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Lessons from orangutans: Upright walking may have begun in trees

May 31, 2007
Courtesy American Association 
for the Advancement of Science 
and World Science staff

By ob­serv­ing wild orang­u­tans, re­search­ers have con­clud­ed that our up­right walk­ing style may have first aris­en in rel­a­tively an­cient, tree-dwelling apes. 

The the­o­ry chal­lenges cur­rent think­ing, which sug­gests two-legged walk­ing emerged in more re­cent hu­man an­ces­tors that had al­ready de­scended to flat lands. The new pro­po­sal ap­pears in the June 1 is­sue of the re­search jour­nal Sci­ence.

An adult fe­male Su­ma­tran Orang­u­tan and her in­fant in the Gu­nung Leuser Na­tion­al Park, In­do­ne­sia. (Cour­te­sy SKS Thorpe)


Up­right walk­ing, or bi­pe­dal­ism, has long been con­sid­ered a de­fin­ing fea­ture of hu­mans and our clos­est an­ces­tors. One of the lead­ing ex­plana­t­ions, known as the sa­van­nah hy­poth­e­sis, sug­gests that the an­ces­tors of chimps, go­ril­las and hu­mans de­scended from the trees and be­gan walk­ing on on all fours.

Over time, this four-leg­ged gait would have evolved in­to the “knuckle-walk­ing” that chimps and go­ril­las still use to­day; up­right, two-legged walk­ing would have been the next stage.

Pa­le­on­tol­o­gists have con­ven­tion­ally used signs of bi­pe­dal­ism as key cri­te­ria for dis­tin­guish­ing early hu­man, or “ho­min­in,” fos­sils from those of oth­er apes. 

But this dis­tinc­tion is com­pli­cat­ed by re­cent fos­sil ev­i­dence that some early ho­minins, in­clud­ing the crea­ture dubbed “Lucy” (Aus­tra­lo­pith­e­cus afaren­sis), lived in wood­lands. Still ear­li­er forms seem to have lived in the for­est can­o­py and moved on two legs.

“Our find­ings blur the pic­ture even fur­ther,” said Rob­in Cromp­ton of the Un­ivers­ity of Liv­er­pool, U.K., one of the study’s au­thors. “If we’re right, it means you can’t rely on bi­pe­dal­ism to tell wheth­er you’re look­ing at a hu­man or oth­er ape an­ces­tor. It’s been get­ting more and more dif­fi­cult for us to say what’s a hu­man and what’s an ape, and our work makes that much more the case.”

Cromp­ton and col­leagues at the Un­ivers­ity of Bir­ming­ham in Bir­ming­ham, U.K., drew their con­clu­sions by watch­ing wild orang­u­tans in Su­ma­tra, In­do­ne­sia. Orangutans spend al­most their whole lives in trees, mak­ing them use­ful mod­els for how our an­ces­tors moved around sev­er­al mil­lion years ago, Cromp­ton and col­leagues said.

To col­lect the da­ta, the Un­ivers­ity of Bir­ming­ham’s Su­san­nah Thorpe spent a year in the Su­ma­tran rainfor­est re­cord­ing vir­tu­ally eve­ry move the orang­u­tans made. Then, she and her col­leagues used these ob­serva­t­ions to test the hy­poth­e­sis that bi­pe­dal­ism would have ben­e­fit­ed tree-dwelling ape an­ces­tors.

Be­cause these an­ces­tors were probably fruit-eaters, as orang­u­tans are, they would have needed a way to nav­i­gate the thin, flex­i­ble branches at the tree’s pe­riph­ery, where the fruit typ­ic­ally is. Mov­ing on two legs and us­ing their arms pri­marily for bal­ance, or “hand-as­sisted bi­pe­dal­ism,” may have helped them trav­el on these branches, the re­search­ers said.

They an­a­lyzed nearly 3,000 ex­am­ples of orang­u­tan move­ment, and found that the orang­u­tans were more likely to use hand-assisted bi­pe­dal­ism when they were on the thinnest branches. When bi­pe­dal, the an­i­mals al­so tended to grip mul­ti­ple branches with their long toes.

