AnthroNotes Volume 31 No. 1 Spring 2010NEW PERSPECTIVES ON THEEVOLUTION OF BIPEDALISMby A.lison S. BrooksE^ arly scholars of natural history recognized that bi-pedalism, tool use, and language were among the^nost important defining characteristics ofour spe-
cies. In 1780, Blumenbach actually classified humans in aseparate order, the "Bimana" or two-handed animals, im-plying only two limbs used for locomotion. Even as late asthe 1950s, many thought that large brains and bipedalismhad evolved together, an impression supported by the largebrain of the "Piltdown Man" forgery. But as recent fossilevidence makes clear, bipedalism actually developed longbefore there was any sign of enlarged brains, tool-making,or any kind of symbolic behavior that would suggest lan-guage abilities.Why did our lineage adopt such an unusual way ofgetting around, a way that is exceedingly rare among mam-mals? There are other bipeds, but kangaroos move byhopping on their hind legs, balanced by a long heavy tail,and many birds such as ostriches run on the ground withbent knees, also balanced by a tail. Humans are the onlystriding, tailless bipeds. Because we started out as taillessapes, our bodies had to change in many ways to balancethe body oy^er that one leg during walking and running.Our head is balanced atop the spine rather than out in frontof it, and our spine itself is S-curved, to balance the massof the upper body over the legs. This alone causes us agreat deal of griefby putting strain on the lower back andneck. Our pelvis changed from a long linear shape withflat blades (ilia) extending up the back to a basin shape thatcradled the abdominal organs but created problems lateron in evolution for birthing a large-brained baby. The newshape of the pelvis changed the position of the musclesattached to those blades, which were used to pull the legback but now extend to the side over the hip joint to keepyou from falling over when you pick up one leg and standon the other. Otherwise (or if those muscles aren't func-tioning well) you would have to walk the way chimpan-zees and some other four-footed animals do—leaning side-ways over the leg you stand on to keep your balance and
prevent falling towards the leg you are lifting. The hipjoints are bigger than in apes of comparable body weightsince they carry our full weight whenever we are movingor standing. To make it easier to balance on one leg, ourknees are positioned directly under the body, which makesthe thigh bone (femur) slant inwards from hip to knee.Finally our foot changed from a grasping appendage to apropulsive one, with a bigger, straighter and stronger bigtoe in line with the shorter lateral toes for pushing off, andwith both a longitudinal and transverse arch to stiffen thefoot for toe-off. This literally puts a spring in our stepswhen we run.Fossil Evidence for BipedalismEven the very fragmentary fossil remains from early inhuman evolution suggest that adaptation for bipedalismwas an early and essential step, so to speak, on the road tobecoming human. At 6-7 million years ago (mya), two earlyancestors represented by the fossils Sahelanthropus tchadensisfrom Chad and Orrorin tugenensis from Kenya appear tohave already changed their way of getting around. Thehole in the base of Sahelanthropus\ skull for its spinal cordhad become reoriented so it points downward rather thanbackward, and it is positioned a little further forward sug-gesting that the head was becoming more balanced on topof a vertical upper spine. The cross-section and width ofthe femur of Orrorin suggests that it was already bearingmore weight, although the hip joint would have beensmaller than in modern humans. In addition, the long nar-row neck of the femur, which joins it to the hip socket,indicates that the pelvis (which is missing) was already muchbroader than in apes. The curved toe and finger bones andthe muscle markings and shape of the upper arm bonesuggest that Orrorin also spent a lot of time climbing aroundin trees, to sleep, eat, or escape from predators.An almost complete skeleton of Krdipithecus ramidusfrom Ethiopia, dating to 4.4 mya and published in Octo-ber 2009, has an even more curious combination of traits.The upper blades of the pelvis (ilia) are shorter and broaderthan in apes, and the pelvis as a whole apparently has abasin shape, suggesting a degree of adaptation for uprightwalking. But the foot retains a fully opposable big toe forgrasping and climbing in trees, and it does not have eithera longitudinal or a transverse arch.
