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  • Human Locomotion and Heat Loss:An Evolutionary PerspectiveDaniel E. Lieberman*1

    ABSTRACTHumans are unique in many respects including being furless, striding bipeds that excel at walkingand running long distances in hot conditions. This review summarizes what we do and do notknow about the evolution of these characteristics, and how they are related. Although manydetails remain poorly known, the first hominins (species more closely related to humans than tochimpanzees) apparently diverged from the chimpanzee lineage because of selection for bipedalwalking, probably because it improved their ability to forage efficiently. However, because bipedalhominins are necessarily slow runners, early hominins in open habitats likely benefited fromimproved abilities to dump heat in order to forage safely during times of peak heat when predatorswere unable to hunt them. Endurance running capabilities evolved later, probably as adaptationsfor scavenging and then hunting. If so, then there would have been strong selection for heat-loss mechanisms, especially sweating, to persistence hunt, in which hunters combine endurancerunning and tracking to drive their prey into hyperthermia. As modern humans dispersed intoa wide range of habitats over the last few hundred thousand years, recent selection has helpedpopulations cope better with a broader range of locomotor and thermoregulatory challenges, butall humans remain essentially adapted for long distance locomotion rather than speed, and todump rather than retain heat. C 2015 American Physiological Society. Compr Physiol 5:99-117,2015.

    IntroductionHumans are distinctive compared to other mammals innumerous respects including being habitually bipedal, andthe ability to walk and run long distances at relatively fastspeeds in hot, arid conditions. The optimal walking speedfor an average-sized human is 1.2 m/s, about 20% faster andfour times more efficient compared to our closest relatives,chimpanzees, and about 20% faster and approximately asefficient as a ponys optimal walking speed (117, 133, 161).In addition, although maximum running speed in humansis unimpressive, about half that of most equivalent-sizedquadrupeds (52), humans are among the few mammalsandthe only species of primatethat can repeatedly run verylong distances at relatively fast speeds under aerobic capacity.Other cursorial (running adapted) quadrupeds can run longdistances at a trot but not a gallop, yet the preferred trottingspeed of a pony, approximately 3 m/s, is only half the speedat which a fit human can run a marathon with nearly equalefficiency (63, 133). Finally, most mammals are able to walkor run long distances only in relatively cool conditions, buthumans are the sole species of mammal that excels at longdistance trekking and running in extremely hot conditions. Nohorse or dog could possibly run a marathon in 30C heat (40).

    The purpose of this review is to summarize the evidencefor the origins of the special nature of human locomotionand heat loss mechanisms, and to make the argumentfirstelucidated by Carrier (24)that these systems share a linkedevolutionary history. Although some details remain murky,

    multiple lines of evidence suggest that there was strongselection on early hominins (species more closely related tohumans than to chimpanzees) to stand and walk efficiently,and that the origins of bipedalism was later followed byadditional selection for long distance walking and then forendurance running. In turn, it is reasonable to hypothesizethat selection for long distance walking and running createda selective advantage for hominins to dump heat effectivelyin hot, arid conditions. After modern humans evolved anddispersed all over the globe, further selection occurred tohelp different populations adapt to a wide range of climaticconditions, but all human populations are variants of a basicadaptive pattern for long-term aerobic exertion in hot habitats.

    I first summarize the evidence for the evolution of humanlocomotion and then heat loss, in both cases drawing on thefossil record as well as comparisons between humans and theAfrican great apes. I then evaluate alternative hypotheses forhow these two distinctive systems evolved, and the extent towhich they are linked. I conclude with a discussion of thecontemporary relevance of the evolutionary bases of theseadaptations.

    *Correspondence to danlieb@fas.harvard.edu1Department of Human Evolutionary Biology, Harvard University,11 Divinity Avenue, Cambridge MA 02138, USAPublished online, January 2015 (comprehensivephysiology.com)DOI: 10.1002/cphy.c140011Copyright C American Physiological Society.