On me­di­um-sized branches, the orang­u­tans used their arms more to sup­port their weight, chang­ing their mov­ing style to in­cor­po­rate hang­ing. They only tended to walk on all fours when nav­i­gat­ing the larg­est branches, the re­search­ers found.

Hand-assisted bi­pe­dal­ism may have of­fered sev­er­al ad­van­tages that al­lowed our tree-dwel­ling an­ces­tors to ven­ture on­to thin branches, Cromp­ton and col­leagues argued. The ani­mals could have gripped mul­ti­ple branches with their toes and dis­trib­ut­ed their cen­ter of gra­vity more ef­fec­tive­ly, while keep­ing one or both of their long arms free to reach for fruits and oth­er sup­ports. 

Orangutans al­so keep their legs straight while stand­ing on bend­ing branches, the au­thors re­ported. The ben­e­fit of the straight legs is un­clear, but when hu­mans run on springy sur­faces, we al­so keep our weight-bearing legs straighter, so this may have an en­er­gy-related ad­van­tage.

“Our re­sults sug­gest that bi­pe­dal­ism is used to nav­i­gate the small­est branches where the tasti­est fruits are, and al­so to reach fur­ther to help cross gaps be­tween trees,” said Thorpe.

The au­thors pro­pose an ev­o­lu­tion­ary sce­nar­i­o that be­gins as oth­er re­search­ers have en­vi­sioned. Some­where to­ward the end of the Mi­o­cene era, 24 to 5 mil­lion years ago, cli­mate in East and Cen­tral Af­ri­ca be­came al­ter­nately wet­ter and dri­er, and the rainfor­est in­creas­ingly patchy. Apes liv­ing in the for­est can­o­py would have be­gun to en­coun­ter gaps be­tween trees that they couldn’t cross through the high can­o­py. 

Hu­man an­ces­tors would thus have aban­doned the high can­o­py for the for­est floor, where they re­mained bi­pe­dal and be­gan eat­ing food from the ground or smaller trees. The an­ces­tors of chimps and go­ril­las, on the oth­er hand, be­came more spe­cial­ized for ver­ti­cal climb­ing be­tween the high can­o­py and the ground. They thus would have de­vel­oped knuckle-walk­ing for cross­ing from one tree to anoth­er on the ground.

The upright style, called “ar­bor­e­al bi­pe­dal­ism, had very strong adap­tive ben­e­fits. So, we don’t need to ex­plain how our an­ces­tors could have gone from be­ing quad­ru­pe­dal to be­ing bi­pe­dal,” Thorpe said. Ob­serva­t­ions of orang­u­tan move­ment should be use­ful for con­serva­t­ion ef­forts, ac­cord­ing to Thorpe. These an­i­mals are se­ri­ously en­dan­gered, mainly be­cause of hab­i­tat de­struc­tion. 

“If you can un­der­stand how they cross gaps in the for­est, you can learn about ef­fects that liv­ing in logged or de­grad­ed hab­i­tat would have on their lo­co­mo­tion. These could af­fect en­er­gy lev­els, for ex­am­ple, if they have to go to the ground, which is in­credibly risky be­cause the Su­ma­tran ti­ger is down there lick­ing its lips. The Su­ma­tran orang­u­tan popula­t­ion is pre­dicted to be ex­tinct in the next dec­ade if hab­i­tat de­grada­t­ion con­tin­ues. Our re­search fur­ther high­lights the need for pro­tect­ing these an­i­mals,” she said.


