The Hominid Pelvis
The most dramatic illustration of bipedalism is the pelvis, and the most dramatic specimen demonstrating pelvic morphology is the relatively complete skeleton from Hadar, Lucy, AL 288-1. This important fossil preserves a nearly complete innominate (hip) bone and sacrum, which have been used to reconstruct the pelvis of this individual. The upper portion of the innominate bone, called the ilium, is short and curving compared to the long, flattened ilium of chimpanzees and other apes. This curvature provides an anterior attachment for the muscles that pull the femur forward during walking or running. Although the ilia are short in length compared to a chimpanzee, they extend more broadly to the side, resulting in a pelvis that is very broad overall. Indeed, this pelvis is nearly as wide as that of a human female, despite the small body size of the australopithecine. This pelvic width contributes to a very different body shape for early hominids than for humans.
The width of the pelvis affects the muscular requirements of walking. Whenever one leg supports the body, the trunk of hominids tends to fall away from the supporting leg. The muscles that prevent the body from falling over attach to the lateral part of the ilium and to the femur, pulling the trunk upward around the hip joint. A wide ilium tends to increase the efficiency of these muscles. Their effectiveness is also aided by a long femur neck, just as long handles on a pair of scissors greatly increase the force with which they can cut. This configuration of muscles is similar to the condition found in living people, but it is much more extreme. In living people, the pelvis is much narrower in proportion to overall body height, and femur necks are much shorter in proportion to femur length.
A number of hypotheses compete to explain why the bipedal adaptation of early hominids should be different from modern human bipedalism in this way. One explanation is that the pelvic width contributes to the length of the stride, by rotation of the pelvis during walking. This rotation would increase stride length without increasing the length of the swing of the legs, allowing an increased stride without lowering the mass of the body--which if lowered would subsequently have to be raised again through greater muscular exertion (Rak 1991). Alternatively, the widely spaced femora may allow a greater mechanical advantage for the muscles that draw the legs medially, toward the midline. Such a configuration would be advantageous for certain leg motions, especially the style of climbing that requires the legs to clamp around a branch or trunk. This kind of climbing would be more necessary to bipeds who lacked the prehensile feet of living apes.
The wide pelvis of early hominids had one consequence beyond those related to bipedalism, by setting the stage for a major evolution of the birth process. During birth, the infant must pass through the birth canal, entering it through the pelvic inlet. Although relatively small compared to the pelvic inlets of the apes, the oval shape of the australopithecine pelvic inlet probably did not create any problems during birth because australopithecine infants likely had head sizes about the same as those of chimpanzees. Even so, Robert Tague and Owen Lovejoy (1986) have speculated that birth in australopithecines may have involved a more complex series of events than chimpanzee births, with the possibility that the infant head may have had to rotate a quarter-turn to be born in line with the sideways oval in a manner similar to the half-rotation of a human infant during birth. Karen Rosenberg and Wenda Trevathan (2002) suggest that at this time, hominid females began to require assistance with the birthing process. The pelvic constraints associated with bipedal locomotion would come to determine much more about the birth process in later hominids.
More on bipedalism in hominids
References:
Rak Y. 1991. Lucy's pelvic anatomy: its role in bipedal gait. J Hum Evol 20:283-290.
Rosenberg KR, Trevathan W. 1996. Bipedalism and human birth: the obstetrical dilemma revisited. Evol Anthropol 4:161-168.
Tague R, Lovejoy C. 1986. The obstetric pelvis of A.L. 288-1 (Lucy). J Hum Evol 15:237-255.
The robotic Lucy model
The BBC is running this article about a new study that evaluates the bipedality of A. afarensis using robotic design software:
Now, a team of scientists from around the UK have used computer robotic techniques to work out the most energy efficient gait for afarensis based on Lucy's skeleton and the Laetoli footprint trails.
They claim to have cleared up the debate by finding that, based on their model, Lucy almost certainly did walk tall.
There has been a long-standing debate about how human Lucy was
"Assuming that the early human relative Australopithecus afarensis was the maker of the Laetoli footprint trails, our study suggests that by 3.5 million years ago at least some of our early relatives - despite their small stature - could sustain efficient bipedal walking at absolute speeds within the range shown by modern humans," co-author Weijie Wang, from Dundee University, told the Scotsman newspaper.
So what we seem to have here is a computer software equivalent of early 1980's science. Perhaps they programmed Owen Lovejoy?
The paper (abstract) is a little more interesting than the BBC description. It does a kind of optimization modeling to find the speed and style of locomotion with the lowest energy cost. That lowest-cost speed was associated with a human-like gait at around 1 meter per second (m/s). Here is the logic:
Rather than trying to interpret the behaviour of such species by a combination of analogies to humans in certain anatomical regions with analogies to other apes elsewhere, it seems sensible to adopt a reverse-engineering approach and determine what kind of locomotion a particular set of body proportions were best 'designed' to perform. Since the locomotor system is concerned primarily with the application of external force by the body, simulation techniques drawn from mechanical engineering are a potential means of predicting the significance of differences in proportions for the motion and force characteristics of bipedal locomotion (Sellers et al. 2005: 2).
