A publication of the Archaeological Institute of America
Can chimpanzees and orangutans tell us anything about why our ancestors stood up?
Five chimpanzees were trained to walk upright on treadmills while wearing oxygen masks. (© 2005 Cary Wolinsky)
While Americans were getting ready for bikini season, chimpanzees were hitting the gym for a different reason. Michael Sockol of the University of California, Davis and David Raichlen of the University of Arizona trained five chimpanzees to walk on treadmills in a study they say shows that early humans began walking upright (i.e. bipedalism) because it saves energy. In another study looking at orangutans, S.K.S. Thorpe of the University of Birmingham argues that bipedalism allows for efficient travel through flimsy branches.
What these theories have in common is that they imply that bipedalism emerged as a way to save energy. Sockol says, "Regardless of what else may have played a role, energy consumption is unavoidable in the day-to-day life of all animals. Any energy saved during any other activity--such as walking--can be spent on growth and reproduction. So, while other factors may have contributed to the initial shift to bipedality, energy consumption is inescapable."
If accepted, these theories could end a century-old debate regarding the origins of bipedalism. Scholars as far back as Charles Darwin have been intensely interested in the issue because it would help them pinpoint when the human lineage began. Robert Eckhardt of Penn State explains, "You can think of upright posture and bipedal locomotion as being the signature for the human lineage. And when we find the earliest evidence for that, we're pretty sure that we have the earliest evidence for the human lineage."
Chimps on Treadmills
In a study published in the Proceedings of the National Academy of Sciences, Sockol and Raichlen compared how much energy humans and chimpanzees use while walking. Five chimpanzees were taught to walk upright on treadmills while wearing oxygen masks. Raichlen said that the training was done over four months, but only when the chimps were cooperating. "These guys are smart enough that they would hit the stop button on the treadmill when they were done. If they didn't want to walk on the treadmill, they'd just hit the stop button or they'd jump off."
Researchers then collected data about the subjects' metabolism and muscle movement. Based on knowledge of how much energy is involved in muscle movement, they were able to calculate the overall energy consumption of each different walk. Bipedalism was found to be costlier for three of the five chimps, which would be expected from an animal that usually walks on all fours. The surprising finding was that bipedalism was equally or more efficient for the other two chimps. Moreover, the human bipedal gait was found to be nearly four times more efficient than chimpanzee quadrupedalism.
Based on their findings, Sockol and Raichlen concluded that the energy cost of walking is grounded in anatomy. Before the study, they predicted that longer limbs would reduce overall energy costs by lengthening the stride. Since force is exerted every time the foot is placed on the ground, lengthening the interval between steps has the effect of reducing the amount of energy used.
Additionally, they predicted that pelvis orientation would affect bipedal efficiency. In humans, the back of the pelvic bone sticks out perpendicular to the ground, which allows the hamstrings to create a moving force while the legs are vertical. For chimps, however, the back of the pelvis points down, causing them to hunch over before their hamstrings can move. Confirming their predictions, the two most efficient bipedal chimps had longer limbs and more perpendicular pelvic bones.
So what does this have to do with humans? These two anatomical traits were also found in early human fossils between 1.5 and 2.5 million years old. According to Sockol and Raichlen, the presence of these traits suggests that bipedalism was more energy efficient for our ancestors too. Early humans who walked upright would have had the advantage of extra energy to gather food, grow, and reproduce. In other words, bipeds would have been stronger candidates for natural selection.
When ARCHAEOLOGY asked Owen Lovejoy of Kent State University about Sockol's study, he sighed and said, "I don't know why we're even talking about it....The study makes no significant contribution to my knowledge." The problem with Sockol's study, in his opinion, is that it demonstrates the exact opposite of what it intends to show. Rather than showing that bipedalism is energy efficient, Sockol reported that the majority (3/5) of chimps were actually less efficient on two legs. When averaged as a group, the chimpanzees showed no significant difference between bipedalism and quadrupedalism.
These findings are consistent with an earlier study, performed in 1973 by C. Richard Taylor of the Harvard Museum of Comparative Zoology. Taylor trained juvenile chimpanzees to walk bipedally and quadrupedally on a treadmill. Like Sockol, he found that chimpanzees, averaged as a group, use about the same amount of energy for each gait. Thus, aside from confirming previous research, Lovejoy does not believe the study adds anything new.
However, Alan Walker of Penn State thinks Lovejoy is being too hard on Sockol. According to Walker, the new data is valuable because it pertains to adult chimpanzees. Taylor's 1973 study involved only juvenile chimps, which expend energy differently. Walker says that "there have always been people who were worried that the numbers weren't right...that the comparisons weren't possible because they were baby chimps."
Not to mention the extreme difficulty of working with adult chimpanzees. "Previous studies... [used] all baby chimps because they were easy to handle," Walker said. "If they start getting grumpy, you can grab them and put them in a cage. You can't do that with an adult chimpanzee. They tear your head off." He concludes, "This is difficult stuff to do and I think that it's better than nothing. It's obviously much better than the previous studies."
And Sockol's response? He thinks the criticism is entirely misguided. The important finding, in his opinion, were the individual results, not the averages. He states that bipedalism "is more costly for some chimps. It is also less costly for others, and that's the whole point of these results; individuals vary in this regard." The significant finding was that bipedalism was more efficient for certain chimpanzees, namely those with longer limbs and perpendicular pelvic bones.