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From 4.4 to ca. 1.9 mya, members of the genusAustralopithecus, whose most complete skeletons so far areLucy and the recently reported skeletons ofAustralopithecus
sediba {Science 4/9/201 0), were more committed to habitualbipedal walking, with an S-curved spine, a very wide ba-sin-shaped pelvis, an inward-slanting femur, a large big toeclose to the other shorter toes, an expanded heel, and somedegree of arch in the foot. That Australopithecus walkedupright is also demonstrated by the footprint trail at Laetoli,(near Olduvai Gorge in northern Tanzania), where a largerand a smaller Australopithecuswalked together through wetmuddy ash 3.6 mya. Their stride may have been differentfrom ours, however, as their toes were still long and curvedfor climbing in the trees, and their legs were short com-pared to their arms.Only with Homo erectus at ca. 1.8 mya do we see a full-commitment to life on the ground and reduction or elimi-nation of many of the features that made it easier to getaround in the trees: longer arms, an upwardly orientedshoulder joint for hanging, and grasping toes with a lim-ited arch. The "Turkana bov" skeleton of an adolescentdated to 1 .5 mya has the long legs and basin-shaped pelvisof a fully bipedal human, while a slightly earlier foot skel-eton from Olduvai Gorge has short toes and a human-likearch. A recendy published set of footprint tracks in Kenyafrom 1.5 mya suggests real "toe-off" striding and a fully-
human arch. Variations in the pelvis after Homo erectus ap-parently have more to do with providing space for a larger-brained infant than with perfection of bipedal locomo-tion.Why Walk on Two Legs?Why did our lineage adopt this peculiar mode of gettingaround? It would not have made it any easier to escapefrom predators—most chimpanzees and all monkeys canoutrun us over short distances. An early argument held thatbipedalism developed to free the hands to make tools. Butas there is no sign of elaborate tool-making for a least thefirst 3-4 million years after the first bipedal members ofour lineage such as Sahelanthropus and Orrorin, it seems morelikely that later tool-makers took advantage of hands thatwere already partly freed. Another argument suggests thatbipedalism developed to facilitate moving between widely-spaced feeding trees, but Ardipithecus seems to have inhab-ited fairly dense woodland. Did we become bipedal to seeover the tall grass? (My own experience in the western RiftValley suggests that the grass in many places is more thansix feet high, and Lucy was about 3.5 feet tall.) Two othertheories that are plausible but difficult to demonstrate are1) that bipedalism makes a primate appear larger and morethreatening to potential challengers and predators (malegorillas run short distances on their hind legs to threatenintruders) or 2) that bipedalism makes it easier to carry
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food and infants to a safe place to feed (but see the com-ment on Ardi's likely closed woodland habitat, above). Andeven ifwe initially became capable of bipedal walking forone of the above reasons, why abandon the safety of thetrees altogether after 1.8 million years ago, since by thenour ancestors had lost their climbing toes, short legs, longarms and upwardly mobile shoulders for hanging frombranches?Born to Run?A new theory proposed by Bramble, Lieberman and col-leagues in a series of papers from 2004 on, is that it is notabout bipedal walking at all. Ratherwe were "born to run,"and it is running that constitutes one of several major ad-aptations that helped shape our bipedal morphology afterAustralopithecus. How can this be, as human runners are rela-tively slow. Human legs and feet are well-adapted, how-ever to endurance running. In jogging or "marathon-mode," the long spring-like tendons in our legs (such as theAchilles tendon) and the arches in our feet store and thenrelease energy like a spring during one part of the runningstep for later release. Our shorter toes are well-adapted forpushing off, while long legs make it possible to cover agiven distance in fewer steps. Finally our long waist andother structures allowed the upper body to counteract thetwisting forces generated by running and stabilize the body.This is why runners feel compelled to pump their armsback and forth in opposite directions to the correspondinglegs when they run. Also, a new (in H. erectus) ligament tothe back of the head stabilizes the head on the spine. Run-ning also makes maximum use ofour large gluteal muscles,which first become enlarged in the genus Homo, while walk-ing uses them only minimally.In addition to these adaptations in our lower limbs,specializations for heat loss allow us to run for exception-
ally long distances even in the middle of the day. Theseadaptations include our elongated bodies compared to apes,hair loss, increased number of sweat glands, mouth-breath-ing while running, and possibly adaptations of the circula-tory system to better cool the brain.Bramble and Lieberman compare our endurancerunning to trotting, which several running mammals areable to maintain for considerable periods of time. Well-conditioned humans, however, "trot" faster than dogs and
can even out trot horses in hot conditions. Mammals canmove faster when galloping than trotting, but they cannotpant and gallop at the same time. Since non-human mam-mals cool mostly by panting, they can die of heat exhaus-tion (hyperthermia) when forced to gallop for extendedperiods of time in the heat. The authors argue that ourhuman ability to run with long strides, sweating, and otherheat-loss adaptations account for why so many humans,even into their 70's and 80's, are able to run marathons.An ability to run long distances in the heat of the daywould have conferred considerable advantages on earlyhumans, who incorporated increasing amounts of meatinto their diets but seem to have lacked sophisticated weap-onry for hunting or for challenging other carnivores at kill
sites. In 2006, Liebenberg published a study of so-calledpersistence hunting by modern San peoples in the KalahariDesert in Botswana. (One of these hunts can be viewed at(http://www.youtube.com/watch?v=fUpo mA5RP8 .)Hunts in the heat of the day last from two to eight hoursof running after the animal through sand and brush overdistances of 25-35 km. at average speeds of 6-10 km/hour, which is not especially fast even by marathon stan-dards. At this point, the animal collapses from heat exhaus-tion, sometimes not even requiring the hunter to finish itoff. Since persistence hunting requires considerable track-ing abilities, has a risk of dehydration, and is rare amongmodern hunters, Pickering and Bunn challenged Brambleand Lieberman's scenarios. Did early humans who lived insavanna woodlands rather than in desert bushlands havethe ability to track animals as the San do? Lieberman andothers replied that even non-human carnivores have the
ability to follow prey, that following a wounded animalalso would have required tracking skills, and that persis-tence hunting even today is often more successful than hunt-ing with a bow and arrow. Further, hunters can carry waterin an ostrich eggshell or a skin bag. Did they actually rundown animals? We can only ask how they might have ac-quired large prey otherwise, without sophisticated weap-onry. Running may have developed out of an earlier adap-tation to bipedal walking as a way to cope with increasingaridity and use of open environments by our ancestorsafter 1.8 mya.