The authors relate their gait findings to the preserved Laetoli footprint trails, and find something very interesting. The trails have footprints that are very close together -- especially for the larger, possibly dual G2 trail. This has previously been interpreted as meaning that the walking speed of the larger individual who made this trail was very slow (Alexander 1984; Charteris et al. 1984) -- only around 0.7 - 0.8 m/s. The model in this study predicts a faster speed of around 1 m/s, which would be close to the optimal walking speed estimated for Lucy's proportions.
They do not address a relevant question, which is whether the larger trail was made by an individual larger than Lucy, who might have had a different optimal walking speed. Indeed, there is a basic assumption of monomorphism. It is partly covered by the fact that living people don't exceed 1.0 m/s for their average walking speed by very much, so the variation between large and small A. afarensis individuals may have been slight. The basic finding seems to be that shorter legs have an optimum gait that involves more, slightly shorter, steps, while longer legs take fewer steps more slowly. This isn't surprising based on a pendular model: shorter pendula have shorter periods, and longer steps with a shorter leg must require more energy-wasting up-and-down motion.
An important weakness of the model is that it considers costs only due to motion in two dimensions: forward and up-and-down. The wide pelvis of A. afarensis might be expected to exert greater costs in a side-to-side dimension compared to recent humans, and that energy effect is not considered.
But the bottom line:
Thus, within the limits of our model, and assuming that Taylor and Rowntree's (1973) data are reliable, the bipedal performance of Australopithecus afarensis, as predicted by our model is not only much closer to that of modern humans than to that of bipedally walking great apes, but at normal walking speeds, shows a clear speed/cost advantage over chimpanzee quadrupedalism. Climbing remains significantly more energetically expensive than terrestrial quadrupedalism for chimpanzees, despite their musculoskeletal adaptations (Pontzer and Wrangham 2004). Pontzer and Wrangham (2004) have shown that the costs of locomotion in chimpanzees are nevertheless dominated by terrestrial walking, because of very high daily travel distances, versus only limited use of climbing. If we make the major (and quite likely incorrect) assumption that the African apes existing at the time of A. afarensis were ecologically, morphologically and physiologically similar to modern common chimpanzees, then our data would tend to support Rodman and McHenry's (1980) argument that the adoption of bipedalism offered energetic advantages to early human ancestors (ibid., 9).
In any event, it won't quiet doubters:
However, Professor [Christopher] Stringer believes the controversy will not vanish overnight.
"There are still some people who argue that, looking at the anatomy of the foot bones of afarensis, that they were unlikely to have made the Laetoli footprints," he told the BBC News website.
"So it doesn't end the argument because there is still the possibility that there were different creatures around at the time."
No doubt soon, Kent State will have a droid afarensis army to finally crush this dissent and bring order to the galaxy.
References:
Sellers WI, Cain GM, Wang W, and Crompton RH. 2005. Stride lengths, speed and energy costs in walking of Australopithecus afarensis: using evolutionary robotics to predict locomotion of early human ancestors. Roy Soc Interface Online
Why be bipedal?
The skeletal adaptation to bipedalism is well documented in early hominids. What is less clear is what events led to this adaptation and its eventual success. Hypotheses about why bipedalism arose have been very common, but most lack the necessary evidence to test them. All apes can walk bipedally, so the behavior itself was within the capabilities of the common ancestors of hominids and chimpanzees. What is necessary is to explain how bipedalism became so essential that it provoked skeletal adaptations that made other forms of locomotion much more difficult.
One argument is efficiency. Human bipedalism is very efficient at normal walking speeds, because forward motion results from gravity swinging each leg forward like a pendulum. The walking biped recaptures this forward momentum by slowing the swinging leg before footfall. As a result, walking at normal speeds on level surfaces requires very little muscular activity, making bipedalism more efficient than knuckle-walking or quadrupedalism (McNeill Alexander 1985). Aside from its energetic efficiency, bipedalism also has the advantages of raising the head, and therefore allowing a wider range of vision in a grassland environment, and of freeing the hands for carrying items or for tool use.
Despite these advantages, bipedalism also has considerable disadvantages. The first is that it makes climbing considerably more difficult. Without the ability to grasp with the feet, hominids are less secure in an arboreal setting. There are many indications that climbing remained an important part of the behavior of early hominids, discussed below. The combination of features found in early hominids reflects a compromise adaptation to climbing, which is based on the presence of morphological adaptations to bipedalism in the pelvis and foot. Part of this compromise was structural, involving much more powerful arms and possibly human-proportioned hands for gripping branches rather than suspending from them. Another part of the compromise was behavioral. The loss of a grasping foot is also a serious problem for child-rearing. In chimpanzees and other primates, the young can use their hands and feet to grasp and cling to their mother's fur. For hominid infants, such clinging would have been much more difficult, if not entirely impossible. One of the adaptations to bipedalism must, then, have been a behavioral change toward carrying dependent offspring until they were old enough to walk.
