note that the biomechanics of running and walking are substantially different, especially for a biped. In addition, the physiological demands of ER are quite different from those of sprinting or walking. Thus, evidence for walking capabilities in early hominins is not necessarily evidence for ER capability. Instead, a diverse array of comparative functional morphological and physiological evidence (Carrier, 1984; Bramble and Lieberman, 2004) suggests that human ER capabilities are not a byproduct of selection on bipedal walking alone. It follows that human ER capabilities demand some explanations for when and why they evolved.
Unfortunately, we cannot pinpoint precisely when ER first evolved. As outlined above and by Bramble and Lieberman (2004), the majority of the fossil evidence points to H. erectus as the first endurance runner. But, as also noted, we cannot rule out the possibility that H. habilis had some ER capabilities, nor can we rule out the hypothesis that later hominins had better performance capabilities than early H. erectus. More definitive answers require more evidence and more research. That said, the available evidence suggests that Australopithecus lacked many, if not most, of the derived features of Homo that improve ER performance. Some of these derived features, such as relatively large anterior and posterior semicircular canals, and the nuchal ligament, are specific to running. Other derived features, such as long legs, would have benefited both running and walking. Thus it is reasonable to speculate that selection for ER occurred in the context of selection for both walking and running long distances.
If there has been any skepticism about the ER hypothesis, then it has been with regard to why ER evolved (see, for example, comments in Carrier, 1984; also Pickering and Bunn, 2007). Because modern humans, including recent hunter-gatherers, no longer require ER in their daily lives, it is hard for many scholars to imagine how ER would have been selected for in the distant past. However, such reliance on the ethnographic present - what Wobst (1978) has referred to as the "tyranny of ethnography" - is problematic since recent inventions (such as the bow and arrow and the domestication of the dog) have substantially changed human hunting strategies in precisely those aspects that relate to ER. Critically, these innovations allow humans to hunt and kill animals from a distance without getting close to large prey. But for most of the history of the genus Homo, it appears that hominins have been able to kill large, prime age, adult prey that would have posed serious risks to any hunter armed solely with an untipped spear. ER, however, would have changed that equation by allowing hunters in the hot, arid and open habitats that have existed in Africa since at least 1.9 Ma, to run their prey into exhaustion, thereby disadvan-taging them sufficiently to be slain with minimal risk and a high probability of success. While ER-based persistence hunting would have required the cognitive skills to track an animal combined with abundant access to water, the energetic costs are surprisingly low in comparison to walking, and well worthwhile in terms of the payoff. Like other methods of hunting, ER and PH would also have required social groups with food-sharing.
Although the extent to which scavenging was an important behavior among early hominins is still debated, it is likely that scavenging played some component of early Homo subsistence strategies, just as it now does among the Hadza and Bushmen. Since carcasses are an evanescent resource in which early access improves the chance of getting something to eat and minimizing competition with other carnivores, then it would have benefited from ER capabilities.
In short, there is a compelling case to be made that ER would have given early Homo the ability to create a new niche within the carnivore guild: that of a diurnal predator within the increasingly open habitats in Africa by 1.9 Ma. In particular, ER would have provided ESA hunters with various means of getting meat at comparatively low risk and low cost. Observations that ER is rare among modern hunter-gatherers who possess weapons (such as the bow and arrow and atlatl) are not disproof of the hypothesis. Instead, the persistent, albeit rare, use of ER in scavenging and persistence hunting by modern hunter-gatherers such as the Bushmen, the Tarahumara and others are testaments to the importance of running in hunting in general, and the effectiveness of persistence hunting in particular, despite the invention of technologies that have made these athletic feats obsolete.
Finally, it is fun to conclude by speculating on a possible scenario for the evolution of ER in the genus Homo. Natural selection works by tinkering (Jacob, 1977). That is, selection can work only by taking advantage of small-scale heritable variations that somehow improve performance within a particular fitness context. One can well imagine circumstances in which the earliest members of the genus Homo or perhaps australopithecines began to scavenge or possibly hunt a little. In such a context, individuals with variations such as larger anterior and posterior semicircular canals, longer legs, narrower waists, more sweat glands, and so on might have enjoyed some fitness benefit because their improved performance in long distance running and/or walking that helped them acquire more meat. Over time - depending on factors such as the strength of selection, how much variation was available, and population size - modern ER capabilities, along with a modern-shaped body evolved, probably first in H. erectus. These capabilities apparently enabled H. erectus to kill medium- to large-sized animals in the hot, open habitats of Africa in the Early Pleistocene without any weaponry more sophisticated than a sharpened wooden stick. After the ESA, more sophisticated projectile technologies evolved (e.g., stone- and bone-tipped spears, bows and arrows, spear throwers and nets) that gave hunters other, less grueling options to bring home the bacon. As a result, persistence hunting has become less important. In addition, many homi-nins started to move out of Africa into temperate zones where PH was no longer possible. But the traces of our ancestry persist in a body well-suited to ER, a behavior that nowadays serves primarily as a means of relaxation and a way to stay healthy.
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