This individual variation is critical to Sockol's argument. Without diversity, he argues, individuals cannot gain the advantage necessary to win battles of natural selection. The diversity "is the most important point because it is just the kind of [variation] that natural selection acts upon." By showing that one anatomically unique chimp was a more efficient biped, Sockol believes he can provide evidence for why certain anatomical traits were selected for in our ancestors.
Orangutans in Trees
Thorpe demonstrates the role of energy using a different approach. Rather than setting up treadmills, she and her team decided to study orangutans in their natural habitat. They spent a year in Indonesia's Gunung Leuser National Park recording 2,811 observations of orangutan movement through different types of branches. Their results, published in the June 2007 issue of Science, suggest that orangutans rely on bipedalism when they encounter flimsy branches or use multiple supports.
An orangutan uses bipedal locomotion to navigate multiple branches. (© Susannah K. Thorpe)
Thorpe believes her observations are relevant to human bipedalism because modern humans and orangutans use similar muscle groups when walking bipedally. Whereas chimpanzees and gorillas walk with flexed limb muscles, humans and orangutans walk with straight limbs. Additionally, they both use the "orthograde clamber" when moving through trees. This means that their trunk is erect and their body weight is mainly supported by their forelimbs when they are hanging onto branches above their head. She believes--and Walker agrees--that these similarities justify a comparison between orangutans and humans in the case of bipedality.
According to Thorpe, bipedalism would have saved energy for our ancestors, just as it saved energy for the orangutans in her study. To give one example, crossing gaps bipedally using thin, peripheral branches consumes less energy than climbing down the tree, crossing the ground, and climbing back up the tree. Additionally, orthograde suspension is efficient because it increases stability and reduces knee and elbow flexion. Thus, bipedalism may have emerged as a way for human ancestors to save energy as they moved through trees.
Thorpe and her team reinforced their observations with evidence from the fossil record. Using fossil morphology, computer modeling, human skeletal development, and experimental studies of human exercise physiology, her team demonstrated that the early human ancestor, Australopithecus afarensis, would have been an efficient biped over short distances. This means that A. afarensis, like orangutans, could have reaped the energetic benefits of navigating tricky branches bipedally. After learning to move upright in trees, our early ancestors could then apply their bipedal repertoire to movement on the ground.
Once again, Lovejoy is not convinced. He argues that orangutans are exceedingly specialized for upper canopy life. So specialized, in fact, that they have actually adopted curled toes to navigate through branches. In his opinion, it is not fair to draw conclusions about human ancestors from a species with anatomy completely foreign to our own. Lovejoy does concede, however, that Thorpe's study provides useful information about the life of an orangutan, even if it does not shed light on the origins of bipedality.
Walker has deeper concerns, questioning Thorpe and Sockol's common assumption that humans must be energy efficient. "The thought that humans have to be efficient is a sort of Western capitalist thought," he says. With all the tasks they must do, Walker states, "We don't expect organisms to be efficient." Even if humans did desperately need to save energy, Walker says they could have used methods other than bipedalism. For example, they could have started hibernating like bears or simply moved less, like sloths.
Moreover, it is not even clear that less energy is better. A cheetah, for example, goes into oxygen debt during the mad dash to its prey, but then consumes a lot of calories at the finish line. Walker goes on, "I mean look at birds. Some birds build up with fat and then they fly. And carry on flying for thousands and thousands of miles....That's not efficient - I mean, to build up all that fat and fly? But it's the only way you can do it if you want a nice, warm climate all year round." Since many other animals have inefficient energy strategies, Walker questions whether humans alone were forced to develop bipedalism in order to simply save energy.
At the end of the day, Thorpe and Sockol's energy hypotheses seem to be just two more possible explanations for the origins of bipedalism. They can't be ruled out, but they aren't generally accepted, either. Other contenders include the theory that humans needed to stand upright to reach food higher in trees and Lovejoy's own hypothesis that male providers needed to carry food while their female mates cared for their young.
If this debate is ever going to end, Lovejoy says, "The fossil record is the only thing that's going to do it." Sockol agrees, admitting that one limitation of their theory is that energy-efficient limb and pelvic bones have not been recovered from fossils more than 4 million years ago. On the upside, he points out, "The results of our current study have allowed us to make predictions about what we should expect to find." In the future, Sockol hopes for archaeological evidence showing that the earliest hominids had longer limbs and perpendicular pelvic bones, which would strengthen the claim that the original shift to bipedalism was motivated by the need to save energy.
Eckhardt's strategy also relies on the fossil record. He believes his Orrorin tugenesis fossils are unique because CT scans reveal unusually intricate details of their internal bone structure. Eckhardt hopes to use this new knowledge of internal architecture to understand how bipeds benefit from thick-walled femur shafts. With the help of a team in France, he has also been able to reconstruct the environment that these early ancestors lived in. He says, modestly, "If we can put together the structure and function and the environment in which those new structures and functions evolved, we should be a bit further than where we are now."
Walker, on the other hand, does not think that any one fossil will settle the debate. He advises scholars to "keep all the hypotheses and strike them down as you find them to be in conflict with the facts." For now, it looks like scientists will have to be satisfied with a collection of possible explanations, rather than one single causal factor.
Laura Sexton is an undergraduate majoring in History, Philosophy, and Social Studies of Science and Medicine at the University of Chicago.