Over time, scientists have devised many different theories to reconstruct the circumstances that led to the evolution of bipedalism. Charles Darwin himself correctly assumed that the African apes are the closest human relatives, and constructed a model for hominid origins that stressed the appearance of bipedalism from an ape-like ancestor. In Darwin's model, bipedalism is seen as the adaptation resulting when a quadrupedal ape is forced to assume a terrestrial adaptation. In Darwin's formulation, this adaptation was partly caused by the advent of a hunting subsistence pattern, where the hands needed to be free to carry weapons and meat. Additionally, Darwin thought that a change in habitat from woodland to savanna left early hominids without the refuge of trees, resulting in less importance of climbing and a greater need for efficient movement on the ground. Other later researchers picked up many of the themes of Darwin's model, stressing other important features of life on the savanna, such as the need to see over tall grass, and the need to adapt to intense solar radiation. Bipedalism has been suggested as an adaptation to both these factors, by placing the head high and upright, and decreasing the exposure of the trunk to direct light from overhead. This model came to be called the savanna model, or stressing the importance of hunting in the model, the killer-ape hypothesis.
Today, we have greater knowledge about the environments that early hominids occupied, and many aspects of the savanna model do not appear to describe the conditions under which bipedalism evolved. All of the sites before 3 million years ago seem to have been partially or fully wooded, and no early hominids are known from full savanna environments. Additionally, the fossil forelimb elements of early hominids demonstrate the continued importance of climbing. The importance of hunting has been questioned because chimpanzees hunt extensively without the adaptations of early hominids, and because no tools, weapons or adaptations to making tools are known from the earliest hominids. These observations imply that bipedalism was not the simple consequence of a single climatic change.
Lovejoy (1981) has suggested that social factors may have been principally responsible for the origin of bipedalism. In his hypothesis, food sharing was an important component of social behavior. Lovejoy speculates that males may have supplied food to females in order to gain mating access or to contribute to the parenting of their own offspring. This behavior would require the use of hands for provisioning. Such a hypothesis must be reconciled with the apparently high level of sexual dimorphism among early hominids, but may provide significant insights.
One reason for the proliferation of hypotheses to explain hominid origins is that we have almost no knowledge about the postcranial anatomy of the immediate ancestors of the hominids. Most hypotheses have assumed that the common ancestors of living African apes and hominids were essentially like chimpanzees, with suspensory locomotion in the trees and knuckle-walking on the ground. Whether hominids originally evolved from a knuckle-walking ape or not has been controversial. Some scientists, like Brian Richmond and David Strait (2000), argue that early hominids like Lucy bear anatomical features that indicate a knuckle-walking ancestry. In this formulation, the occasional bipedal locomotion of chimpanzees and gorillas is a model for how obligate bipedalism originated. The anatomical changes that characterize the known hominid fossils grow from a more intensive use of this basic hominoid behavior.
Other scientists point to the possibility that knuckle-walking evolved in parallel in chimpanzees and gorillas. The manner of arboreal locomotion in living and extinct apes seems to have been greatly influenced by body size. Known early hominids average slightly less than chimpanzees in body size, and it is possible that their common ancestor was small, rather than chimpanzee-sized. Wolpoff (1999) has suggested that the ancestors of hominids may have been small apes who often walked or ran bipedally above large branches, as well as on the ground. From this perspective, the knuckle-walking of chimpanzees and gorillas and the bipedalism of hominids represent different strategies for ground locomotion related to body size. Under this hypothesis, the large apes developed a suspensory adaptation in response to increases in body size, with locomotion on the ground occurring later than or secondary to this increase. In contrast, early hominids adapted more fully to the ground before their body size increased, resulting in an anatomical adaptation to bipedalism, with climbing secondary.
None of the factors here excludes any of the others, and probably the origin of hominid bipedalism involved a complex combination of these and possibly others. Until scientists have more knowledge about the anatomy of the first hominids and their ancestors, we will be unable to rigorously test these hypotheses. Nevertheless, even as the record of hominid evolution has been pushed back to six million years, bipedalism remains the hallmark adaptation of our lineage. For this reason, explanations for its origin remain one of the most important parts of paleoanthropology.
References:
Lovejoy CO. 1981. The origin of man. Science 211:341-350. JSTOR
McNeill Alexander Ra. 1992. Human locomotion. In: Jones S, Martin R, Pilbeam D, editors, The Cambridge encyclopedia of human evolution. Cambridge: Cambridge University Press. p 80-85.
Richmond BG, Strait DS. 2000. Evidence that humans evolved from a knuckle-walking ancestor. Nature 404:382-385. PubMed
Wolpoff M. 1999. Paleoanthropology. Second edition. New York: McGraw-Hill.
John Hawks Department of Anthropology
University of Wisconsin—Madison
Copyright © 2007 John Hawks