Energetics of biPEdaLism possibLE impucation in its oricin
A more parsimonious—and therefore more scientifically attractive—explanation of bipedalism was proposed by Peter
Rodman and Henry McHenry of the University of California at Davis in a 1980 publication. Very simply, they suggest that bipedalism might have evolved, not as part of a change in the nature of the diet or social structure, but merely as a result of a change in the distribution of existing dietary resources. Specifically, in the more open habitats of the late Miocene, hominoid dietary resources became more thinly dispersed in some areas; the continued exploitation of these resources demanded a more energy-efficient mode of travel—hence the evolution of bipedalism. In this scenario, the evolution
FICURE 17.6 The basicranium: Because the skull is perched atop a vertical spine in a biped, the foramen magnum (through which the spinal cord enters the cranium) is located toward the center of the cranium; it is found toward the back in apes. The occipital condyles articulate with the first vertebra (atlas vertebra) of the axial spine.
- FICURE 17.7 Hypothesized causes of bipedalism: Perhaps the defining characteristic of hominins, bipedalism has inevitably long been the focus of speculation as to its evolutionary cause. Some of the main ideas are shown here.
of bipedalism reflects the improved locomotor efficiency associated with foraging, and nothing else. (Interestingly, efficiency of foraging is invoked in a recent description of bipedal features in the Miocene ape Oreopithecus bambolii.)
Rodman and McHenry's proposal is based on a few simple points. First, although human bipedalism is less energy-efficient than conventional quadrupedalism at high speeds, it is just as efficient—or more so—at walking speeds. Second, chimpanzees are roughly 50 percent less energy-efficient than conventional quadrupeds when walking on the ground, whether they employ knuckle-walking or move bipedally. Therefore, noted Rodman and McHenry, "there was no energetic Rubicon separating hominoid quadrupedal adaptation from hominin bipedalism." (See figure 17.8.)
For bipedalism to evolve among hominoids, only a selective advantage favoring improved energetic efficiency of locomotion was necessary. A more dispersed food resource could provide such a selection pressure.
Rodman and McHenry's hypothesis has recently been challenged on several counts, particularly by Karen Steudel of the University of Wisconsin. She points out that this scenario implicitly assumes that the common ancestor of humans and African apes was a knuckle-walker, which may not be correct (see unit 15). In addition, the postcranial skeleton of the early hominins differed from that of modern humans, specifically in including a significant degree of arboreal adaptation. The energy efficiency of bipedal locomotion in these creatures is therefore likely to have been lower than in modern humans, upon which the above energetic calculations were based. There is, concludes Steudel, "no reason to suppose that our quadrupedally-adapted ancestors would have reaped energetic advantages when they shifted to an upright stance."
Robin Crompton, of the University of Liverpool, and his colleagues have also weighed in over issues of energy efficiency of early hominins. The skeletal remains of Australopithecus afarensis have been widely interpreted to imply a bent knee, bent hip (BKBH) mode of gait, as opposed to the striding gait of modern humans. Their investigations show that a BKBH gait is considerably less energy-efficient than a striding
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FICURE 17.8 Energetics of locomotion: The solid line represents the energy cost of running (at 3.83 meters per second) in mammals of different body size; the dotted line shows the cost of walking (at 1.25 meters per second). Note that chimpanzees are less efficient than other mammalian quadrupeds at both running and walking, while humans are less efficient at running but more efficient at walking. gait. In computer simulations and treadmill measurements, Crompton and his colleagues show that oxygen consumption increases by 20 percent, and core body temperature rises almost 2°C after 15 minutes of BKBH locomotion. They argue that "a very substantial (and in our view as yet unidentified) selective advantage would have had to accrue, to offset the selective disadvantages of 'bent-hip, bent-knee' gait in terms of energy transformation." Jack Stern, of the State University of New York, challenged the conclusion, and pointed out that even BKBH locomotion would be effective for moving between dispersed food sources, if not for longdistance journeys. Rodman and McHenry maintain that, although their hypothesis may have oversimplified the situation, it remains valid. Lynne Isbell, of Rutgers University, and Truman Young, of Fordham University, recently extended the evolutionary context of the energy-efficiency hypothesis to other African hominoids. If, as the hypothesis argues, Miocene climate change made hominoid dietary resources less densely distributed, then hominoids would have been forced to become more efficient in exploiting them. Isbell and Young accept that bipedalism represents one potential adaptation to this situation, which inevitably requires an increase in the daily travel distance while foraging for dispersed resources. A second strategy is to reduce the required daily travel distance, which is achieved by diminishing group size. (A large group requires more total food resources each day than a small group, and must therefore travel further to harvest it.) This strategy, argue Isbell and Young, was adopted by chimpanzees, which exhibit a fission-fusion group structure. As part of their argument, they cite field observations of gorillas and chimpanzees in Gabon, where the apes feed heavily on fruits. When these resources become scarce, gorillas maintain their group size, but switch their dietary emphasis to leaves. In contrast, chimpanzees continue to eat fruits, but forage in smaller groups or even alone. (See figure 17.9.) Isbell and Young's analysis is important because it puts hominin bipedalism within a general evolutionary ecology context of different behavioral adaptations by African hom-inoids to the same environmental circumstances, that is, food resources becoming more widely distributed as a result of climate change. A key issue, of course, is what exactly were the environmental conditions when bipedalism originated, not what they were when the new mode of locomotion was well developed.
FICURE 17.9 Alternative adaptations: The solid lines depict the relationship between daily distance traveled and group size under conditions of low resource availability (upper line) and high availability (lower line). At low resource availability, a hominoid group of a particular size must travel a longer distance each day to harvest those resources. A shift from high to low resource availability occurred during the late Miocene. Chimpanzees might have adapted to the change by reducing group size, while the hominin adaptation involved the evolution of a more energy-efficient mode of locomotion—that is, bipedalism. (Adapted from Isbell and Young.) A hypothesis developed by Kevin Hunt, of Indiana University, shifts the focus away from foraging efficiency to feeding efficiency. From more than 600 hours of field observations of chimpanzees and their bipedal behavior—which included stationary feeding of fruits from bushes and low branches in small trees, and locomotion from one spot to another—Hunt made the following observations: 80 percent of bipedal behavior was related to stationary feeding; only 4 percent was observed during direct locomotion. Hunt suggests, therefore, that the hominin bipedal adaptation was primarily a feeding adaptation; only later in hominin history did it become a specifically locomotor adaptation. The plethora of hypotheses offered to explain the evolution of bipedalism reflects both a fertility of ideas among anthropologists and the difficulty of using available evidence to discriminate between them. Any attempt to test hypotheses must encompass the possibility that hominin bipedalism arose in a heavily wooded or forested environment, rather than in open woodland or grassland savannah as was once thought to be the case. | |
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