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Further work on running, much of it by Lieberman,his colleagues and students, has demonstrated that shortertoes lower the energetic cost of running but not of walk-ing, and that habitual barefoot runners (studied in Americaand Kenya) land on the balls of their feet (or sometimeson the mid-foot) rather than on the heel. Even with a highlycushioned modern running shoe, landing on the heel gen-erates more "shock" or force that travels up the leg, whilelanding on the forefoot almost entirely eliminates any colli-sional force on impact, making barefoot running comfort-able and easy to do on even the hardest surfaces. Studieson the developmental history of these adaptations in child-hood and on physiological effects of hormones on bonegrowth have also added to our understanding of this fun-damental and unique human adaptation.
ReferencesBennett, M.R., etal. 2009. Early Hominin Foot Morphol-ogy Based on 1 .5 Million-Year-Old Footprints from Ileret,Kenya. Science 323: 1 197-1201
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Berger, L.R., eta/. 2010. Australopithecus sediba: A New Spe-cies of Homo-Like Australopith from South Africa. Science328: 195-204.Bramble, D.M., and Lieberman, D.E. 2004. EnduranceRunning and the Evolution of Homo. Nature 432: 345-352.Devlin, M. J., and D.E. Lieberman. 2007. Variation in Es-tradiol Level Affects Cortical Bone Growth in Responseto Mechanical Loading in Sheep. Journal of Experimental Bi-ology 2 10: 602-613.Liebenberg, L. 2006. Persistence Hunting by ModernHunter-Gatherers. CurrentAnthropology 47 (6): 1017-1025.Lieberman, D.E., and DM. Bramble. 2007. The Evolu-tion of Marathon Running Capabilities in Humans. SportsMedicine 37(4-5): 288-290.Lieberman D.E., eta/. 2007. The Evolution of EnduranceRunning and the Tyranny of Ethnography: A Reply toPickering and Bunn. Journal of Human Evolution 53: 439-442.Lieberman D.E., et a/. 2006. The Human Gluteus Maxi-mus and Its Role in Running. Journal of ExperimentalBiology209:2143-2155.
Lieberman, D.E. etal. 2010. Foot Strike Patterns and Col-lision Forces in Habitually Barefoot Versus Shod Runners.Nature 463: 531-535.
Pickering, T.R., and H.T Bunn. 2007. The Endurance Run-ning Hypothesis and Hunting and Scavenging in Savanna-Woodlands. Journalof Human Evolution 53: 434-438.Richmond, B.G, and W.L. Jungers. 2008. Orrorin tugenensisFemoral Morphology and the Evolution of Hominin Bi-pedalism. Science 318: 1662-1665,
Rolian, C, etal. 2009. Walking, Running and the Evolutionof Short Toes in Humans. Journal of Experimental Biology212:713-721.White, T.D., et al. 2009. Ardipithecus ramidus and thePaleobiology of Early Hominids. Science 326: 75-86. (Seealso articles in the same issue by Lovejoy, O. et al?)ZollikoferC.P.E., et al. 2005. Virtual Cranial Reconstruc-tion of Sahelanthropus tcbadensis. Nature 434: 755-759.
Alison S. Brooks is professor of anthropology at GeorgeWashington University and editor of 'AnthroNotes"
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