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By observing wild orangutans, researchers have concluded that our upright walking style may have first arisen in relatively ancient, tree-dwelling apes. The theory challenges current thinking, which suggests two-legged walking emerged in more recent human ancestors that had already descended to flat lands. The new theory appears in the June 1 issue of the research journal Science. Upright walking, or bipedalism, has long been considered a defining feature of humans and our closest ancestors. One of the most popular explanations, known as the savannah hypothesis, suggests that the ancestors to chimps, gorillas and humans descended from the trees and began walking on the ground on all fours. Over time, this four-legged gait would have evolved into the “knuckle-walking” that chimps and gorillas still use today and then into upright, two-legged walking in humans. Paleontologists have conventionally used signs of bipedalism as key criteria for distinguishing early human, or “hominin,” fossils from those of other apes. But this distinction is complicated by recent fossil evidence that some early hominins, including the creature dubbed “Lucy” (Australopithecus afarensis), lived in woodlands, while still earlier forms seem to have lived in the forest canopy and moved on two legs. “Our findings blur the picture even further,” said Robin Crompton of the University of Liverpool in Liverpool, Great Britain, one of the study’s authors. “If we’re right, it means you can’t rely on bipedalism to tell whether you’re looking at a human or other ape ancestor. It’s been getting more and more difficult for us to say what’s a human and what’s an ape, and our work makes that much more the case.” Crompton and colleagues at the University of Birmingham in Birmingham, U.K., came to their conclusions by observing wild orangutans in Sumatra, Indonesia. Orangutans spend almost their whole lives in trees, making them useful models for how our ancestors moved around several million years ago, Crompton and colleagues said. To collect the data, the University of Birmingham’s Susannah Thorpe spent a year in the Sumatran rainforest recording virtually every move the orangutans made. Then, she and her colleagues used these observations to test the hypothesis that bipedalism would have benefited tree-dwelling ape ancestors. Because these ancestors were probably fruit-eaters, as orangutans are, they would have needed a way to navigate the thin, flexible branches at the tree’s periphery, where the fruit typically is. Moving on two legs and using their arms primarily for balance, or “hand-assisted bipedalism,” may have helped them travel on these branches. The researchers analyzed nearly 3,000 examples of observed orangutan movement, and found that the orangutans were more likely to use hand-assisted bipedalism when they were on the thinnest branches. When bipedal, the animals also tended to grip multiple branches with their long toes. On medium-sized branches, the orangutans used their arms more to support their weight, changing their moving style to incorporate hanging. They only tended to walk on all fours when navigating the largest branches, the researchers found. Hand-assisted bipedalism may have offered several advantages that allowed our arboreal ancestors to venture onto thin branches. They could have gripped multiple branches with their toes and distributed their center of gravity more effectively, while keeping one or both of their long arms free to reach for fruits and other supports. Orangutans also keep their legs straight while standing on bending branches, the authors report. The exact benefit of the straight legs is still unclear, but when humans run on springy surfaces, we also keep our weight-bearing legs relatively straight, so this may have an energy-related advantage. “Our results suggest that bipedalism is used to navigate the smallest branches where the tastiest fruits are, and also to reach further to help cross gaps between trees,” said Thorpe. The authors propose an evolutionary scenario that begins as other researchers have envisioned. Somewhere toward the end of the Miocene epoch (24 to 5 million years ago), climate in East and Central Africa became alternately wetter and drier, and the rainforest grew increasingly patchy. Apes living in the forest canopy would have begun to encounter gaps between trees that they could not cross at the canopy level. Thorpe and colleagues suggest early human ancestors responded to this by abandoning the high canopy for the forest floor, where they remained bipedal and began eating food from the ground or smaller trees. The ancestors of chimps and gorillas, on the other hand, became more specialized for vertical climbing between the high canopy and the ground and thus developed knuckle-walking for crossing from one tree to another on the ground. This style, called “arboreal bipedalism, had very strong adaptive benefits. So, we don’t need to explain how our ancestors could have gone from being quadrupedal to being bipedal,” Thorpe said. Observations of orangutan movement should be useful for conservation efforts, according to Thorpe. These animals are seriously endangered, primarily due to habitat destruction. “If you can understand how they cross gaps in the forest, you can learn about effects that living in logged or degraded habitat would have on their locomotion. These could affect energy levels, for example, if they have to go to the ground, which is incredibly risky because the Sumatran tiger is down there licking its lips. The Sumatran orangutan population is predicted to be extinct in the next decade if habitat degradation continues. Our research further highlights the need for protecting these animals,” she said.