    Volume 5, January 2015 99

  • Evolution of Human Locomotion and Heat Loss Comprehensive Physiology

    BipedalismFossil evidence discovered over the last few decades indicatesthat selection for bipedalism may have been the initial sparkthat set the human lineage on a different evolutionary pathfrom the African great apes. In fact, the presence of adapta-tions for bipedalism is a major reason for classifying manyof these fossils as hominins (a problem of circular logic ifbipedalism evolved more than once). Bipedalism, however,is an unusual form of locomotion that has a long, complexevolutionary history involving several stages, each of whichwas contingent upon previous stages, and likely driven byselective pressures caused by changing climatic conditions.Therefore, before reviewing the evolutionary transformationsthat led to modern human walking and running, it is useful tobegin with a consideration of the phylogenetic and ecologicalcontexts in which hominin bipedalism first evolved.

    Evolutionary contextAs Figure 1 illustrates, molecular data unambiguouslyindicate that humans and chimpanzees share a last commonancestor (LCA) that diverged approximately 5 to 8 millionyears ago (Ma), and that gorillas diverged from the human-chimpanzee clade approximately 8 to 12 Ma. Molecularevidence that humans and chimpanzees are sister taxa wasinitially a surprise to paleontologists because chimpanzeesand gorillas share many similarities in their cranial andpostcranial anatomy, with many differences attributable tothe effects of size (9, 54, 55, 151). Among their many sim-ilarities, chimpanzees and gorillas habitually knuckle walk, adistinctive form of locomotion that involves resting the fore-limbs on the dorsal surface of the middle phalanges of a flexedhand. Knuckle walking is commonly interpreted as a way to

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    H. sapiens P. troglodytes P. paniscus G. gorilla

    LCA of humans,chimps and gorillas

    LCA of humans and chimps

    Figure 1 Evolutionary tree showing the relationships amonghumans, chimpanzees, and gorillas, as well as the Last CommonAncestors (LCA) of humans and chimps, and of humans, chimps, andgorillas. The dates of the divergences are only approximate.

    locomote quadrupedally while maintaining adaptations forarboreal climbing such as short hind limbs, long upper limbs,and long, curved manual phalanges (for review, see Ref. 46).Given the unique nature of knuckle walking in combinationwith additional similarities between chimpanzees and goril-las, it was assumed before molecular data indicated otherwisethat these species were closely related cousins who shareda common knuckle-walking ancestor, and that the LCA ofhumans and the African great apes was unlikely to have been aknuckle walker, but instead a generalized quadruped or possi-bly even some sort of brachiator with an orthograde (upright)trunk (see Ref. 60).

    The fact that chimpanzees and humans are more closelyrelated to each other than to gorillas has necessitated a re-evaluation of hominin origins. From a phylogenetic perspec-tive, the most parsimonious scenario is that knuckle walkingand other similarities between the African great apes evolvedjust once in the LCA of chimpanzees, humans, and gorillas,and that the more recent LCA of just humans and chimpanzeeswas also a knuckle walker that resembled chimpanzees andgorillas in many respects (125). If not, then the many simi-larities between chimpanzees and gorillas must have evolvedindependently, which is highly unlikely.

    Reconstructing the LCA as a knuckle-walker has impor-tant implications for hypotheses about the origins of bipedal-ism, but this reconstruction is unsubstantiated and is the sub-ject of much debate for three reasons. First, although theLCA of humans and chimpanzees has never been discovered,there are many species of fossil great apes from the Miocene(23-5 Ma), and few of them closely resemble either chim-panzees or gorillas, especially in terms of their locomo-tor anatomy. Instead, some such as Proconsul are gener-alized quadrupeds, and others such as Morotopithecus areorthograde climbers (97, 171). Second, chimpanzees andgorillas knuckle walk in a slightly different manner, lead-ing some scholars to speculate that this mode of locomotionevolved independently in the two species (77). Third, althoughthere is almost no fossil record of chimpanzee and gorilla evo-lution, paleontologists have discovered a number of putativeearly hominins, and some scholars have argued that theseearly members of the human lineage do not resemble chim-panzees or gorillas (4, 181). Although this view cannot bediscounted entirely, the balance of evidence suggests that theLCA was a knuckle-walker. Most importantly, interpretationsof the earliest hominins as unlike chimpanzees or gorillasare problematic because these fossils actually resemble chim-panzees and gorillas in most aspects for which they are unlikelater hominins (91,186). In addition, while Miocene apes arediverse, most of these species are unlikely to be closely relatedto the LCA of humans and chimps, which means that theirlocomotor adaptations do not refute recons